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Applied and Environmental Microbiology, September 2008, p. 5759-5768, Vol. 74, No. 18
0099-2240/08/$08.00+0     doi:10.1128/AEM.01030-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Physiological and Transcriptional Responses to High Concentrations of Lactic Acid in Anaerobic Chemostat Cultures of Saccharomyces cerevisiae

Derek A. Abbott,^1,2 Erwin Suir,^1,2 Antonius J. A. van Maris,^1,2* and Jack T. Pronk^1,2

Department of Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands,¹ Kluyver Centre for Genomics of Industrial Fermentation, Julianalaan 67, 2628 BC Delft, The Netherlands²

Received 8 May 2008/ Accepted 22 July 2008

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Based on the high acid tolerance and the simple nutritional requirements of Saccharomyces cerevisiae, engineered strains of this yeast are considered biocatalysts for industrial production of high-purity undissociated lactic acid. However, high concentrations of lactic acid are toxic to S. cerevisiae, thus limiting its growth and product formation. Physiological and transcriptional responses to high concentrations of lactic acid were studied in anaerobic, glucose-limited chemostat cultures grown at different pH values and lactic acid concentrations, resulting in a 50% decrease in the biomass yield. At pH 5, the yield decrease was caused mostly by osmotically induced glycerol production and not by the classic weak-acid action, as was observed at pH 3. Cultures grown at pH 5 with 900 mM lactic acid revealed an upregulation of many genes involved in iron homeostasis, indicating that iron chelation occurred at high concentrations of dissociated lactic acid. Chemostat cultivation at pH 3 with 500 mM lactate, resulting in lower anion concentrations, showed an alleviation of this iron homeostasis response. Six of the 10 known targets of the transcriptional regulator Haa1p were strongly upregulated in lactate-challenged cultures at pH 3 but showed only moderate induction by high lactate concentrations at pH 5. Moreover, the haa1 mutant exhibited a growth defect at high lactic acid concentrations at pH 3. These results indicate that iron homeostasis plays a major role in the response of S. cerevisiae to high lactate concentrations, whereas the Haa1p regulon is involved primarily in the response to high concentrations of undissociated lactic acid.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Lactic acid has traditionally been used in many industrial applications, including food preservation and production of cosmetics and pharmaceuticals (5). Today, concerns for environmental issues and oil availability contribute to an increased interest in polylactic acid as a bioplastic produced from renewable feedstocks (11). Lactic acid is currently produced predominantly with various species of lactic acid bacteria (5). Due to their complex nutritional requirements and sensitivity to low pH, these bacteria are suboptimal for the production of the high-purity, undissociated lactic acid (pK_a = 3.86 [12]) required for polylactic acid production (5, 10).

Engineered Saccharomyces cerevisiae strains are under evaluation as possible alternative lactic acid producers. Deletion of one or more of the functional genes encoding pyruvate decarboxylase in combination with the expression of a heterologous lactate dehydrogenase has resulted in S. cerevisiae strains with reduced or eliminated ethanol formation that are capable of producing L-lactic acid (3, 26, 46, 50, 60). The comparatively high acid tolerance of S. cerevisiae and its simple nutritional requirements should facilitate the production of undissociated lactic acid instead of the lactate anion that is formed at a higher pH in bacterial fermentations.

In the food industry, lactic acid is commonly used as an acidulant and a preservative (6, 54). In comparison with other weak acid preservatives, S. cerevisiae is relatively insensitive to lactate, and strong inhibitory effects require relatively high lactate concentrations (1, 18, 40, 52). As has been demonstrated for several other weak organic acid preservatives, the mechanism by which food is preserved by lactic acid is thought to be based, at least to some degree, on the intracellular accumulation of protons mediated by the diffusion of undissociated acid into the cells (14, 39). Although the inhibitory actions of lactic acid on yeast growth and metabolism are pH dependent and are accompanied by changes in the intracellular pH, there are indications that the mechanism underlying its toxicity differs from that of other weak acid preservatives (39) and involves the toxicity of the lactate ion (56). Based on transcriptional regulation studies, genes pertaining to cell wall architecture and a set of genes controlled by the transcriptional regulator Aft1p, which are involved in iron uptake and metabolism, have been implicated in resistance to lactic acid (29). However, phenotypic screening of deletion mutants in genes pertaining to iron homeostasis did not reveal an increased sensitivity to lactic acid or acetic acid (29).

Lactic acid toxicity at high concentrations or low pH (as required for the production of free acid) is likely to represent a major challenge for the industrial production of lactic acid, where very high concentrations of free lactic acid are desired. However, lactate toxicity and tolerance in S. cerevisiae are not only relevant for industrial lactate production. In addition, lactic acid is commonly found in industrial yeast fermentations, where the proliferation of contaminant lactic acid bacteria leads to lactic acid accumulation (38, 40). Furthermore, the presence of lactic acid in combination with other stressors has been shown to synergistically inhibit yeast growth and metabolism (52) and, thus, to affect many yeast-based industrial fermentations (40).

The aim of the present study was to analyze the physiological and transcriptional responses of S. cerevisiae to lactic acid stress. To dissect the organism's responses to lactic acid from those to the lactate anion, experiments were performed at pH 3 and pH 5. Anaerobic, glucose-limited chemostat cultures were used to enable a quantitative comparison of the transcriptional regulation and physiological effects of lactic acid at a fixed specific growth rate. To further facilitate the comparison, the concentrations of lactic acid used in the chemostat cultivation experiments were chosen such that they resulted in the same reduction of the biomass yield on glucose at both pH 3 and 5.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Chemostat growth conditions.
The laboratory reference strain CEN.PK 113-7D (MATa) was grown at 30°C in 1.5-liter chemostat fermentors (Applikon, Schiedam, The Netherlands). A comparable degree of weak acid stress was ensured by decreasing the biomass yield to approximately 50% of the reference condition (no added lactic acid) with the addition of the appropriate amount of L-lactic acid to the medium vessel prior to sterilization. All cultures, including that of the reference strain, were fed with synthetic medium as described by Verduyn et al. (61), with 25 g liter^–1 glucose as the limiting nutrient and 0.15 ml liter^–1 silicone antifoam (BDH, Poole, England) to prevent excessive foaming. The dilution rate was set to 0.10 h^–1, and the pH was controlled at 5.0 or 3.0 with the automatic addition (ADI 1031 bio controller; Applikon) of 2 M KOH. The stirrer speed was set at 800 rpm, and anaerobicity was maintained by sparging the fermentor with N₂ gas at 500 ml min^–1. To prevent diffusion of oxygen, the fermentor was equipped with Norprene tubing and Viton O-rings, and the medium vessel was also flushed with N₂ gas.

Analytical methods.
Chemostat cultures were assumed to be in at steady state when, after at least five volume changes, the culture dry weight and specific carbon dioxide production rate changed by less than 2% over two volume changes. Steady-state samples were taken between 10 and 14 volume changes after inoculation to avoid possible evolutionary adaptation during long-term cultivation. Culture dry weight measures were determined in duplicate via filtration onto dry, preweighed nitrocellulose membranes. Samples were dried in a microwave oven for 20 min at 360 W. Culture supernatants were obtained by centrifugation of the chemostat broth or by a rapid sampling method using precooled (–20°C) steel beads (34). For the purpose of flux determination and carbon recovery, supernatants and media were analyzed via high-performance liquid chromatography using an Aminex HPX-87H ion exchange column with 5 mM H₂SO₄ as the mobile phase. Off gas was first cooled with a condenser (2°C) and then dried with a Perma Pure dryer (model no. PD-625-12P). CO₂ and O₂ concentrations in the off gas were measured with an NGA 2000 Rosemount gas analyzer.

Microarray analysis.
Sampling of chemostat cultures at pH 5 was performed by instantly quenching the yeast culture in liquid nitrogen, as described previously (44). However, in contrast to sampling cultures at pH 5, for which the protocol was optimized, sampling cultures at pH 3, especially in the presence of high lactic acid concentrations, with this method did not yield sufficient quantities of mRNA to proceed with cDNA synthesis. Therefore, pH 3 cultures were quenched in ice-cold Tris-EDTA (TE) buffer at pH 8 (using 5x the volume of sample) and then washed in ice-cold TE buffer (with 2x the volume), followed by a wash in ice-cold demineralized water (with 2x the volume). Finally, cells were resuspended in acetate-EDTA buffer, sodium dodecyl sulfate, and acid phenol-chloroform as previously described (44). Probe preparation and hybridization to Affymetrix GeneChip microarrays were performed as described previously (2).

Data acquisition, quantification of array images, and data filtering were performed with Affymetrix Microarray Suite version 5.0, MicroDB version 3.0, and Data Mining Tool version 3.0. All arrays were scaled by normalizing the average signal from all probes to a value of 150. Since transcripts with values below 12 cannot be measured accurately, their levels were set to 12 for statistical analysis (44). Groups of genes which were up- or downregulated in the presence of lactic acid at pH 3 or pH 5 were examined for enrichment, using MIPS database functional annotation (35) and significant transcription factor binding, as previously described (2). To enable further study of these data by other researchers, the data from Affymetrix GeneChip microarrays used in this study are available via Gene Expression Omnibus series accession number GSE10066 (http://www.ncbi.nlm.nih.gov/projects/geo/query/acc.cgi?acc=GSE10066).

Strain construction.
Gene deletions were generated in the genetic background of the prototrophic strain CEN.PK 113-7D by using standard yeast media and genetic techniques (8). The KanMX marker was amplified by using pUG6 as a template (19) and specific primers. The resulting disruption cassettes, containing sequences homologous to the targeted genes, were transformed using the protocol for high-efficiency transformation of yeasts (8). After recovering, the cells were plated on YPD medium containing G418 (200 µg/ml). Confirmation of successful gene disruptions was performed using colony PCR.

To generate the tpo2 tpo3 double deletion, the hphNT1 marker was amplified using pFA6a-hphNT1(27) as a template and specific primers targeting TPO3. The TPO3 disruption cassette was transformed into the tpo2 strain, and transformants were selected on YPD medium containing 20 µg/ml hygromycin B. Deletion of both TPO2 and TPO3 was confirmed by colony PCR.

Anaerobic batch cultivation for phenotypic screening.
The preinoculum for the anaerobic batch cultures was produced by performing an anaerobic chemostat for each strain at pH 3 and bringing it to steady state (the medium and conditions were as described above, with no lactic acid added). The fermentor was then emptied until only approximately 50 ml remained as the inoculum for batch growth. Then, fresh medium containing 500 mM lactic acid was added, and batch fermentation was initiated. Specific growth rates in the batch phase were calculated based on continuous off-gas CO₂ measurements (as described above). Upon completion of the batch culture (depletion of glucose), the fermentor was once again emptied and refilled with fresh medium without lactic acid, and chemostat cultivation resumed until steady state was established at a dilution rate of 0.1 h^–1. Then, the fermentor was emptied and refilled once more, and growth rates were determined for each strain in an anaerobic batch culture containing 750 mM lactic acid at pH 3 (as described above for 500 mM lactic acid).

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Physiological responses to lactic acid.
Chemostat cultivation makes it possible to study the effect of environmental stimuli at a fixed specific growth rate. Therefore, quantitative comparisons of the physiological effects and transcriptional responses to lactic acid at different pH values were performed with anaerobic, glucose-limited chemostat cultures. Anaerobic conditions were utilized to prevent lactate consumption via respiratory metabolism (33). To this end, experiments were performed in which the concentration of lactic acid in the medium was titrated to reduce the biomass yield on glucose to approximately 50% of the biomass yield in anaerobic, glucose-limited reference cultures (no lactic acid added) at each pH value.

For cultures grown at pH 5, 900 mM of lactic acid (61 mM undissociated acid) was required to decrease the biomass yield on glucose to 50% of the biomass yield in reference cultures (Fig. 1). Assuming that the undissociated species determines weak organic acid toxicity, the Henderson-Hasselbach equation (assuming a pK_a value of 3.86 for lactic acid [12]) can be used to estimate the concentration of lactic acid required to obtain a similar yield reduction at pH 3.5. This led to the prediction that a lactic acid concentration of 85 mM (60 mM undissociated acid at pH 3.5) should cause a 50% reduction of the biomass yield at pH 3.5 (Fig. 1). However, experiments showed that the required concentration was almost 9-fold higher (750 mM) (Fig. 1). Further experiments at pH 3 showed that, even at this low pH, 500 mM lactic acid was required to reduce the biomass yield to approximately 50% of that of the reference condition (Table 1). In contrast, benzoic acid at total concentrations of 2 mM (pH 5) and 0.3 mM (pH 3.5), corresponding to 0.27 mM and 0.25 mM undissociated acid, respectively, showed the same degree of reduction of the biomass yield, thus confirming that for benzoic acid, toxicity is mediated predominantly by the undissociated species.

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FIG. 1. Effect of lactic acid on biomass formation in glucose-limited anaerobic chemostat cultures of S. cerevisiae CEN.PK 113-7D. Each data point represents an independent chemostat culture which was grown to steady state at a dilution rate of 0.10 h–1. Based on the data obtained at pH 5.0 (•), the Henderson-Hasselbach equation was used to estimate an equivalent stress at pH 3.5 (dashed line), assuming that the undissociated acid was solely responsible for the observed decrease of the biomass concentration. Experimental data obtained at pH 3.5 () did not correlate with this prediction.

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TABLE 1. Physiological analysis of chemostat cultures of S. cerevisiae in the presence and absence of high lactate concentrationsa

Using lactate concentrations that resulted in approximately 50% reduction of the biomass yield, triplicate anaerobic chemostat fermentations were performed at pH 3 and 5, and results were compared to those of reference fermentations without lactic acid. Steady-state fluxes and carbon recoveries were calculated from substrate and metabolite concentrations in growth media and culture supernatants (Table 1).

Residual concentrations of glucose in cultures grown in the presence of lactic acid were higher than those in the reference cultures. In microorganisms, the specific rate of consumption of the growth-limiting substrate, q_s, often exhibits saturation kinetics with respect to its concentration, C_s (36). These kinetics can be described by the modified Monod equation,

Thus, the increased rate of glucose consumption by the cultures may be at least partially responsible for the increased residual glucose concentration. However, especially in the chemostat cultures grown at pH 3, this phenomenon cannot fully account for the drastic increase of the residual glucose concentration in cultures grown with lactic acid. This suggests that lactic acid alters the kinetics of glucose transport, for example, by altering the expression or kinetic parameters (K_s or V_max) of plasma-membrane glucose transporters (56) or by perturbation of the plasma membrane structure (9). Since the residual glucose concentrations remained well below 5 mM in the cultures at pH 5, no substantial impact of glucose repression on gene expression was anticipated (62), nor was it observed in the current study. At pH 3 in the presence of lactate, the residual glucose concentration was elevated to levels that have previously been reported to cause glucose repression, but extensive downregulation of glucose-repressible genes was not observed.

Specific rates of glycerol production were higher for cultures challenged with lactate than for reference cultures. The differences were most notable at pH 5, where the high concentration of (dissociated) lactic acid, combined with the large amounts of KOH required to achieve pH 5 after lactic acid was added to the medium, resulted in a dramatically increased salt concentration. As glycerol is a well-known compatible solute that counteracts osmotic pressure in yeast cells (22), the elevated levels of glycerol were likely an artifact caused by the presence of the high salt concentrations. Conversion of 0.5 mol glucose into 1 mol of glycerol requires the input of 1 mol of ATP. Under anaerobic conditions, this ATP has to be provided by the dissimilation of an additional 0.5 mol of glucose through glycolysis. In cultures grown at pH 5, the increase in glucose consumption for glycerol production could largely account for the major part of the increased specific rate of glucose consumption and, hence, for the decreased biomass yield on glucose (Table 1). Glycerol production was less pronounced in lactate-challenged cultures grown at pH 3, to which less lactic acid and, especially, less KOH were added (Table 1). Under these conditions, only a small part of the observed decrease of the biomass yield could be attributed to glycerol production.

The increased acetate flux in lactate-challenged cultures is probably linked directly to the elevated glycerol production. Glycerol production leads to increased formation of NAD⁺ (59), which can be balanced by the formation of oxidized products such as acetate. Consistent with this interpretation, the increased acetate production was less pronounced at pH 3 (Table 1). Carbon recovery in lactate-challenged cultures grown at pH 5 was only 93% (Table 1). With a CO₂ production rate that was higher than that expected from growth, ethanol and acetate formation, and a gap in the redox balance, the possible formation and evolution of acetaldehyde were investigated. Acetaldehyde formation results in the net production of 1 mol of NADH per mole and, thus, represents another means of balancing the NADH requirement for glycerol production in response to osmotic stress. Consequently, off-gas trapping was performed with 0.5 M TRIS-HCl (pH 9.0) (15, 37), and a flux of approximately 1.0 mmol acetaldehyde g^–1 h^–1 was observed. This represents a 2.4% increase in carbon recovery.

Transcriptional profiling: data quality and overall response.
To obtain statistically reliable transcriptome data, triplicate chemostat cultures and oligonucleotide array analyses were carried out for lactic acid-challenged scenarios, as well as for the corresponding reference scenarios at pH 3 and pH 5. The average coefficient of variation for triplicate arrays under each condition was less than 18%. To allow for a comparison between the present data and that of previous chemostat-based transcriptome studies of other organic acids (2), a false-discovery rate of 0.5% with a twofold change was applied as the selection criterion used to identify significantly changed transcripts. A comparison of lactic acid-challenged cultures with those grown under the appropriate reference conditions revealed that in contrast to cultures grown at pH 3, cultures grown at pH 5 showed a larger number of transcripts that responded to lactic acid (Table 2). The role of the more strongly increased osmotic pressure at pH 5 in this response was substantiated by the upregulation of many genes involved in glycerol synthesis.

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TABLE 2. Transcriptional responses to high concentrations of lactic acida

Approximately 2.3% of the entire genome (146 genes) showed a qualitatively similar response to lactate at pH 3 and pH 5. Of these consistently lactate-responsive genes, 51 genes were commonly upregulated and 95 downregulated at both pH values. Finally, a significant number of genes exhibited a response to lactic acid that was specific for one of the pH values (Table 2). Genes that shared the same qualitative transcriptional response (up or down) to lactate and/or culture pH were clustered in groups, which were analyzed in more detail and are discussed below.

pH-independent transcriptional responses.
Comparisons of the overall transcriptional responses to lactate at pH 3 and pH 5 revealed major effects of culture pH. To identify pH-independent (commonly regulated) changes in transcript levels, two clusters of genes were defined that were consistently up- or downregulated at both pH 3 and 5. Genes that showed a transcriptional response to lactate only at pH 3 or at pH 5 were considered to be part of the pH-dependent lactate response (see below). The clusters thus identified were examined for enrichment of transcription factor binding (20) and MIPS functional categories (35).

Genes in the functional categories "siderophore iron transport" and "amine/polyamine transport" were significantly overrepresented among the genes that showed a pH-independent transcriptional upregulation in response to lactate (Table 3). The functional categories "metabolism" (including a number of subcategories), "energy," and "amino acid transport" were enriched in the set of commonly downregulated genes.

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TABLE 3. MIPS functional categories overrepresented among lactic acid-responsive transcriptsa

Genes that have been shown to bind the transcription factors Hap1p and Rcs1p/Aft2p were overrepresented in the set of genes that showed a pH-independent upregulation in lactate-challenged cultures, while only Stp1p/Bap1p was overrepresented among the downregulated genes (Table 4). Hap1p and Rcs1p/Aft2p are key determinants of iron homeostasis, and their identification in this gene set is corroborated by enrichment of the corresponding functional categories. Likewise, identification of enrichment for Stp1p/Bap1p binding in the pH-independent downregulated gene set can be linked directly to the downregulation of amino acid metabolism.

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TABLE 4. Overrepresentation of transcription factor binding sites among lactic acid-responsive genesa

pH-dependent transcriptional responses.
While the Rcs1p/Aft1p and Hap1p transcription factor binding sites were already significantly enriched in the set of genes that showed a pH-independent response to lactate (Table 4), genes involved in the homeostasis of iron and other metals were even more strongly overrepresented among the genes that showed a specific transcriptional upregulation at pH 5 (Table 3). Conversely, the genes that showed a transcriptional response to lactate only at pH 3 were not significantly enriched in any particular functional category (Table 3). This may reflect the fact that the lactate response at pH 5 is composed of the response to free lactic acid (as observed at pH 3), as well as additional responses related to the lactate anion.

Genes that are bound by the transcription factors Sko1p, Skn7p, and Cin5p were specifically overrepresented among the genes that were transcriptionally upregulated in lactate-challenged cultures grown at pH 5 (Table 4). These transcription factors are involved in the regulation of osmotolerance and salt tolerance, and their overrepresentation among these genes is probably a consequence of the experimental setup, which, at pH 5, involved high concentrations of lactate anions and potassium cations. The overrepresentation of the MIPS categories "ionic and cationic homeostasis" in this gene set (Table 4) further supports this interpretation.

Binding of several transcription factors involved in cell morphology was overrepresented among genes that showed a lactate response (of either up- or downregulation) at pH 5 only. However, the regulation patterns observed gave no clear indication for either transcriptional induction or repression of pseudohyphal growth. For example, the upregulated gene set at pH 5 shows enrichment of genes that are bound by both negative (Nrg1p, Sok2p) and positive (Phd1p) regulators of pseudohyphal growth. Furthermore, two additional transcription factors involved in cell morphology (Ste12p, Tec1p) were overrepresented among genes that were specifically downregulated at pH 5. Given the elevated concentrations of salts in these cultures, osmotic responses mediated by the Sho1 receptor may contribute to enhanced signaling of pseudohyphal growth-related transcriptional responses (41).

The involvement of genes pertaining to the cell cycle and pseudohyphal growth was also apparent at pH 3, but the identity of the transcription factors for which binding was overrepresented was different than that at pH 5. For instance, targets of Ace2p, which controls the cell cycle by activating the expression of early G₁-specific genes, were enriched at pH 3. In addition to the role of Ace2p in cell cycle control, Gancedo (17) suggested that Ace2p may also influence pseudohyphal growth. Furthermore, targets of Swi5p, a transcription factor that activates transcription of genes expressed in G₁ phase and at the G₁/M boundary, were overrepresented at low pH. Although extensive transcriptional events related to cellular morphology and cell cycle were observed, routine phase-contrast microscopy of the chemostat cultures did not reveal clear morphological differences.

High concentrations of lactate anions cause an "iron status" transcriptional response.
At pH 5, the majority of the transcriptional response to lactate focused on metal ion homeostasis. In particular, many genes related to the cellular iron status often showed a very strong upregulation in lactate-challenged cultures (Table 5). Essentially, this transcriptional response to lactate is strikingly similar to the response that would be observed for iron-limited cultures (28, 43). Although copper and iron homeostasis are linked (55), both copper-independent and copper-dependent mechanisms were upregulated in the lactate-challenged cultures, indicating that the changes observed were not a secondary effect of copper status. This "iron status response" observed for lactate is not observed for four other organic acids tested under similar conditions (2). Furthermore, this response was almost completely alleviated at pH 3, where the total acid concentration was lower (Table 5) and the concentration of anionic species was drastically lower (840 mM at pH 5; 60 mM at pH 3).

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TABLE 5. Changes in expression of genes involved in iron homeostasis in lactate-challenged chemostat cultures compared to reference anaerobic chemostat culturesa

It is well known that lactate chelates iron (and, to a lesser degree, other metal cations) in a pH-dependent manner (54). To confirm the decreased availability of iron in the culture medium in the presence of lactic acid, a simple ferrozine assay was performed. Ferrozine binds with free Fe²⁺ to produce a purple color (47). Although approximately 10 µM FeSO₄ was added to the growth medium, the presence of EDTA in the medium (required to keep metal cations in solution) likely binds the majority of iron. Consequently, the free-iron levels in the growth medium were not detectable with the ferrozine assay. However, upon supplementation of all growth media with additional FeSO₄ (150 µM), a signal could be detected. Growth media containing a range of lactic acid concentrations (at each pH) were supplemented with iron and analyzed spectrophotometrically. The addition of 250 µM ferrozine (Sigma) revealed a clear relationship between lactic acid concentration and "free" iron (Fig. 2). Furthermore, pH dependency was demonstrated as the decrease in iron was much more severe at pH 5 than at pH 3.

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FIG. 2. Relative free-iron concentration, as indicated by an optical density at 570 nm (OD570nm) in complete synthetic medium supplemented with increasing concentrations of lactic acid at pH 5 (•) and pH 3 (). For each condition, 150 µM FeSO4 was added, and unbound Fe2+ was detected with ferrozine (250 µM), which results in absorbance at 570 nm upon interaction with Fe2+ (47). Decreased OD570nm indicates decreased availability of Fe2+ in growth medium containing increasing concentrations of the lactate anion. However, the quantitative relationship between free Fe2+ and OD570nm is unknown.

To investigate the physiological relevance of the iron homeostasis response to lactate, AFT1, which encodes a key transcriptional regulator, was deleted. The deletion mutant failed to grow in shake-flask cultures (pH 5) on standard synthetic medium with glucose when 900 mM lactate was added. Supplementation of the synthetic medium with 10-fold or 25-fold higher FeSO₄ concentrations partially rescued this lactate-induced growth deficiency (Fig. 3), while growth of the reference strain was unaffected by iron supplementation (data not shown).

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FIG. 3. Growth of the S. cerevisiae mutant strain (aft1) in glucose synthetic medium containing 900 mM lactic acid. Iron sulfate concentrations in the synthetic medium were 10 µM (standard concentration, •), 100 µM (), and 250 µM (). In contrast to the aft1 strain, the reference strain (CEN.PK 113-7D) did not exhibit a growth deficiency in the presence of lactic acid with the standard concentration of iron (). The pH of all shake flasks was set to 5, and urea was utilized as the nitrogen source to prevent acidification of the growth medium (21).

Within the categories that are more highly enriched at, or specific to, pH 5, there are many genes coding for enzymes that require iron and/or other divalent cations for activity (based on the BRENDA database [www.brenda-enzymes.info/], using the enzyme classification code). Many enzymes did not have data available for S. cerevisiae, but divalent metal cations were extensively required in other organisms. In reference to amino acid metabolism, 2 of the 12 genes (17%) downregulated in the common response encode enzymes which require divalent metal cations, while 14 of the 18 enzymes (78%) which were downregulated only at pH 5 showed the same requirement. Similar trends were observed for metabolism of energy reserves and C compound and carbohydrate metabolism, where 92% and 75% of the genes downregulated at pH 5, respectively, indicated a requirement for metal cations.

Haa1p regulon: transcriptional analysis and phenotypic screening.
The transcription factor Haa1p is not included in a published compendium of transcription factor-binding data (20) and was therefore not included in the statistical analysis of the sets of lactate-responsive genes. However, previous studies of yeast responses to poorly lipophilic organic acids (16) suggest that Haa1p may be involved in the tolerance of lactic acid. The overrepresentation of amine/polyamine transport among the lactate-upregulated genes is also indicative of the importance of this regulon, as the known Haa1p targets TPO2 and TPO3 are involved in polyamine and organic acid transport. Indeed, 6 of the 10 genes which have previously been shown to be regulated by Haa1p were significantly upregulated in response to lactic acid at pH 3 (Table 6). Even though the concentration of total acid is almost twofold higher at pH 5, the role of Haa1p appears to be more pronounced at pH 3, strongly suggesting that this regulon responds primarily to undissociated lactic acid. Although the transcription factor itself was not strongly induced by lactic acid, a number of target genes were highly upregulated. The transporters of the major facilitator superfamily (TPO2 and TPO3) along with YGP1 (a poorly characterized cell wall glycoprotein implicated in other stress responses [13, 32]) and YRO2 (homologous to HSP30) were highly upregulated in the presence of lactic acid. Similarly, the importance of TPO2, TPO3, YGP1, and, to a lesser extent, YRO2, was also highlighted in response to other organic acids with low to moderate membrane affinities (16).

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TABLE 6. Transcriptional modulation of previously identified target genes of the Haa1p transcriptional regulatora

Given the strong transcriptional induction of this regulon, a haa1 mutant and a set of isogenic strains that carried deletions in the most highly upregulated Haa1p targets were screened for sensitivity to lactic acid at pH 3. Duplicate anaerobic batch cultures of the mutant strains were compared to the reference strain at pH 3 (Table 7). A significant growth defect was observed for haa1 in the presence of 500 mM lactic acid. Furthermore, the haa1 strain had not shown any growth after almost 200 h in the presence of 750 mM lactic acid, while the reference strain grew at 0.19 h^–1. Consistent with these data, a previous study with haa1 showed a prolonged lag phase in the presence of acetic acid, and the duration of the lag phase was directly correlated to increased intracellular accumulation of the acid (16). Strangely, the deletion of TPO2 (the most highly upregulated gene of the regulon) did not result in a lactate-induced growth defect. We hypothesized that TPO3, which is 89% identical to TPO2 (30), was compensating for the lack of TPO2. However, a tpo2 tpo3 double mutant did not exhibit a growth defect in the presence of high lactic acid concentrations (Table 7). Similarly, the deletion of YGP1 or YRO2 had no effect on growth rates in the presence of lactic acid.

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TABLE 7. Growth rates of S. cerevisiae CEN.PK 113-7D and mutant strainsa

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Is lactic acid an atypical weak-organic-acid preservative?
By analyzing physiological responses to lactic acid (pK_a = 3.86) at pH 3 and at pH 5, we attempted to dissect the effects of the lactate anion (the predominant species at pH 5) from those of the undissociated lactic acid (predominant at pH 3). It is well documented that the antimicrobial actions of weak-organic-acid preservatives such as sorbate, benzoate, and propionate are conferred primarily by extracellular undissociated acid. This species causes dissipation of the pH gradient across the plasma membrane and intracellular accumulation of the anion (25, 31, 48, 51). Defense mechanisms against this mode of toxicity include increased activity of the plasma membrane ATPase and ATP-driven export of organic acids (23, 24, 42, 45). Indeed, the effects of benzoate on biomass yields at pH 3 could be accurately predicted from experimental data obtained at pH 5 by (i) assuming that the undissociated acid is the sole extracellular species responsible for toxicity and (ii) applying the Henderson-Hasselbach equation for benzoic acid dissociation.

The observation that biomass formation in anaerobic chemostat cultures was not directly correlated to the concentration of undissociated lactic acid in the cultures (Fig. 1) suggested that undissociated lactic acid was not the only species affecting the biomass yield on glucose. This clear difference between lactate and other weak organic acids is probably related to the low lipid solubility of lactic acid (the octanol-water partition coefficient [log P = –0.60]). Lipid solubility is strongly correlated with weak-organic-acid toxicity. For example, reducing the biomass yield of S. cerevisiae to 50% of that of the reference condition required 105 mM of acetate (log P = –0.31), 2 mM of benzoate (log P = 1.87), 20 mM of propionate (log P = 0.33), or 1.3 mM of sorbate (log P = 1.33) at pH 5 (2), while under the same conditions, 900 mM of lactate was required to achieve the same effect (Fig. 1; this study). At these high concentrations of the lactate anion, a substantial fraction of the consumed glucose was redirected toward glycerol formation as an osmoregulation response (22) to the high osmotic strength of the growth medium used for the experiments at pH 5. While, at first glance, osmotic response might seem to be a trivial consequence of the experimental design, it represents a realistic concern for the high-level industrial production of lactate at pH values above that of the pK_a of 3.86, where lactic acid formation has to be titrated with hydroxide or carbonate salts.

Iron homeostasis: involvement of the Aft1p regulon in lactate tolerance.
The strong and coordinated induction of a large number of target genes of the Rcs1p/Aft2p and Aft1p transcription factors, as well as the heme-responsive regulator Hap1, indicated a strong impact of lactate on the regulation of iron homeostasis. The physiological functions of these genes indicated a general remodeling of iron metabolism, including uptake, retention, and incorporation (28). Transcriptional effects of lactate on iron homeostasis genes were also recently reported for shake-flask cultures of S. cerevisiae (29). The present chemostat study demonstrated that, while this effect was very pronounced at pH 5, it was largely alleviated at pH 3. Ferrozine assays provided further support for the hypothesis that this iron homeostasis response was caused by chelation of free iron at high concentrations of lactate anions, thus severely restricting its bioavailability, and probably not, as previously proposed, by an increased iron requirement of lactate-stressed yeast cells (29).

Experiments with the aft1 strain indicated that the Aft1p regulon, involved in iron homeostasis, is essential for lactate tolerance in S. cerevisiae. The clear lactate sensitivity of the aft1 mutant, which was not found in a previous study (29), could be complemented by iron supplementation of the medium. The absence of a measurable effect of iron supplementation on the specific growth rate of lactate-stressed cultures of a wild-type reference strain indicates that the observed transcriptional reprogramming of iron homeostasis genes is sufficient to counter the effects of iron chelation by lactate. In view of these results, especially at low pH, iron chelation is unlikely to represent a major issue for industrial implementation of industrial processes for the production of lactate with engineered S. cerevisiae strains.

Transcriptional responses to undissociated lactic acid: involvement of the Haa1p regulon in lactic acid tolerance.
The majority of the genes known to be regulated by the Haa1p regulon, which has previously been implicated in tolerance to other weak organic acids (16), were strongly upregulated in the presence of lactate. This effect was most pronounced at pH 3, consistent with the notion that the Haa1p regulon is involved in tolerance to the undissociated acid.

The identification of lactate export mechanisms is highly relevant because energy costs for export have been implicated in the inabilities of engineered "homolactic" S. cerevisiae strains to gain a net ATP yield from lactate fermentation (60). Of the 10 genes hitherto identified as Haa1p targets, TPO2 and TPO3 encode H⁺ antiporters (49) belonging to the major facilitator superfamily. Originally, TPO2 and TPO3 were thought to be polyamine transporters localized to the vacuole (57); however, there is evidence for plasma membrane localization (4), and deletion of TPO2 has been correlated to increased accumulation of intracellular acetate (16). Involvement of Tpo2p and/or Tpo3p in lactate/proton antiporter activity, combined with proton expulsion via the plasma membrane ATPase, which has an ATP/proton stoichiometry of 1 (7, 53), would be consistent with a zero net ATP yield for lactate fermentation (60). However, since even a double deletion of TPO2 and TPO3 did not result in impaired growth in the presence of lactic acid, either the encoded transporters are not involved in lactate tolerance or redundant exporters are encoded by the S. cerevisiae genome.

The strong growth defect of the haa1 strain in the presence of lactic acid (Table 7) was not found for single-deletion strains in the transcriptionally upregulated targets of Haa1p. This indicates either that the proteins encoded by the Haa1p regulon have to act synergistically to achieve lactate tolerance or, alternatively, that other as-yet-unknown targets of the Haa1p regulon are involved in lactate tolerance. A thorough investigation of the composition of the Haa1p regulon and its mechanistic contributions to lactate tolerance is therefore warranted.

 
We thank Carlos Gancedo and Carmen Lisset-Flores for valuable discussions of mRNA extraction from low-pH cultures, Jean-Marc Daran for helpful advice on strain construction, and Theo Knijnenburg for assistance with analysis of microarray data.

We thank Tate & Lyle Ingredients Americas, Inc., for financial support.

The Kluyver Center for Genomics of Industrial Fermentation is supported by The Netherlands Genomics Initiative.

 
* Corresponding author. Mailing address: Department of Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands. Phone: 31 15 2782412. Fax: 31 15 2782355. E-mail: a.j.a.vanmaris{at}tudelft.nl

Published ahead of print on 1 August 2008.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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Applied and Environmental Microbiology, September 2008, p. 5759-5768, Vol. 74, No. 18
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