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Applied and Environmental Microbiology, August 2008, p. 4727-4736, Vol. 74, No. 15
0099-2240/08/$08.00+0     doi:10.1128/AEM.00297-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

High-Throughput Identification and Validation of In Situ-Expressed Genes of Lactococcus lactis

Herwig Bachmann,^1,2 Michiel Kleerebezem,^1,2,3,4 and Johan E. T. van Hylckama Vlieg^1,2,3*

NIZO food research, P.O. Box 20, 6710 BA Ede, The Netherlands,¹ Kluyver Centre for Genomics of Industrial Fermentation, Delft, The Netherlands,² TI Food & Nutrition, P.O. Box 557, 6700 AN Wageningen, The Netherlands,³ Wageningen University, Microbiology Department, Dreijenplein 10, 6703 HB Wageningen, The Netherlands⁴

Received 4 February 2008/ Accepted 24 April 2008

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Understanding the functional response of bacteria to their natural environment is one of the current challenges in microbiology. Over the past decades several techniques have been developed to study gene expression in complex natural habitats. Most of these methods, however, are laborious, and validation of results under in situ conditions is cumbersome. Here we report the improvement of the recombinase-based in vivo expression technology (R-IVET) by the implementation of two additional reporter genes. The first one is an -galactosidase gene (melA), which facilitates the rapid identification of in vivo-induced genes. Second, the bacterial luciferase genes (luxAB) are transcriptionally coupled to the resolvase gene, which allows rapid validation and characterization of in vivo-induced genes. The system is implemented and validated in the industrially important lactic acid bacterium Lactococcus lactis. We demonstrate the applicability of the advanced R-IVET system by the identification and validation of lactococcal promoter elements that are induced in minimal medium compared to the commonly used rich laboratory medium M17. R-IVET screening led to the identification of 19 promoters that predominantly control expression of genes involved in amino acid and nucleotide metabolism and in transport functions. Furthermore, the luciferase allows high-resolution transcription analysis and enabled the identification of complex medium constituents and specific molecules involved in promoter control. Rapid target validation exemplifies the high-throughput potential of the extended R-IVET system. The system can be applied to other bacterial species, provided that the reporter genes used are functional in the organism of interest.

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Understanding the responses of microorganisms in their natural environments is one of the big challenges of the postgenomic era. Complex niches such as soil, decaying plant material, and the (mammalian) gastrointestinal tract show a rich bacterial diversity and complex microbial activity patterns. Sampling complexity and environmental dynamics hamper comprehensive transcriptional analysis of a specific microorganism residing in such a complex environment. To overcome these challenges, a number of tools to study in vivo gene expression were developed during the past decades (25). One of the most powerful technologies is termed in vivo expression technology (IVET) and is based on transcriptional fusions of genomic DNA fragments to a selectable reporter gene (32). Recombinase-based IVET (R-IVET) is an IVET variation that employs a resolvase as the primary reporter gene. Upon expression of the resolvase, a chromosomally located marker, situated between two recombination sites, is excised from the genome (1). This irreversible step "records" in situ promoter activity and acts as a promoter-trap system, allowing the accumulation of promoter activity detection over extended in situ incubation periods. Moreover, the system is able to detect very low and transient gene expression in complex environmental conditions. An inherent property of IVET systems is that only the upregulation of genes can be detected. Efforts were made to broaden the spectrum of detectable genes in R-IVET systems by mutating the ribosomal binding site of the resolvase and using these mutants to construct multiple libraries (23). Another limitation of IVET systems is that the validation of induced target sequences in vivo is in many cases laborious and requires advanced transcript detection methodology (27).

In this study we addressed these restrictions, and here we describe the implementation of an advanced R-IVET system in the lactic acid bacterium Lactococcus lactis, which is one of the best characterized low-GC gram positive bacteria and is extensively used in complex food fermentations. This improved system employs an -galacotsidase gene (melA) for positive primary clone selection without selection pressure and the luciferase genes luxAB as a secondary reporter for the validation of selected target genes. Bacterial luciferase allows sensitive detection of promoter activity in vitro and in situ after the primary R-IVET selection process. To exemplify the applicability of the system, 19 promoters specifically activated in a minimal medium compared to a rich laboratory medium were identified, and their regulation by medium components was evaluated. The identified sequences regulate predominantly the expression of genes involved in amino acid and nucleotide metabolism and genes coding for transporters. The added value of the presence of luxAB in the recombinase vector is also illustrated by the high-resolution analysis of the purCSQLF promoter activity pattern in different media and during the different phases of growth.

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Bacterial strains and plasmids.
Bacterial strains and plasmids are listed in Table 1. Escherichia coli was grown under aerobic conditions in TY broth (33). Lactococcus lactis MG1363 was grown in M17 medium (Merck, Darmstadt, Germany) supplemented with 0.5% (wt/vol) glucose. L. lactis MG5267 and its derivatives were grown in M17 medium supplemented with 0.5% (wt/vol) lactose. Escherichia coli MC1061 (6) and Top10 (Invitrogen, Breda, The Netherlands) were used as an intermediate cloning host for the construction of pNZ5512 and pNZ5517. L. lactis MG1363 (12) was used as an intermediate cloning host for pNZ7125, pNZ5515, and pNZ5518. L. lactis NZ5500 was used as a cloning host for pNZ5519 and pNZ5520. The minimal medium for L. lactis that was used for R-IVET library screening was prepared as described by Poolman and Konings (30), with the modification that only eight amino acids were added (glutamate, histidine, isoleucine, leucine, methionine, valine, asparagine, and glutamine), which are essential for exponential growth of L. lactis (14). When appropriate, antibiotics were added to the media, at concentrations of 10 µg/ml chloramphenicol, 200 µg/ml erythromycin and 50 µg/ml kanamycin for Escherichia coli, and 5 µg/ml chloramphenicol and 5 µg/ml erythromycin for L. lactis unless indicated differently.

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TABLE 1. Bacterial strains, plasmids, and oligonucleotides used in this study

DNA techniques.
Plasmid preparation from E. coli and L. lactis was performed using Jetstar columns according to the manufacturer's manual (Genomed GmbH, Loehne, Germany). For L. lactis the manufacturer's protocol was modified by using a solution of 50 mM Tris, 10 mM EDTA, 100 µg/ml RNase, and 4 mg/ml lysozyme at pH 8.0 for the first step in the cell lysis protocol. Cell suspensions were incubated at 50°C for 1 h, and subsequently plasmid DNA was purified according to the manufacturer's recommendations. DNA techniques were performed as described by Sambrook et al. (33). Restriction endonucleases, DNA polymerases and DNA ligase were purchased from Invitrogen (Breda, The Netherlands), New England Biolabs (Leusden, The Netherlands), and Stratagene (La Jolla, CA) and used according to the manufacturer's recommendations. Oligonucleotides were purchased from Proligo (Paris, France) and Invitrogen (Breda, The Netherlands). Transformation of L. lactis was performed as described by Wells et al. (41).

Construction of Lactococcus lactis NZ5500.
The intergenic region between the convergent genes llmg_0760 and llmg_0761 in the genome of Lactococcus lactis MG1363 was selected as the target site for introduction of the R-IVET resolution cassette. This locus contains a unique BpuI restriction site between the 3' ends of the convergent genes which was used during cloning procedures (see below). The llmg_0760 and llmg_0761 3' encoding fragments and intergenic region were amplified with PWO Superyield DNA polymerase (Roche Diagnostics GmbH, Mannheim, Germany) PCR, using the primers lc0012FW1 and lc0323Rev1 and L. lactis MG1363 genomic DNA as the template. The 1.6-kbp amplicon was cloned into pCRblunt (Invitrogen, Breda, The Netherlands), yielding pNZ5511, and the identity and intactness of the cloned fragment were verified by DNA sequencing. To construct a loxP-ery-loxP resolution cassette, the erythromycin resistance gene cassette was obtained as a PstI fragment from pUC18ery (38) and subsequently treated with T4 DNA polymerase. The fragment was then digested with BamHI and cloned in BamHI-HincII-digested pUC19lox2 (4). From the resulting plasmid, the loxP-ery-loxP cassette was recovered as a PstI fragment, which was treated with T4 DNA polymerase and cloned in Bpu10I-digested and Klenow-fragment treated pNZ5511. The resulting plasmid was designated pNZ5513 and contains an llmg0760-loxP-ery-loxP-llmg0761 R-IVET resolution cassette. This cassette was recovered from pNZ5513 as a HindIII-PstI fragment, which was treated with T4 polymerase and cloned in ClaI-digested and Klenow fragment-treated pNZ5510, yielding pNZ5514. Plasmid pNZ5510 is a pNZ7101 (31) derivative from which the erythromycin gene was removed by HindIII digestion, Klenow treatment, and self-ligation. To obtain the -galactosidase-encoding gene (melA) of Lactobacillus plantarum under control of a strong lactococcal promoter, the melA coding region and its ribosome binding site were PCR amplified using the primers melAF94 and melAF95 and cloned in the SmaI-digested derivative of vector pIL253 (35), from which the EcoRI fragment had been removed by EcoRI digestion followed by self-ligation. The resulting vector (pNZ3250) was digested with BamHI and XbaI, treated with S1 nuclease, and religated. This was followed by the insertion of a multiple cloning site (composed of the oligonucleotides pRB056_122 and pRB056_123) into the EcoRI site upstream of the melA gene, generating the melA reporter plasmid pNZ3251. The melA gene was subsequently placed under transcriptional control of the strong lactococcal promoter of the usp45 gene (Pusp45) (37). Pusp45 was obtained as a 224-bp amplicon using the usp45FW3 and usp45REV3 as primers and chromosomal DNA of L. lactis MG1363 as a template. The Pusp45 amplicon was digested with NcoI and XhoI and cloned in equally digested pNZ3251, resulting in pNZ5515. The Pusp45-melA fragment was recovered from pNZ5515 as a DraI-NcoI fragment, treated with Klenow fragment, and cloned in pCRblunt (3), generating pNZ5516. The Pusp45-melA cassette was isolated from pNZ5516 as an EcoRI fragment, and protruding ends were removed by Klenow treatment. This fragment was cloned downstream of the erythromycin resistance gene into the R-IVET resolution cassette in SmaI-digested pNZ5514, yielding pNZ5517. This final R-IVET resolution vector contains a chromosomal integration cassette composed of the loxP-ery-Pusp45-melA-loxP resolution cassette flanked by llmg_0760 and llmg_0761 regions. Strain MG5267 was transformed with pNZ5517, and integrants were selected on LM17 supplemented with 2.5 µg/ml erythromycin. Replica plating revealed that out of 63 primary integrants, three colonies displayed chloramphenicol sensitivity, suggesting direct double-crossover integrants. The anticipated genetic organization in these candidate double-crossover integrants was verified by PCR using the primer combinations melAFW-lc0012FW2 and lc0323REV2-eryREV, leading to the identification of the desired R-IVET double-crossover integrant (designated NZ5500) that harbors the loxP-ery-Pusp45-melA-loxP cassette between the convergent genes llmg_0760 and llmg_0761.

Library construction.
A random chromosomal R-IVET library for L. lactis was constructed based on the previously described R-IVET vector pNZ7125 (4) that contains the cre gene as recombinase reporter, which was supplemented by the secondary reporter luxAB. The luxAB genes were isolated as a 2.5-kbp HindIII-PsiI fragment from pJIM2374 (8), treated with Klenow fragment, and cloned in pCRblunt (Invitrogen, Breda, The Netherlands), yielding pNZ5512. The luxAB fragment was reisolated from pNZ5512 as an EcoRV-HindIII fragment, protruding ends were removed by Klenow treatment, and subsequently the fragment was cloned downstream of the cre gene in PmlI-digested pNZ7125, resulting in pNZ5518. To verify the functionality of the cre-luxAB fragment in pNZ5518, the L. lactis usp45 promoter was amplified from genomic DNA of MG1363 with the primers usp45REV1 and usp45FW2. The amplified fragment was restricted with Sau3AI and ligated into BglII-digested pNZ5518, resulting in pNZ5519. A blunt-end cloning site was introduced upstream of the cre gene by oligonucleotide linker ligation (the linker was composed of AdaptVI-1 and AdaptVI-2) in NcoI-BglII-digested pNZ5518, yielding pNZ5520. This R-IVET vector is based on a low-copy replicon (pIL252 [35]) and contains a blunt-end cloning site (SrfI/SmaI) upstream of the cre-luxAB expression detection cassette.

To construct the R-IVET library, genomic DNA of MG1363 was partially digested with AluI and fragments ranging from 0.5 to 1.5 kilobase pairs were isolated from an agarose gel. The isolated DNA fragments were ligated into the SmaI-digested pNZ5520. The ligation mixture was digested with SrfI (the genome of MG1363 contains only two SrfI restriction sites) to eliminate self-ligated plasmids prior to transformation to NZ5500. Transformants were plated on LM17 containing 5 µg erythromycin/ml and 5 µg chloramphenicol/ml and incubated for 3 days. Colonies of approximately 23,000 transformants were collectively resuspended in 100 ml LM17 broth supplemented with chloramphenicol. Subsequently, multiple aliquoted glycerol stocks containing approximately 10¹⁰ CFU each were prepared and stored at –80°C. The excision rate during the counterselection procedure was determined by plating an aliquot of the primary transformation of the R-IVET library on LM17 supplemented with chloramphenicol and 5-bromo-4-chloro-3-indolyl--D-galactopyranoside (X--Gal) (MP Biomedicals, Amsterdam, The Netherlands). The excision rate was determined as the percentage of colonies showing a white phenotype on this melA-indicator medium.

Screening procedure.
Ten milliliters of minimal medium was inoculated with approximately 10⁶ cells of the R-IVET library and incubated at 30°C for 24 h. Cells were plated on LM17 supplemented with chloramphenicol and 30 µg/ml X--Gal and incubated for 3 days. Colonies that had undergone excision of the loxP-ery-melA-loxP fragment during the screening procedure in minimal medium showed a white phenotype, and they were transferred to 96-well microplates and glycerol stocks were prepared. Luminescence screening in LM17 was performed throughout growth as described below. A subset of identified clones, covering the whole range of observed luminescence activity levels, was selected for detailed analysis. Individual clones were precultured in minimal medium, and luminescence screens were performed in subcultures in minimal medium or minimal medium supplemented with 0.25% yeast extract (Difco, Sparks, MD), 0.25% Bacto tryptone (Difco, Sparks, MD), 0.5% meat extract (Merck, Darmstadt, Germany), or 0.5% soy broth (Difco, Sparks, MD). To ensure growth in the microplate reader under nonanaerobic conditions, the amino acid arginine (125 mg/liter) was added to the minimal medium in those cases where no rich-nutrient supplements were added.

For the supplementation of M17 medium with purines, adenine, guanine, and xanthine (Fluka, Zwijndrecht, The Netherlands), the components were dissolved in a 0.1 molar solution of sodium hydroxide at a concentration of 10 g/liter for each compound. Using these purine stock solutions, growth media were supplemented to final concentrations as indicated. Single-amino-acid supplementation of minimal medium was done with the following concentrations: serine, 1 g/liter; glycine, 0.5 g/liter; threonine, 0.68 g/liter; and tryptophan, 0.1g/liter. Experiments with the arcA clone were performed under anaerobic conditions in minimal medium with either 10, 2, 0.4, 0.08, 0.016, or 0 mM of arginine added, and samples were taken during logarithmic growth.

DNA sequencing.
Total DNA from candidate clones was isolated using the InstaGene matrix (Bio-Rad, Hercules, CA) according to the manufacturer's instructions. Amplification of the DNA inserts of the library plasmid pNZ5520 was performed by Herculase-PCR (Stratagene, La Jolla, CA) or Taq-PCR (Promega, Madison, WI) using the primers creREV2 and BglIICre (Table 1). Amplicons were purified using the Wizard SV gel and PCR clean-up system (Promega, Madison, WI) and used for end sequencing of the insert, using primer creREV2 as a sequencing primer and the ABI Prism Big Dye Terminators v 3.1 cycle sequencing kit (Applied Biosystems, Foster City, CA), according to the manufacturer's instructions.

Detection of luminescence.
The detection of luminescence was performed in white microplates with a transparent bottom (Greiner, Alphen a/d Rijn, The Netherlands). Cells were grown at 30°C in a Safire II microplate reader (Tecan, Zürich, Switzerland) and nonanal (Sigma, Zwijndrecht, The Netherlands) was supplied as a volatile by placing 50 µl of 1% nonanal diluted in silicon oil (Fluka, Zwijndrecht, The Netherlands) in all the empty spaces between the wells of the mircroplate. The plates were covered with a lid to entrap the volatile in the plate, and the luminescence and optical density at 600 nm (OD₆₀₀) were measured at 10-min intervals throughout growth. The measured values are expressed in lux/OD₆₀₀ (in arbitrary units). The maximum values of the lux/OD₆₀₀ measurements were taken for primary data analysis. For all luminescence measurements riboflavin was added to the medium to improve the luminescence assay (2). Luminescence measurements for the anaerobically grown arcA clone were performed as described elsewhere (2).

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Adapted R-IVET screen for L. lactis.
The cre-loxP system is functional in L. lactis (5), which suggests that the cre resolvase is suitable for the construction of an R-IVET screening system in this organism. For R-IVET screening purposes, a cre resolution target locus, loxP-ery-Pusp45-melA-loxP, was constructed and integrated into the chromosome of L. lactis MG5267, resulting in strain NZ5500 (Fig. 1). This strain is a derivative of L. lactis MG1363 that harbors the lactose operon integrated into its chromosome (39) and was chosen based on the fact that a full genome sequence is available (40) and because of its ability to utilize lactose as a carbon source. The cre resolution cassette confers erythromycin resistance to L. lactis NZ5500. The Lactobacillus plantarum WCFS1 gene melA, encoding an -galactosidase (18) under control of the lactococcal usp45 promoter, leads to a blue phenotype of NZ5500 on media containing X--Gal (15). The introduction of the cre resolution cassette in the chromosome of MG5267 did not affect the growth rate (data not shown). To verify the chromosomal stability of the cre resolution cassette in NZ5500, cells were propagated in LM17 for 200 generations and subsequently approximately 1,000 colonies were plated on LM17 supplemented with X--Gal. Only a single white colony that still displayed erythromycin resistance was observed, suggesting that a mutation in the melA gene had occurred and corroborating that the loxP-ery-Pusp45-melA-loxP locus is stable.

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FIG. 1. Schematic representation of the R-IVET system. On the chromosome, the integration locus of the loxP-ery-Pusp45-melA-loxP fragment is shown. The plasmid pNZ5520 harbors a cre-luxA-luxB construct which is flanked by the terminators Tlas and TpepN. The SmaI/SrfI restriction site upstream of cre was used for the insertion of chromosomal fragments.

Library construction.
An advanced R-IVET promoter-trap vector based on the previously described R-IVET vector pNZ7125 was constructed (4). This novel R-IVET vector (pNZ5520) harbors a promoterless cre-luxAB cassette downstream of an SfrI/SmaI cloning site. Introduction of pNZ5520 into NZ5500 resulted in a stable erythromycin-resistant and MelA-positive phenotype that remained unaffected by 200 generations of growth. The pNZ5520 derivative harboring the strong usp45 promoter upstream of the cre-luxAB cassette (pNZ5519) was also introduced in this strain. As anticipated, introduction of this plasmid in strain NZ5500 instantly led to an erythromycin-sensitive and MelA-negative phenotype in all transformant colonies. Moreover, the NZ5500 derivative harboring pNZ5519 displayed a >1,000-fold-increased level of luminescence in comparison to its pNZ5520-containing counterpart (data not shown). These experiments confirm the functionality of the cre/loxP resolution system as well as the luxAB reporter and validate the suitability of the constructed system for R-IVET screening. During further experiments, the pNZ5519- and pNZ5520-harboring derivatives of strain NZ5500 were used as luminescence-positive and -negative controls, respectively.

A plasmid library was constructed by cloning 0.5- to 1.5-kbp random fragments of MG1363 chromosomal DNA into the SfrI/SmaI site of pNZ5520. The resulting R-IVET library was transformed into NZ5500 cells, and transformants were plated on LM17 supplemented with chloramphenicol and erythromycin for counterselection. During this counterselection, active promoter elements (e.g., promoters of housekeeping genes upstream of cre) will lead to the expression of the resolvase, leading to clones with an erythromycin-sensitive and MelA-negative phenotype. Thereby, counterselection on medium containing erythromycin will eliminate these clones from the library. During the counterselection process, approximately 10% of all clones showed excision of the loxP-ery-Pusp45-melA-loxP fragment as determined by blue/white screening on plates lacking erythromycin and containing X--GAL. The growth and culture manipulation steps during counterselection were kept to a minimum, aiming to maintain the broadest possible diversity of library clones. PCR analysis, amplifying the insert present in the upstream regions of individual NZ5500 transformants containing a specific pNZ5520 derivative, showed that more than 95% of all R-IVET library clones contained an insert, with an estimated average size of 1.2 kbp. Based on these numbers, the genomic coverage of the library was calculated according to the formula of Clarke and Carbon (7) and was estimated to be higher than 99%. To analyze the diversity of the library inserts, the inserts of 90 randomly selected library clones were amplified by PCR and subjected to Sau3AI and AluI digestion. No common restriction patterns could be identified among these 90 clones, confirming the library's diversity. In addition, the inserts of 30 randomly picked library clones were subjected to single-end sequencing, establishing that the inserts were randomly distributed over the chromosome of L. lactis MG1363 (data not shown).

Plating the library that was retained after primary counterselection on LM17 supplemented with chloramphenicol and X--Gal resulted in 1% white colonies. These colonies can be considered false positives and were most likely the consequence of the limited counterselection procedure applied during library construction. The sensitivity of the luminescence measurements was demonstrated by randomly picking 25 of those false positives and determining their luminescence signals when grown on LM17 medium, showing that the vast majority (24 out of 25) of the white colonies generated significantly higher luminescence signals than those observed with the negative control, NZ5500 harboring pNZ5520. These measurements establish the sensitivity of the luminescence measurements and illustrate that false-positive clones can readily be identified in R-IVET screens using this secondary reporter.

Screen on minimal medium.
To demonstrate the functionality of the advanced R-IVET system, it was applied to identify promoters of L. lactis that are specifically induced when cells grow on minimal medium compared to the rich laboratory medium used during library construction. In short, during library construction a counterselection was performed to eliminate all genes induced on the rich medium M17, which is achieved by the addition of erythromycin to the medium. The R-IVET library obtained was then cultured in minimal medium and subsequently plated on LM17 medium supplemented with chloramphenicol and X--Gal. After 3 days of incubation, about 3,000 colonies were screened for the excision of the loxP-ery-Pusp45-melA-loxP cassette, and 163 (5.4%) white colonies were identified and transferred to a 96-well microplate. All 163 clones were subjected to luminescence screening in M17. Luminescence and OD were measured at 10-min intervals throughout the growth curve. Based on the results, a subset of 28 clones covering the entire range of luminescence signal strengths was selected for further characterization and sequencing (Table 2). The luminescence measurements of these individual clones in M17 displayed an almost 200-fold difference between the strongest and the weakest signals observed, indicating the presence of a broad range of promoters in the library (Fig. 2) We were able to sequence 27 out of the 28 clones, identifying clones containing genes involved in amino acid metabolism (7 clones), nucleotide metabolism (5 clones), transport functions (5 clones), and several other functions (10 clones). One out of the 27 sequences did not contain a genuine promoter sequence. This clone (llmg_1168) contained a chromosomal fragment that was apparently wrongly oriented, since it corresponded to the antisense strand of locus llmg_1168. The finding of target sequences located on the antisense strand of an open reading frame is not an uncommon result of IVET screens (32, 34).

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TABLE 2. Effect of addition of rich medium constituents to minimal medium on a subset of R-IVET clones activated during growth in chemically defined medium

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FIG. 2. Luminescence signal in M17. The clones are of a subset of the initial screen, and they were chosen to represent the entire range of luminescence activity levels observed. Luminescence was measured at 10-min intervals throughout growth, and the maximum values were used for data analysis. Each bar shows the average of two biological replicates. Error bars show standard deviations. a.u., arbitrary units.

The subset of 28 clones was subjected to a detailed characterization in which the main components of M17 were added separately to minimal medium at a concentration identical to that present in M17. The M17 component added to minimal medium was either yeast extract (0.25%), meat extract (0.5%), soy broth (0.5%), or tryptone (0.25%). The results showed that 19 out of the 28 selected clones displayed significantly decreased luminescence levels upon the addition of at least one of these rich medium constituents compared to the level observed in minimal medium (Table 2). In general, yeast and meat extracts repressed the luxAB expression of the selected clones more effectively than soy broth and tryptone. Several genes are differentially regulated, depending on the rich medium component added. This is exemplified by the results observed with bcaP, encoding a branched-chain amino acid permease (9), which is downregulated 14- and 22-fold upon the addition of yeast extract and meat extract, respectively. At the same time, this clone showed no significant change of promoter activity upon the addition of soy broth. Another example is the clone containing a fragment encoding the O-acetylhomoserine sulfhydrylase (cysD), which was significantly downregulated only by the addition of meat extract and not by the other components. Notably, two of the selected clones, corresponding to inserts from the pyrE and pyrDB loci, displayed upregulation of the luminescence signal upon addition of any of the M17 constituents. These two clones rank among the clones with the highest luminescence signal in M17, indicating that they are expressed at a relatively high level also in this rich laboratory medium, which was also employed during counterselection (Fig. 2). Either these clones may be false positives or they may have escaped elimination from the library during the counterselection procedure due to variation in the conditions employed (i.e., solid versus liquid medium). Importantly, most clones that express high levels of luxAB luminescence in M17 also failed to show significant differential regulation in minimal medium upon the addition of single constituents from the rich medium, supporting their classification as false positive.

Supplementation with individual compounds.
To further analyze the transcriptional response of identified clones, we performed additional experiments where we aimed at identifying the molecules regulating the differential expression of target sequences. One of the identified clones harbored the promoter region of trpE, a gene involved in tryptophan biosynthesis. With this clone a significant decrease of the luminescence signal was observed upon addition of any of the M17 components tested, with yeast and meat extracts having the largest effect, leading to a 10-fold decrease of the luminescence signal (Table 2). The supplementation of minimal medium with the amino acid tryptophan (100 mg/liter) also resulted in a 10-fold decrease of luminescence, indicating that tryptophan may be the regulatory molecule in these complex medium components.

We also investigated the effect of arginine on the expression of the arginine deiminase gene arcA, a clone which was downregulated 16- and 11-fold by the addition of yeast and meat extracts, respectively (Table 2). The addition of arginine to the medium clearly showed an upregulation of arcA (Fig. 3), which is consistent with previously published results (22). Furthermore, we investigated the effect of serine, glycine, and threonine on the phosphoserine aminotransferase gene serC. Bacto tryptone and yeast extract led to a three- and fivefold downregulation of serC, respectively (Table 2), while no effect was observed upon the addition of serine, glycine, and threonine to the minimal medium (data not shown).

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FIG. 3. Luciferase signal of the arcA R-IVET clone (llmg_2313) grown anaerobically in minimal medium. Arginine was added to the medium at the indicated concentrations. Samples were taken from exponentially growing cells, and the results of the luciferase measurements are expressed in arbitrary units (a.u.) per unit of OD600. Error bars show standard deviations (n = 4).

A clone containing the promoter sequence of the purCSQLF operon upstream of luxAB showed the highest expression level when grown on M17 (Table 2) and displayed a very distinct expression pattern throughout the growth curve. Its luminescence signal increased approximately 1,000-fold in less than 2 h during the mid-logarithmic growth phase. To exemplify this rapid switch of gene expression, the activity pattern of this clone was compared to that of a control construct where luxAB expression is controlled by the lactococcal usp45 promoter (Fig. 4). Since the purCSQLF operon is involved in purine biosynthesis (16), the influence of the addition of purines to M17 was also investigated. The addition of either adenine, guanine, or xanthine to M17 clearly led to a decrease of the luminescence signal, where the quantitative effect for equal concentrations decreases from adenine to guanine to xanthine (Fig. 5). To investigate how the purC target sequence, which displays such a high expression level of luciferase, can escape elimination during the counterselection process, we isolated plasmid DNA of the clone and reintroduced it into the original R-IVET strain NZ5500. The transformants were plated on medium containing X--Gal and supplemented either with chloramphenicol or with chloramphenicol and erythromycin. The results showed more than 99% blue colonies, and that the addition of erythromycin had an effect on neither the amount of colonies nor the colony size, indicating that the resolvase was not expressed at high levels in these clones. Subsequently, a single colony was isolated and propagated overnight in LM17 broth. When a sample of this culture was plated, we found that all colonies showed a MelA-negative phenotype. These findings show that the purine shortage which triggers expression of the purCSQLF operon is encountered only during the mid-logarithmic growth phase in M17 liquid medium and not on the same medium used in agar plates.

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FIG. 4. Effect of purine addition on luciferase activity during growth in M17 of the R-IVET clone llmg_0973 containing a plasmid with the purCSQLF promoter sequence. Luminescence and OD600 were measured at 10-min intervals. The luminescence signals per unit of OD600 are depicted for the clone harboring the promoter sequence of purCSQLF upstream of the cre-luxAB genes with no additional adenine in the medium () or with the supplementation of 36.7 mg/liter adenine to the growth medium (). As a control, strain NZ5500(pNZ5519) harboring the usp45 promoter sequence upstream of the cre-luxAB genes was included (). The corresponding OD600 measurement is shown for NZ5500(pNZ5519) (), which is representative of all three clones. Each data point represents the average of multiple biological replicates: n = 11 (); n = 3 (); and n = 8 (). a.u., arbitrary units.

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FIG. 5. Luciferase expression controlled by the purCSQLF promoter in clone llmg_0973 after the addition of purines to M17 medium. Luminescence was measured at 10-min intervals throughout growth, and the maximum values were used for data analysis. Black bars indicate the addition of adenine (n = 3), white bars the addition of guanine (n = 2), and gray bars the addition of xanthine (n = 2) to the medium. As a control experiment, equal amounts of a solution in which no purines were dissolved (0.1 molar NaOH) were added, and the results showed no effect on growth and purCSQLF expression (data not shown). Error bars show standard deviations. a.u., arbitrary units.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
IVET has been used widely, and a number of variant versions of the system have been developed (32). R-IVET systems especially have been successfully applied to study regulatory responses of various bacterial species in their natural habitats, where the analysis of gene expression with alternative methods is difficult (32). However, the currently used R-IVET systems also have some drawbacks, several of which were addressed in the present study. First, the introduction of an -galactosidase encoding gene (melA) between the loxP sites of the excision cassette on the chromosome allows simple blue-white screening under nonselective conditions on media containing X--Gal. This enabled simple quality control, without the need for replica plating of the library during counterselection, as well as rapid and reliable detection of clones from which the loxP-ery-Pusp45-melA-loxP cassette was excised. Second, a major challenge in many R-IVET-based studies is the validation of promoter activation and cognate gene expression patterns in the natural environment that follows the primary R-IVET clone selection. The R-IVET system presented here overcomes this drawback by the inclusion of the bacterial luciferase-encoding luxAB genes of Vibrio harveyi as a secondary promoter probe in combination with the primary promoter-trap resolvase (cre). Bacterial luciferase (LuxAB) catalyzes the reaction of a long-chain aldehyde, reduced flavin mononucleotide, and molecular oxygen, yielding the corresponding carboxylic acid, flavin mononucleotide, water, and light (490 nm). It is functional in many microbes (28), and detection methods are very sensitive and allow in situ measurement in complex environments, including several tissues of living animals (13) and food products (26). Including luciferase as a secondary reporter allowed us to analyze promoter activities in a quantitative and high-throughput manner. Furthermore, the developed assay allows the systematic screening of R-IVET clones in vitro to identify the biochemical trigger for the activation of gene expression.

The R-IVET system developed was implemented in L. lactis and used to identify promoters activated in minimal medium compared to rich laboratory medium. The 163 R-IVET clones identified in the primary R-IVET screen were further analyzed by measuring luminescence continuously throughout the growth curve in rich medium. A subset consisting of 28 clones representing luminescence signal levels covering the entire range observed for all clones was selected for further evaluation. The detailed analysis of this subset showed that the luciferase reporter gene allows the rapid identification of potential false positives through their relatively high luciferase signals in the rich laboratory medium used during counterselection. Moreover, we also show that some promoter sequences are only partially repressed in rich medium e.g., arcA, serC, and bcaP. These moderately expressed genes would normally be considered false positives and would therefore be expected to be eliminated from an R-IVET library that has been subjected to extensive counterselection. This demonstrates that the luciferase-based secondary attribute broadens the detection range of the developed R-IVET system. Measurement of luciferase activity in minimal medium supplemented with specific components present in the rich laboratory medium used during counterselection allowed the identification of rich medium constituents and even specific molecules that regulate the activity of specific R-IVET promoter clones. This is exemplified by the downregulation of the trpE-R-IVET clone by all rich medium components and by the subsequent demonstration that this effect is achieved specifically by the tryptophan present in these components. These findings are consistent with previous reports that describe the tryptophan-mediated transcriptional control of the trpEFG locus in L. lactis (11). Analogously, we evaluated the effect of arginine on the promoter element of the arcA. The results clearly showed an upregulation of arcA with an increasing arginine concentration in the medium, which is consistent with the literature (22). However, arcA was downregulated by the addition of all rich M17 components. We currently cannot explain which compound of the rich components causes this effect. Furthermore, we investigated the effect of serine, glycine, and threonine on the phosphoserine aminotransferase gene serC. In this case we did not observe an effect of the amino acid addition on their regulation, even though the rich nutritional sources clearly reduced their activity. This may be explained by the energetically less efficient uptake of free amino acids compared to uptake of oligopeptides containing these amino acids, which are the predominant nitrogen source in the rich medium components employed (19). Another possible explanation could be that the interconversion of amino acids influences the expression levels of the promoter sequences investigated.

Additional analyses focused on the strongest promoter sequence identified in the initial screening. This purC R-IVET clone displayed a very distinct expression pattern compared to most other clones. This is evident in an almost 1,000-fold increase of the luminescence signal during mid-logarithmic growth phase, reaching activity levels similar to that observed for the luciferase-positive control. The strict temporal expression pattern observed for this clone demonstrates that the luciferase reporter is available in its active form extremely rapidly upon promoter activation. This is an important attribute of the luciferase reporter, making it very valuable for the validation of in situ expression of genes. Using the purC R-IVET clone, we also showed a clear dose-response relationship following the addition of the purines adenine, guanine, and xanthine. The demonstrated effect of purines on purC is consistent with literature findings (16), and it was recently found that purine addition has a profound effect on high-density cultures even in a rich culture medium (20). Notably, our control experiments have unambiguously shown that the positive control R-IVET vector that contains the cre-luxAB cassette under control of the strong usp45 promoter resulted in 100% resolution of the chromosomal target locus (loxP-ery-Pusp45-melA-loxP), suggesting that it is highly unlikely that a promoter of similar strength would escape elimination from the R-IVET library during counterselection. This consideration led us to reintroduce the purC R-IVET clone into the original R-IVET strain and investigate its expression pattern on agar plates in more detail. The results clearly showed that on a solid medium this promoter sequence is not activated to a level that would lead to the resolution of the two lox sites. However, liquid medium cultures with the same medium led to a 100% resolution of the target locus of the clones recovered from that culture. Unraveling the differential activity patterns of the purCSQLF promoter in solid and liquid media was experimentally straightforward and demonstrates the strength of the current system in validating identified R-IVET clones.

The organism chosen for this study, L. lactis, is extensively used in (multispecies) starter cultures in the dairy industry for the production of cheese. The R-IVET method presented here will enable elucidation of gene activation in L. lactis in such complex dairy fermentation environments. The system can be directly applied in product matrices, as these are compatible with luminescence measurements because the substrate of the luminescence reaction can be supplied either as a volatile or by submerging the product in, e.g., silicone oil containing the substrate. Moreover, for in vivo luminescence measurements in the gastrointestinal tract, the luxCDE genes, coding for substrate generation of the luminescence reaction, could readily be incorporated in the system and thereby provide the luciferase substrate from endogenous metabolism (10). Such autotrophic luciferase constructions have been shown to offer tremendous possibilities for in situ measurements in living model animals (13).

The R-IVET system described here overcomes some of the R-IVET shortcomings by broadening the detection range of promoter activity and allowing high-throughput validation of target sequences in vitro and in situ. Based on the literature, many of the system's components are functional in other bacterial species, which should allow the transfer of the developed tool to other microorganisms. For example, the cre-loxP resolvase (17, 24, 36, 21) and luxAB reporter system (28) have been successfully used in a variety of bacteria. It can be anticipated that the usp45 promoter will be active in various backgrounds (29), and the pIL252 replicon has been reported as a broad-host-range replicon (35). However, depending on the target organism, some changes might be needed. Taken together, our findings indicate that the system presented here offers significant advantages over the existing R-IVET systems based on its dual-selection possibilities and that it could potentially be applied to other bacterial species.

 
We thank Peter Bron, Roger Bongers, and Iris van Swam for fruitful discussions and for kindly supplying plasmids pNZ7125, pNZ3251, and pUCloxery. We also thank Oscar Kuipers for access to parts of the genomic sequence of L. lactis MG1363 prior to its publication.

This project was carried out within the research program of the Kluyver Centre for Genomics of Industrial Fermentation, which is part of The Netherlands Genomics Initiative/Netherlands Organization for Scientific Research.

 
* Corresponding author. Mailing address: NIZO food research, P.O. Box 20, 6710 BA Ede, The Netherlands. Phone: 31-318659511. Fax: 31-318650400. E-mail: Johan.van.hylckama.vlieg{at}nizo.nl

Published ahead of print on 6 June 2008.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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Applied and Environmental Microbiology, August 2008, p. 4727-4736, Vol. 74, No. 15
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