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Applied and Environmental Microbiology, March 2002, p. 1305-1311, Vol. 68, No. 3
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.3.1305-1311.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Cloning, Characterization, and Transcription of Three Laccase Genes from Gaeumannomyces graminis var. tritici, the Take-All Fungus

Anastasia P. Litvintseva and Joan M. Henson*

Department of Microbiology and Thermal Biology Institute, Montana State University, Bozeman, Montana 59717

Received 1 May 2001/ Accepted 30 November 2001


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ABSTRACT
 
Gaeumannomyces graminis var. tritici, a filamentous ascomycete, is an important root pathogen of cereals that causes take-all disease and results in severe crop losses worldwide. Previously we identified a polyphenol oxidase (laccase) secreted by the fungus when induced with copper. Here we report cloning and partial characterization of three laccase genes (LAC1, LAC2, and LAC3) from G. graminis var. tritici. Predicted polypeptides encoded by these genes had 38 to 42% amino acid sequence identity and had conserved copper-binding sites characteristic of laccases. The sequence of the LAC2 predicted polypeptide matched the N-terminal sequence of the secreted laccase that we purified in earlier studies. We also characterized expression patterns of these genes by reverse transcription-PCR. LAC1 was transcribed constitutively, and transcription of LAC2 was Cu inducible. All three genes were transcribed in planta; however, transcription of LAC3 was observed only in planta or in the presence of host (wheat) plant homogenate.


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INTRODUCTION
 
Laccases (EC 1.10.3.2) are widely distributed oxidoreductases that catalyze the biological oxidation-reduction of polyphenols with a concomitant reduction of molecular oxygen to water. They are blue copper oxidases, which are characterized by having four bound copper atoms, of which at least one is contained in a type 1 site or "blue" copper site (29). Cu-binding domains are highly conserved among laccases. Recently, numerous genes coding for laccase-like proteins have been cloned and characterized (6, 7, 11, 18-20, 25-27, 32, 57).

In higher plants, laccases are involved in lignification of xylem tissues (10, 26); however, various functions have been reported for fungal laccases. Laccase of the animal pathogen Cryptococcus neoformans oxidizes dihydroxyphenylalanine into a melanin-like pigment (37, 51). Aspergillus nidulans laccase is required for pigment biosynthesis during conidial development and maturation (48). In the white-rot basidiomycetes Trametes versicolor, Lentinus edodes, and Pleurotus ostreatus, laccase is involved in lignin degradation (2, 9, 21). Laccase from Pycnoporus cinnabarinus functions in both lignin degradation and the biosynthesis of cinnabarinic acid, an antimicrobic with activity against a variety of bacterial species (14, 16, 44). Other functions, such as oxidation of humic acids (7, 40) and oxidation of Mn2+ to Mn3+ (5, 24) have also been proposed for fungal laccases. Nevertheless, the biological role of the majority of fungal laccases remains uncertain.

Many filamentous fungi produce several laccase isozymes encoded by multiple genes. For example, Rhizoctonia solani has four laccase genes (50), T. versicolor has five (53), the basidiomycete I-62 has three (27), Agaricus bisporus has two (42), P. ostreatus has at least three (20), and Podospora anserina has at least four (18). Quite often these genes are expressed under different environmental conditions (18, 28, 40, 42, 57) or during different stages of the fungal life cycle (18, 28, 48). Differential expression of laccase genes implies that laccases have alternative functions, but to our knowledge this has not been shown.

Many G. graminis var. tritici isolates secrete at least one laccase (13, 47). We recently purified and partially characterized a secreted laccase from G. graminis var. tritici (13). The active protein has an apparent molecular mass of 190 kDa, consists of two heavily glycosylated 60-kDa subunits, and is inducible by copper and xylidine. The purified protein catalyzed the polymerization of 1,8-dihydroxynaphthalene, a natural fungal melanin precursor, into high-molecular-weight melanin and catalyzed the oxidation of the dye poly B-411, a lignin-like polymer. We report here cloning and partial characterization of three laccase genes from G. graminis var. tritici.


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MATERIALS AND METHODS
 
Strains, media, and culture conditions.
G. gramnis var. tritici strain DM528 was isolated by D. Mathre from Montana wheat (Triticum aestivum) and purified by single-spore isolation. Cultures are available from the U.S. Department of Agriculture (Peoria, Ill.) agricultural fungus collection. The fungal strain was maintained on Luria-Bertani (LB) medium (38) plus 1.2% (wt/vol) agar at 24 to 26°C. Long-term stocks were maintained on potato dextrose broth (PDB; Difco, Detroit, Mich.) slants with 1.2% (wt/vol) agar stored at 4°C.

For protein purification, fungal cultures were grown in LB liquid medium supplemented with 400 µM CuSO4 with constant agitation (200 rpm) at 23 to 25°C. To determine the effects of different growth media on gene expression, the fungus was grown in LB broth with and without 400 µM CuSO4 for 4 to 5 days, in PDB for 4 to 5 days, and in starvation basal medium for 10 days (43). For Cu induction experiments, the fungus was grown in Fahraeus minimal liquid medium (17) with sucrose substituted for glucose; CuSO4 was added to a final concentration of 400 µM on the second day of incubation.

Wheat plants were infected with G. graminis var. tritici as described earlier with a few modifications (23). Briefly, wheat seeds were surface sterilized with 1% AgNO3 for 10 min, washed 10 times with sterile water, and planted in sterile vermiculite with fungal inoculum in magenta boxes (Sigma-Aldrich, St. Louis, Mo.). The inoculum consisted of approximately one agar plug (1-cm diameter) per seed. Agar plugs were cut from young mycelia grown on solid LB medium and spread near seeds approximately 2 cm below the surface. Control (uninfected) wheat plants were treated with sterile LB agar plugs. All plants were grown with a 12-h photoperiod with 3,000 lx.

For plant homogenate induction experiments, wheat plants were grown aseptically in magenta boxes on sterile vermiculite for 7 to 10 days. From 10 to 20 plants were harvested, quick-frozen and ground in liquid N2, and resuspended in 20 ml of sterile water. The plant suspension was filter sterilized and added to fungal cultures to a final concentration of 30% (vol/vol).

Laccase purification and sequencing.
The enzyme was purified from 5-day-old copper-induced LB cultures with a Bio-Rad (Hercules, Calif.) Prep Cell apparatus and eluted after 6 to 7 h as determined by 2,6-dimethoxyphenol assays, as described previously (13). Samples of laccase (30 µl) in 2x loading buffer (13) were heated at 95°C and electrophoresed on a sodium dodecyl sulfate (SDS)-7.5% (wt/vol) polyacrylamide gel electrophoresis (PAGE) gel using the Tris-tricine discontinuous gel system (39). The gel was electroblotted onto a polyvinylidene difluoride membrane (Millipore, Bedford, Mass.) using the Bio-Rad semidry electroblotter according to the manufacturer's instructions. The membrane was stained in 0.15% (wt/vol) Coomassie blue R250 in 50% (vol/vol) methanol for 5 min, destained in 50% methanol, washed in water, and air dried, and the protein was subjected to Edman degradation. Protein sequencing was performed on an Applied Biosystems 475 protein sequencer (Applied Biosystems, Foster City, Calif.) by the Laboratory for Bioanalysis and Biotechnology at Washington State University (Pullman, Wash.).

DNA and RNA purification.
Fungal genomic DNA was prepared as previously described (30). RNA was purified from fungal mycelia grown in different culture media, from 10-day-old infected wheat plants, and from 10-day-old uninfected control plants. Infected and uninfected wheat plants and filtered mycelia were quick-frozen and ground in liquid N2. Total RNA was purified with a Qiagen (Chatsworth, Calif.) RNeasy plant minikit according to the manufacturer's instructions. All RNA preparations were treated with RQ1 DNase (Promega, Madison, Wis.) for 2 h at 37°C, according to the manufacturer's instructions, to remove any contaminating DNA, and repurified with the same kit.

Genomic library construction and screening.
A custom genomic library of G. graminis var. tritici was obtained from Stratagene (La Jolla, Calif.). The library was constructed in vector {lambda} DASH II and propagated in Escherichia coli XL1-Blue MRA(P2). A subcloned laccase gene from Neurospora crassa (kindly provided by H. Inoue) was used as a probe for genomic library screening. Briefly, a 4.5-kb BglII DNA fragment was excised from the pBL1 plasmid (56), gel purified using the QIAquick gel extraction kit (Qiagen), and labeled with digoxigenin-11-dUTP (Boehringer Mannheim, Indianapolis, Ind.) by the random-primed method according to the manufacturer's instructions. Approximately 20,000 plaques were screened by plaque hybridization (38) with the probe at a concentration of 25 ng/ml under high-stringency conditions: 5x SSC (1x SSC is 150 mM NaCl, 15 mM sodium citrate, pH 7)-1% blocking reagent (Boehringer Mannheim)-0.1% N-lauroylsarcosine-0.2% SDS at 65°C overnight. Filters were washed twice in 2x SSC-0.1% SDS at room temperature and twice in 0.5x SSC-0.1% SDS at 65°C.

Plaques that hybridized with the N. crassa laccase probe were selected and purified, and fungal DNA fragments were subcloned into the NotI site of plasmid pGEM-5Zf(+) (Promega) according to standard protocols (38). Both strands of putative laccase genes and their 5' and 3' untranslated regions were sequenced by the primer-walking approach (38).

RT-PCR.
Single-tube Access reverse transcription (RT)-PCR (Promega) was used to monitor the differential transcription of laccase genes. Two micrograms of total RNA (determined spectrophotometrically at A260) was used for each RT-PCR. Reactions were carried out in 25-µl volumes with 1x AMV/Tfl reaction buffer (0.2 mM deoxynucleoside triphosphates, 2.5 mM each primer, 1 mM MgSO4, 2.5 U of avian myeloblastosis virus [AMV] reverse transcriptase, and 2.5 U of Tfl DNA polymerase). RT-PCR parameters were 45 min at 48°C and 3 min at 94°C, followed by 40 cycles of 1 min at 94°C, 1 min at 50 to 60°C (depending on the specific primer pair), and 1 min at 72°C, followed by 1 cycle of 7 min at 72°C.

Primers specific for each laccase gene were designed based on the sequences reported in this article. LAC1-specific primers were 5'-GAGCTGGAACGCGATGGCTA-3' and 5'-GCATCATCCGCTACGACAAG-3', with an annealing temperature of 55°C. LAC2-specific primers were 5'-CGCATCATCTTTTGTGCTCC-3' and 5'-AGCGCCAACTACGACGAGGA-3', with an annealing temperature of 52°C. LAC3-specific primers were 5'-GCTATGCGGCACCAGCCTTA-3' and 5'-GTGGCCGTGCAGGTGGAT-3', with an annealing temperature of 50°C.

To verify that the same amounts of total mRNA were used for each RT-PCR, transcription of the housekeeping DNA polymerase {gamma} (POLG) gene from G. graminis var. tritici was monitored with primers 5'-ATGAGCAGGTGGAGGTAGTCA-3' and 5'-CGCAGCTGCTGAGACAGTTCAA-3', which were designed after we sequenced a POLG gene fragment of G. graminis var. graminis. Sequences of the primers used for determining intron positions and for sequencing are available upon request. Amplification products were separated on 2% agarose gels in TBE (90 mM Tris, 90 mM boric acid, 2 mM EDTA), stained with ethidium bromide, and visualized under UV light. For each RT-PCR, a negative control PCR without AMV reverse transcriptase was performed to rule out DNA contamination. PCR products were subcloned into the pCR4-TOPO vector (Invitrogen, Carlsbad, Calif.) and sequenced using M13 forward and reverse primers to verify their identity.

Competitive PCR.
Transcript levels of G. graminis laccase genes were determined by competitive RT-PCR (4, 42, 57). A plasmid containing subcloned LAC1 from G. graminis var. tritici, a plasmid containing a subcloned G. graminis var. graminis LAC2 gene that contains one intron not present in G. graminis var. tritici, and a plasmid containing a subcloned portion of the G. graminis var. graminis POLG gene with a 100-bp KpnI-XhoI insertion were used as competitor templates. Competitor templates with introns or an insertion were larger than cDNA templates and separated from cDNA PCR products on agarose gels. Serial 10-fold dilutions of competitor plasmids were made and added to RT-PCRs. Two micrograms of total RNA purified from fungal cultures growing in the presence or absence of Cu was used for each RT-PCR. Three independent spectrophotometric measurements were performed to determine the concentration for each RNA preparation. LAC2-specific primers were 5'-TGGCAGTGCATGATCCAG-3' and 5'-ATCGGC/GCAGCGCTACGACGT-3', with an annealing temperature of 50°C. RT-PCR conditions, cycling parameters, and LAC1- and POLG-specific primers were the same as for other RT-PCR experiments.

DNA sequencing and analysis.
Sequencing reactions were performed using the ABI PRISM Big Dye terminator cycle sequencing ready reaction kit (Applied Biosystems) according to the manufacturer's instructions. Sequences were analyzed on an Applied Biosystems 310 genetic analyzer using a 47-mm capillary column. Sequences were determined in both directions, edited, and assembled using the Sequencher 3.1.1 program (Gene Codes Corporation, Ann Arbor, Mich.). Sequence homology searches were performed using the BlastX search engine (1).

Nucleotide sequence accession numbers.
Nucleotide sequences of the G. graminis var. tritici LAC1, LAC2, and LAC3 genes were deposited in the EMBL nucleotide sequence database under accession numbers AJ417685, AJ417686, and AJ417687, respectively.


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RESULTS
 
Cloning and analysis of laccase genes.
We identified and subcloned three putative laccase genes that were designated LAC1, LAC2, and LAC3 (55). The overall exon-intron structures of all three laccase genes in G. graminis var. tritici were distinct. LAC1 contained a 1,977-bp open reading frame (ORF) that was interrupted by two introns of 87 and 80 bp. LAC2 contained a 1,731-bp ORF with no introns, and LAC3 contained a 2,212-bp ORF that was interrupted by six introns ranging from 63 to 87 bp. Positions of putative introns were determined by analysis of nucleotide sequence and later verified by partial sequencing of corresponding cDNAs.

Characterization of deduced laccases.
We predicted that LAC1, LAC2, and LAC3 encoded polypeptides of 609, 577, and 581 amino acids, respectively. The deduced amino acid sequences showed 35.8, 35.4, and 48.5% identity with N. crassa laccase (EMBL P06811), respectively, and 42.6% (Lac1-Lac2), 38.1% (Lac1-Lac3), and 39.9% (Lac2-Lac3) identity with each other. All three predicted gene products contained four conserved putative Cu-binding sites that are characteristic of laccases (Fig. 1). The deduced primary polypeptides also began with 18- to 20-amino-acid signal sequences typical of extracellular enzymes as predicted by the Neilsen et al. algorithm (31), and they contained six to eight potential glycosylation sites as predicted by the Hansen et al. algorithm (22) (Fig. 1).



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  		FIG. 1. Alignment of the deduced Lac1, Lac2, and Lac3 amino acid sequences, performed with the ClustalW method (46). Four putative copper-binding domains are shaded. Possible N-glycosylation sites are in bold and underlined, putative signal sequences are shown in italic, suggested cleavage sites are marked with thin arrows, the cleavage site in mature Lac2, determined by protein sequencing, is shown with a thick arrow, and the sequenced polypeptide is underlined.

N-terminal peptide sequence analysis.
In liquid LB culture, when induced with Cu, G. graminis var. tritici produced only one major secreted laccase protein. We purified this protein according to the previously described protocol (13) and subjected it to Edman degradation and sequencing. The first 15 amino acid residues at the amino terminus were TATTRSKWCDFDIKS. This sequence was identical to residues 41 to 56 of the precursor protein encoded by LAC2 (Fig. 1) and implied that the protein undergoes posttranslational proteolytic cleavage at position 40.

Characterization of the upstream regulatory regions of LAC1, LAC2, and LAC3.
Sequence analysis of 5' nontranscribed regions of LAC1, LAC2, and LAC3 revealed typical promoter sequences (49). Transcription of many laccase genes is regulated by copper ions (8, 18, 34). We screened the 5' nontranscribed regions of three laccase genes for similarity with consensus eukaryotic sequences known to mediate transcriptional activation by copper ions (45). Metal-responsive elements (MRE) are found in the upstream regions of the Saccharomyces cerevisiae Cu-metallothionein and CuZn-superoxide dismutase genes (45). Putative MRE sequences are also present in several promoter regions of previously characterized laccase genes (18, 28). We located four putative MRE consensus sequences situated around the TATA box of the LAC2 gene promoter and two consecutive putative MRE sequences located 751 and 690 bp upstream of the TATA box of the LAC1 gene promoter.

ACE1 is a eukaryotic copper-responsive transcription factor that activates Cu-dependent transcription of the S. cerevisiae metallothionein genes (45). We identified one putative ACE1 consensus sequence located 108 bp upstream of the TATA box in the LAC1 gene promoter and two consecutive ACE1 consensus sequences flanking the CT-rich box in the promoter of LAC2 downstream of the TATA box. We did not identify any MRE or ACE1 elements in the LAC3 gene promoter.

Regulation of laccase transcription.
We used an RT-PCR approach to characterize transcription of the three laccase genes of G. graminis var. tritici. Total RNA was purified from young actively growing mycelia, at the peak of secreted laccase activity. The transcription pattern of the LAC1 gene is shown in Fig. 2A. Fungal cultures were grown in starvation medium (lane 2), Fahraeus minimal medium with and without Cu (lanes 3 and 4, respectively), PDB (lane 5), LB with and without Cu (lanes 6 and 7, respectively), LB supplemented with sterile plant homogenate (lane 8), and the host wheat plant (lane 9). With primers specific for LAC1, similar levels of the LAC1 transcript were observed in all conditions tested.



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  		FIG. 2. (A) Transcription patterns of the LAC1 gene. RT-PCR was performed with LAC1-specific primers, and total RNA was isolated from G. graminis grown in starvation medium (lane 2), minimal medium supplemented with 400 µm CuSO4 (lane 3), minimal medium without copper (lane 4), PDB (lane 5), LB medium supplemented with 400 µM CuSO4 (lane 6), LB medium without copper (lane 7), and LB medium supplemented with sterile plant homogenate (lane 8). Lane 9 shows RT-PCR with LAC1-specific primers and RNA purified from wheat plants infected with G. graminis; lane 10 shows RT-PCR with LAC1-specific primers and RNA purified from uninfected control wheat plants. Lane 11 shows amplified genomic LAC1, which contains two introns. Lane 1 shows a 100-bp ladder (Promega) as molecular size markers. (B) Transcription patterns of the LAC2 gene. RT-PCR was performed with LAC2-specific primers and total RNA isolated from G. graminis grown in starvation medium (lane 2); minimal medium supplemented with 400 µM CuSO4 (lane 3); minimal medium without copper (lane 5); PDB (lane 7); LB medium supplemented with 400 µm CuSO4 (lane 8); and LB medium without copper (lane 10). Lane 12 shows RT-PCR with primers specific for LAC2 and RNA purified from the wheat plants infected with G. graminis; lane 14 shows RT-PCR with primers specific for LAC2 and RNA purified from the uninfected control wheat plants; and lane 15 shows amplified genomic LAC2, which has no introns. Control reactions without reverse transcriptase are shown in lanes 4, 6, 9, 11, and 13. Lane 1 shows 100-bp ladder molecular size markers. (C) Transcription patterns of the LAC3 gene. RT-PCR was performed with LAC3-specific primers, and total RNA was isolated from wheat plants infected with G. graminis var. tritici (lane 2); G. graminis var. tritici grown in minimal medium supplemented with 30% (vol/vol) sterile plant homogenate (lane 3); minimal medium with 400 µM CuSO4 (lane 6); minimal medium without copper (lane 7); starvation medium (lane 8); PDB (lane 9); LB with 400 µM CuSO4 (lane 10); and LB without copper (lane 11). Lane 4 shows amplified genomic LAC3, which contains an intron. Lane 5 shows RT-PCR with primers specific for LAC3 and RNA purified from the uninfected control wheat plants. Lane 1 shows 100-bp ladder molecular size markers (Promega). (D) Transcription patterns of the POLG gene. RT-PCR was performed with POLG-specific primers and total RNA isolated from G. graminis grown in starvation medium (lane 2); PDB (lane 4); minimal medium supplemented with 400 µM CuSO4 (lane 6); minimal medium without copper (lane 8); LB medium supplemented with 400 µM CuSO4 (lane 10); and LB medium without copper (lane 12). Lane 14 shows RT-PCR with primers specific for POLG and RNA purified from the wheat plants infected with G. graminis; lane 16 shows RT-PCR with primers specific for POLG and RNA purified from the uninfected control wheat plants; lane 17 shows amplified genomic POLG, which has no introns. Control reactions without reverse transcriptase are shown in lanes 3, 5, 7, 9, 11, 13, and 15. Lane 1 shows 100-bp ladder molecular size markers.

Transcription patterns of LAC2 are shown in Fig. 2B. The LAC2 gene was transcribed in Fahraeus minimal medium supplemented with Cu (lane 3), in LB medium with or without Cu (lanes 8 and 10, respectively), and in planta (lane 12). It was transcribed at a very low level in Fahraeus minimal medium without copper (lane 5), and was not transcribed in starvation medium (lane 2) or in PDB (lane 7).

Transcription of LAC3 was observed only in planta (Fig. 2C, lane 2) or in culture medium with homogenized, sterile plant tissue (Fig. 2C, lane 3). The LAC3 transcript was detected in infected root, leaf, and stem tissues (data not shown).

Transcription patterns of the housekeeping POLG gene are shown in Fig. 2D. Similar high levels of housekeeping gene transcripts were observed in all conditions tested.

Regulation of LAC1 and LAC2 transcription with copper.
The results of the discriminatory LAC1/LAC2 competitive RT-PCR from Fahraeus minimal medium cultures with and without 400 µM CuSO4 are shown in Fig. 3. LAC1 transcription was independent of copper; cultures grown with and without CuSO4 had the same level of LAC1 transcript (Fig. 3A). In contrast, transcription of LAC2 was increased at least 10-fold in the presence of 400 µM CuSO4 (Fig. 3B) and corresponded to an 8-fold increase in secreted laccase activity (data not shown). Transcription of POLG was also independent of copper and indicated that the same amounts of mRNA were used for each RT-PCR. (Fig. 3C).



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  		FIG. 3. Transcription of LAC1 (A), LAC2 (B), and POLG (C) in the presence and absence of 400 µM CuSO4. The amplified competitive templates and cDNAs are indicated with arrows. The amounts of the competitive templates are indicated above the gels (in micrograms per microliter). A 100-bp ladder (Promega) was used as molecular size markers.

Figure 3 demonstrates that in the presence or absence of copper, LAC1 was transcribed at a level (10-3 to 10-4 µg per µg of total RNA) similar to or higher than that of POLG; in the absence of copper, LAC2 was transcribed at a level (10-5 µg per µg of total RNA) lower than that of POLG, and in the presence of the inducer, LAC2 was transcribed at a level similar to that of POLG (10-4 µg per µg of total RNA). Transcription of laccase genes was not induced by zinc or manganese (data not shown). All competitive PCR experiments were repeated three times with similar results.


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DISCUSSION
 
We identified and partially characterized three laccase genes from a plant-pathogenic fungus, G. graminis var. tritici. Each gene had a distinct structure, with different numbers of introns, ranging from zero (LAC2) to six (LAC3). Intron positions were also unique for each gene and did not overlap between the genes. The deduced protein sequences showed 38 to 42% amino acid sequence identity.

Laccases are characterized by having four conserved copper-binding sites. They contain one type 1 (T1) Cu atom, bound as a mononuclear center that gives the proteins their blue color, as well as type 2 (T2) and type 3 (T3) Cu atoms, which together form a trinuclear center. The T1 site functions as a primary electron acceptor, extracting electrons from the substrate. Electrons are then transferred to the T2/T3 center, where reduction of molecular oxygen to water takes place (12, 29). According to a recent analysis of the Coprinus cinereus laccase crystal structure (12) and site-directed mutagenesis studies of Myceliophthora thermophila and R. solani laccases (52), a pentapeptide segment located downstream of the second conserved histidine in the T1 site (H560 in Lac1, H537 in Lac2, and H533 in Lac3; Fig. 1) has a major effect on the redox potential and specificity of the enzyme. The amino acid composition in this region was unique for all three of the deduced laccases in G. graminis var. tritici (Fig. 1), which implies that the enzymes have different substrate specificities and probably different functions in the fungus.

Recently, we purified and partially characterized a secreted, copper-induced laccase from G. graminis var. tritici (13). Here we show that the N terminus of this protein was identical to the predicted protein encoded by the LAC2 gene. LAC2 transcription was significantly induced by Cu and corresponded to a similar increase in the total laccase activity of induced cultures. In PDB or starvation medium, in which LAC2 was not transcribed, a low level of secreted laccase activity was detected. This activity was independent of the copper concentration in the medium (data not shown) and likely corresponded to LAC1 expression, since it was the only laccase gene transcribed in these conditions. However, we have not purified Lac1 or Lac3 laccases because of their low concentrations. Recently it was shown that extracellular proteases may affect the concentrations of secreted laccases (33), which could explain why we were unable to obtain larger amounts of Lac1 and Lac3.

Our study indicated that the laccase genes in G. graminis var. tritici were induced in different conditions. LAC1 was transcribed constitutively in all conditions tested. Constitutive transcription has been reported for several fungal laccase genes (15, 54, 57). In contrast, transcription of LAC2 was copper inducible. The promoter region of LAC2 contained four putative MRE consensus sites, two of which were inverted and 51 bp apart. In addition, this promoter sequence contained two putative ACE1 binding sites situated between the TATA box and ATG codon.

Copper induction has been reported for the P. anserina (18), T. versicolor (25), and P. ostreatus (34) laccase genes. MRE consensus sequences are found in the promoter regions of two fungal laccase genes, those of P. anserina (18) and Phanerochaete chrysosporium (4). One consensus sequence that has some similarities with the binding site of the ACE1 transcription factor from S. cerevisiae was reported for the lcc1 promoter from the basidiomycete PM1 (28). Nevertheless, the role of MRE and ACE1 sites in laccase promoters requires additional investigation, including site-directed mutagenesis to remove these sites.

Transcription of LAC3 was observed only when the fungus was grown in association with the host plant. We were unable to induce LAC3 transcription in a culture except by the addition of sterile, homogenized plant filtrate to the medium. To our knowledge, this is the first report of a fungal laccase gene whose transcription depends on the presence of a host. However, laccase from Botrytis cinerea is inducible in culture by the addition of pectin, suggesting that its expression is host dependent (35). This laccase also oxidizes plant stilbene phytoalexins in vitro and may be a pathogenicity factor (35). The animal pathogen C. neoformans expresses laccase that is involved in the biosynthesis of a melanin-like pigment in mouse tissues; laccase-negative lac1 (Mel-) knockout mutants of C. neoformans are defective in pigment production and nonpathogenic (37, 51).

Although the biological functions of several fungal laccases have been determined (14, 16, 35, 48), the functions of the majority of laccases are still uncertain. However, their abundance suggests that they play important and diverse roles. G. graminis var. tritici produces melanized macrohyphae on its host's root surfaces, and a laccase(s) could be involved in melanin polymerization in these hyphae (41). A laccase(s) could also degrade lignin depositions produced by the plant in response to invasion, and/or oxidize-reduce phytoalexins and other toxic plant compounds. Other functions, such as sequestration or oxidation of manganese, required by the plant for lignin synthesis (36) are also possible.

Previously, we demonstrated that the major secreted laccase in culture, here shown to be encoded by LAC2, is capable of oxidizing lignin and polymerizing melanin precursors in vitro. The ability of laccase to oxidize or reduce polyphenolic compounds such as lignin or melanin precursors depends on the redox potential of the surrounding environment. Both functions can be carried out by the same enzyme in different situations; alternatively, oxidation and reduction may be split between different laccases expressed in different environments or compartments of the infected host. Mutants in which each of these genes are knocked out would help identify the functions of the individual laccases of G. graminis var. tritici, but thus far we have been unsuccessful in generating disruption mutants with cloned laccase genes. Homologous recombination with extraneous DNA is rare in G. graminis (3), and thus, alternative methods for reducing the expression of laccase genes may be required to determine their functions.


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ACKNOWLEDGMENTS
 
We thank Bill Edens and Tim Magnuson for help with protein purification, Kathy Sheehan for critical reading of the manuscript, and Mary Bateson for help with DNA sequencing.

This work was supported in part by the U.S. Army Research Office (grant DAAH04-96-1-01194) and the National Science Foundation (grant MCB9977922).


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FOOTNOTES
 
* Corresponding author. Mailing address: Dept. of Microbiology, 109 Lewis Hall, Montana State University, Bozeman, MT 59717. Phone: (406) 994-4690. Fax: (406) 994-4926. E-mail: jhenson{at}montana.edu. Back


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Applied and Environmental Microbiology, March 2002, p. 1305-1311, Vol. 68, No. 3
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.3.1305-1311.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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