Use of laccase as a novel, versatile reporter system in filamentous fungi

Is Laccase derived from Pleurotus ostreatus effective in microplastic degradation? A critical review of current progress, challenges, and future prospects

Exploration of Pleurotus ostreatus as a biological agent in the degradation of persistent plastics like polyethylene, polystyrene, polyvinyl chloride, and polyethylene terephthalate, revealing a promising avenue toward mitigating the environmental impacts of plastic pollution. Leveraging the intrinsic enzymatic capabilities of this fungus, mainly its production of laccase, presents a sustainable and eco-friendly approach to breaking down complex polymer chains into less harmful constituents. This review focused on enhancements in the strain's efficiency through genetic engineering, optimized culture conditions, and enzyme immobilization to underscore the potential for scalability and practical application of this bioremediation process. The utilization of laccase from P. ostreatus in plastic waste management demonstrates a vital step forward in pursuing sustainable environmental solutions. By using the potential of fungal bioremediation, researchers can move closer to a future in which the adverse effects of plastic pollution are significantly mitigated, benefiting the health of our planet and future generations.

Production and characterization of novel thermostable CotA-laccase from Bacillus altitudinis SL7 and its application for lignin degradation

Laccases are multi-copper oxidases and found in ligninolytic bacteria catalyzing the oxidation of both phenolic and non-phenolic compounds, however its application in lignin degradation suffers due to low oxidation rate, which have intensified the search for new laccases. In the present study, spore coat A protein (CotA) encoding gene having laccase like activity from Bacillus altitudinis SL7 (CotA-SL7) was cloned and expressed in Escherichia coli. The purified CotA-SL7 was active at wide range of temperature and pH with optimum activity at 55 °C and pH 5.0. The kinetic parameters of CotA-SL7 was determined with Km, Vmax, and kcat values 0.4 mM, 2777 μmol/min/mg, and 5194 s-1, respectively. Molecular docking revealed the presence of Pro, Phe, Asp, Asn, His, and Ile residues at the active site taking part in the oxidation of ABTS. The purified CotA-SL7 reduced lignin contents by 31 % and changes in lignin structure were analyzed through fourier transformed infrared spectroscopy (FTIR), scanning electron microsscopy (SEM) and gas chromatography mass-spectrometry (GC-MS). The appearance of low molecular size compounds clearly indicates the cleavage of lignin polymer and opening of the benzene ring by purified CotA-SL7. Thus, high catalytic efficiency of CotA-SL7 makes it a suitable bio-catalyst for remediation of lignin contaminated wastewater from pulp and paper industries with clear insights into lignin degradation at molecular level.

Development of an efficient protein expression system in the thermophilic fungus Myceliophthora thermophila

BACKGROUND

Thermophilic fungus Myceliophthora thermophila has been widely used in industrial applications due to its ability to produce various enzymes. However, the lack of an efficient protein expression system has limited its biotechnological applications.

RESULTS

In this study, using a laccase gene reporting system, we developed an efficient protein expression system in M. thermophila through the selection of strong constitutive promoters, 5'UTRs and signal peptides. The expression of the laccase was confirmed by enzyme activity assays. The results showed that the Mtpdc promoter (Ppdc) was able to drive high-level expression of the target protein in M. thermophila. Manipulation of the 5'UTR also has significant effects on protein expression and secretion. The best 5'UTR (NCA-7d) was identified. The transformant containing the laccase gene under the Mtpdc promoter, NCA-7d 5'UTR and its own signal peptide with the highest laccase activity (1708 U/L) was obtained. In addition, the expression system was stable and could be used for the production of various proteins, including homologous proteins like MtCbh-1, MtGh5-1, MtLPMO9B, and MtEpl1, as well as a glucoamylase from Trichoderma reesei.

CONCLUSIONS

An efficient protein expression system was established in M. thermophila for the production of various proteins. This study provides a valuable tool for protein production in M. thermophila and expands its potential for biotechnological applications.

Enhancing the production of a heterologous Trametes laccase (LacA) by replacement of the major cellulase CBH1 in Trichoderma reesei

The laccases from white-rot fungi exhibit high redox potential in treating phenolic compounds. However, their application in commercial purposes has been limited because of the relatively low productivity of the native hosts. Here, the laccase A-encoding gene lacA of Trametes sp. AH28-2 was overexpressed under the control of the strong promoter of cbh1 (Pcbh1), the gene encoding the endogenous cellobiohydrolase 1 (CBH1), in the industrial workhorse fungus Trichoderma reesei. Firstly, the lacA expression cassette was randomly integrated into the T. reesei chromosome by genetic transformation. The lacA gene was successfully transcribed, but the laccase couldn't be detected in the liquid fermentation condition. Meanwhile, it was found that the endoplasmic reticulum-associated degradation (ERAD) was strongly activated, indicating that the expression of LacA probably triggered intense endoplasmic reticulum (ER) stress. Subsequently, the lacA expression cassette was added with the downstream region of cbh1 (Tcbh1) to construct the new expression cassette lacA::Δcbh1, which could replace the cbh1 locus in the genome via homologous recombination. After genetic transformation, the lacA gene was integrated into the cbh1 locus and transcribed. And the unfolded protein response (UPR) and ERAD were only slightly induced, for which the loss of endogenous cellulase CBH1 released the pressure of secretion. Finally, the maximum laccase activity of 168.3 U/l was obtained in the fermentation broth. These results demonstrated that the reduction of secretion pressure by deletion of endogenous protein-encoding genes would be an efficient strategy for the secretion of heterologous target proteins in industrial fungi.

ONE-SENTENCE SUMMARY

The reduction of the secretion pressure by deletion of the endogenous cbh1 gene can contribute to heterologous expression of the laccase (LacA) from Trametes sp. AH28-2 in Trichoderma reesei.

A Laccase Gene Reporting System That Enables Genetic Manipulations in a Brown Rot Wood Decomposer Fungus Gloeophyllum trabeum

Brown rot fungi are primary decomposers of wood and litter in northern forests. Relative to other microbes, these fungi have evolved distinct mechanisms that rapidly depolymerize and metabolize cellulose and hemicellulose without digesting the more recalcitrant lignin. Its efficient degradative system has therefore attracted considerable attention for the development of sustainable biomass conversion technologies. However, there has been a significant lack of genetic tools in brown rot species by which to manipulate genes for both mechanistic studies and engineering applications. To advance brown rot genetic studies, we provided a gene-reporting system that can facilitate genetic manipulations in a model fungus Gloeophyllum trabeum. We first optimized a transformation procedure in G. trabeum, and then transformed the fungus into a constitutive laccase producer with a well-studied white rot laccases gene (from Trametes versicolor). With this, we built a gene reporting system based on laccase gene's expression and its rapid assay using an 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) indicator dye. The laccase reporter system was validated robust enough to allow us to test the effects of donor DNA's formats, protoplast viability, and gene regulatory elements on transformation efficiencies. Going forward, we anticipate the toolset provided in this work would expedite phenotyping studies and genetic engineering of brown rot species. IMPORTANCE One of the most ubiquitous types of decomposers in nature, brown rot fungi, has lacked robust genetic tools by which to manipulate genes and understand its biology. Brown rot fungi are primary decomposers in northern forests helping recycle the encased carbons in trees back to ecosystem. Relative to other microbes, these fungi employ distinctive mechanisms to disrupt and consume the lignified polysaccharides in wood. Its decay mechanism allows fast, selective carbohydrate catabolization, but without digesting lignin-a barren component that produces least energy trade back for fungal metabolisms. Thus, its efficient degradative system provides a great platform for developing sustainable biotechnologies for biomass conversions. However, progress has been hampered by the lack genetic tools facilitating mechanistic studies and engineering applications. Here, the laccase reporter system provides a genetic toolset for genetic manipulations in brown rot species, which we expect would advance relevant genetic studies for discovering and harnessing the unique fungal degradative mechanisms.

Stachybotrys chartarum-A Hidden Treasure: Secondary Metabolites, Bioactivities, and Biotechnological Relevance

Fungi are renowned as a fountainhead of bio-metabolites that could be employed for producing novel therapeutic agents, as well as enzymes with wide biotechnological and industrial applications. Stachybotrys chartarum (black mold) (Stachybotriaceae) is a toxigenic fungus that is commonly found in damp environments. This fungus has the capacity to produce various classes of bio-metabolites with unrivaled structural features, including cyclosporins, cochlioquinones, atranones, trichothecenes, dolabellanes, phenylspirodrimanes, xanthones, and isoindoline and chromene derivatives. Moreover, it is a source of various enzymes that could have variable biotechnological and industrial relevance. The current review highlights the formerly published data on S. chartarum, including its metabolites and their bioactivities, as well as industrial and biotechnological relevance dated from 1973 to the beginning of 2022. In this work, 215 metabolites have been listed and 138 references have been cited.

Transcriptome Profiling Reveals Differential Gene Expression of Laccase Genes in Aspergillus terreus KC462061 during Biodegradation of Crude Oil

Fungal laccases have high catalytic efficiency and are utilized for the removal of crude oil because they oxidize various aliphatic and aromatic hydrocarbons and convert them into harmless compounds or less toxic compounds, thus accelerating the biodegradation potential of crude oil. Laccases are important gene families and the function of laccases genes varied widely based on transcription and function. Biodegradation of crude oil using Aspergillus terreus KC462061 was studied in the current study beside the transcription level of eight laccase (Lcc) genes have participated in biodegradation in the presence of aromatic compounds, and metal ions. Time-course profiles of laccase activity in the presence of crude oil indicated that the five inducers individual or combined have a very positive on laccase activity. In the status of the existence of crude oil, the synergistic effect of Cu-ABTS compound caused an increase in laccase yields up to 22-fold after 10 days than control. The biodegradation efficiencies of A. terreus KC462061 for aliphatic and aromatic hydrocarbons of crude oil were 82.1 ± 0.2% and 77.4 ± 0.6%, respectively. The crude oil biodegradation efficiency was improved by the supplemented Cu-ABTS compound in A. terreus KC462061. Gas chromatography-mass spectrometry was a very accurate tool to demonstrate the biodegradation efficiencies of A. terreus KC462061 for crude oil. Significant differences were observed in the SDS-PAGE of A. terreus KC462061 band intensities of laccase proteins after the addition of five inducers, but the Cu-ABTS compound highly affects very particular laccase electrophoresis. Quantitative real-time polymerase chain reaction (qPCR) was used for the analysis of transcription profile of eight laccase genes in A. terreus KC462061 with a verified reference gene. Cu²⁺ ions and Cu-ABTS were highly effective for efficient laccase expression profiling, mainly via Lcc11 and 12 transcription induction. The current study will explain the theoretical foundation for laccase transcription in A. terreus KC462061, paving the road for commercialization and usage.

The small-secreted cysteine-rich protein CyrA is a virulence factor participating in the attack of Caenorhabditis elegans by Duddingtonia flagrans

Nematode-trapping fungi (NTF) are a diverse and intriguing group of fungi that live saprotrophically but can switch to a predatory lifestyle when starving and in the presence of nematodes. NTF like Arthrobotrys oligospora or Duddingtonia flagrans produce adhesive trapping networks to catch and immobilize nematodes. After penetration of the cuticle, hyphae grow and develop inside the worm and secrete large amounts of hydrolytic enzymes for digestion. In many microbial pathogenic interactions small-secreted proteins (SSPs) are used to manipulate the host. The genome of D. flagrans encodes more than 100 of such putative SSPs one of which is the cysteine-rich protein CyrA. We have chosen this gene for further analysis because it is only found in NTF and appeared to be upregulated during the interaction. We show that the cyrA gene was transcriptionally induced in trap cells, and the protein accumulated at the inner rim of the hyphal ring before Caenorhabditis elegans capture. After worm penetration, the protein appeared at the fungal infection bulb, where it is likely to be secreted with the help of the exocyst complex. A cyrA-deletion strain was less virulent, and the time from worm capture to paralysis was extended. Heterologous expression of CyrA in C. elegans reduced its lifespan. CyrA accumulated in C. elegans in coelomocytes where the protein possibly is inactivated. This is the first example that SSPs may be important in predatory microbial interactions.

Intercellular communication is required for trap formation in the nematode-trapping fungus Duddingtonia flagrans

Nematode-trapping fungi (NTF) are a large and diverse group of fungi, which may switch from a saprotrophic to a predatory lifestyle if nematodes are present. Different fungi have developed different trapping devices, ranging from adhesive cells to constricting rings. After trapping, fungal hyphae penetrate the worm, secrete lytic enzymes and form a hyphal network inside the body. We sequenced the genome of Duddingtonia flagrans, a biotechnologically important NTF used to control nematode populations in fields. The 36.64 Mb genome encodes 9,927 putative proteins, among which are more than 638 predicted secreted proteins. Most secreted proteins are lytic enzymes, but more than 200 were classified as small secreted proteins (< 300 amino acids). 117 putative effector proteins were predicted, suggesting interkingdom communication during the colonization. As a first step to analyze the function of such proteins or other phenomena at the molecular level, we developed a transformation system, established the fluorescent proteins GFP and mCherry, adapted an assay to monitor protein secretion, and established gene-deletion protocols using homologous recombination or CRISPR/Cas9. One putative virulence effector protein, PefB, was transcriptionally induced during the interaction. We show that the mature protein is able to be imported into nuclei in Caenorhabditis elegans cells. In addition, we studied trap formation and show that cell-to-cell communication is required for ring closure. The availability of the genome sequence and the establishment of many molecular tools will open new avenues to studying this biotechnologically relevant nematode-trapping fungus.

Nematode-trapping fungi are fascinating microorganisms, because they are able to switch from saprotrophic growth to a predatory lifestyle. Duddingtonia flagrans forms adhesive trap systems and conidia and resistant chlamydospores. Chlamydospores are ideal for dissemination in the environment to control nematode populations in the field. We show that D. flagrans is able to catch C. elegans but also the very large wine-pathogenic nematode Xiphinema index. We sequenced the D. flagrans genome and show that it encodes about 10,000 genes with a large proportion of secreted proteins. We hypothesize that virulence effector proteins are involved in the interkingdom organismic interaction and identified more than 100 candidates. In order to investigate the molecular biology of D. flagrans and its interaction with nematodes, we established a transformation system and several molecular tools. We show that cell-to-cell communication and hyphal fusion are required for trap formation. Finally, we show that one putative virulence effector protein targets nuclei when expressed in C. elegans.

A Novel Rapid Fungal Promoter Analysis System Using the Phosphopantetheinyl Transferase Gene, npgA, in Aspergillus nidulans

To develop a convenient promoter analysis system for fungi, a null-pigment mutant (NPG) of Aspergillus nidulans was used with the 4'-phosphopantetheinyl transferase (PPTase) gene, npgA, which restores the normal pigmentation in A. nidulans, as a new reporter gene. The functional organization of serially deleted promoter regions of the A. nidulans trpC gene and the Cryphonectria parasitica crp gene in filamentous fungi was representatively investigated to establish a novel fungal promoter assay system that depends on color complementation of the NPG mutant with the PPTase npgA gene. Several promoter regions of the trpC and crp genes were fused to the npgA gene containing the 1,034-bp open reading frame and the 966-bp 3' downstream region from the TAA, and the constructed fusions were introduced into the NPG mutant in A. nidulans to evaluate color recovery due to the transcriptional activity of the sequence elements. Serial deletion of the trpC and crp promoter regions in this PPTase reporter assay system reaffirmed results in previous reports by using the fungal transformation step without a laborious verification process. This approach suggests a more rapid and convenient system than conventional analyses for fungal gene expression studies.

Immobilization of LccC Laccase from Aspergillus nidulans on Hard Surfaces via Fungal Hydrophobins

Fungal hydrophobins are small amphiphilic proteins that can be used for coatings on hydrophilic and hydrophobic surfaces. Through the formation of monolayers, they change the hydrophobicity of a given surface. Especially, the class I hydrophobins are interesting for biotechnology, because their layers are stable at high temperatures and can only be removed with strong solvents. These proteins self-assemble into monolayers under physiological conditions and undergo conformational changes that stabilize the layer structure. Several studies have demonstrated how the fusion of hydrophobins with short peptides allows the specific modification of the properties of a given surface or have increased the protein production levels through controlled localization of hydrophobin molecules inside the cell. Here, we fused the Aspergillus nidulans laccase LccC to the class I hydrophobins DewA and DewB and used the fusion proteins to functionalize surfaces with immobilized enzymes. In contrast to previous studies with enzymes fused to class II hydrophobins, the DewA-LccC fusion protein is secreted into the culture medium. The crude culture supernatant was directly used for coatings of glass and polystyrene without additional purification steps. The highest laccase surface activity was achieved after protein immobilization on modified hydrophilic polystyrene at pH 7. This study presents an easy-to-use alternative to classical enzyme immobilization techniques and can be applied not only for laccases but also for other biotechnologically relevant enzymes.

IMPORTANCE

Although fusion with small peptides to modify hydrophobin properties has already been performed in several studies, fusion with an enzyme presents a more challenging task. Both protein partners need to remain in active form so that the hydrophobins can interact with one another and form layers, and so the enzyme (e.g., laccase) will remain active at the same time. Also, because of the amphiphilic nature of hydrophobins, their production and purification remain challenging so far and often include steps that would irreversibly disrupt most enzymes. In our study, we present the first functional fusion proteins of class I hydrophobins from A. nidulans with a laccase. The resulting fusion enzyme is directly secreted into the culture medium by the fungus and can be used for the functionalization of hard surfaces.

Alternaria alternata transcription factor CmrA controls melanization and spore development

Melanin is a black pigment widely distributed across the kingdoms, from bacterial to human. The filamentous fungus Alternaria alternata is a typical 'black fungus', which produces melanin in its hyphal and especially its asexual spore cell walls. Its biosynthesis follows the dihydroxynaphthalene (DHN) pathway with 1,8-DHN as an intermediate. Two genes, encoding a polyketide synthase (pksA) and a 1,3,8-trihydroxynaphthalene (THN) reductase (brm2), along with a putative transcription factor, CmrA, comprise a small gene cluster. Here we show that CmrA controls the expression of pksA and brm2, but that it also controls the expression of a scytalone dehydratase encoding gene (brm1) located elsewhere in the genome. The regulatory function of CmrA was shown in a reporter assay system. Al. alternata CmrA was expressed in the filamentous fungus Aspergillus nidulans where it was able to induce the expression of a reporter construct under the control of the putative pksA promoter. This suggests direct binding of CmrA to the promoter of pksA in the heterologous system. Likewise, silencing of cmrA in Al. alternata led to white colonies due to the lack of melanin. In addition, hyphal diameter and spore morphology were changed in the mutant and the number of spores reduced. Silencing of brm2 and inhibition of melanin biosynthesis by tricyclazole largely phenocopied the effects of cmrA silencing, suggesting a novel regulatory function of melanin in morphogenetic pathways.

Fungal laccases and their applications in bioremediation

Laccases are blue multicopper oxidases, which catalyze the monoelectronic oxidation of a broad spectrum of substrates, for example, ortho- and para-diphenols, polyphenols, aminophenols, and aromatic or aliphatic amines, coupled with a full, four-electron reduction of O₂ to H₂O. Hence, they are capable of degrading lignin and are present abundantly in many white-rot fungi. Laccases decolorize and detoxify the industrial effluents and help in wastewater treatment. They act on both phenolic and nonphenolic lignin-related compounds as well as highly recalcitrant environmental pollutants, and they can be effectively used in paper and pulp industries, textile industries, xenobiotic degradation, and bioremediation and act as biosensors. Recently, laccase has been applied to nanobiotechnology, which is an increasing research field, and catalyzes electron transfer reactions without additional cofactors. Several techniques have been developed for the immobilization of biomolecule such as micropatterning, self-assembled monolayer, and layer-by-layer techniques, which immobilize laccase and preserve their enzymatic activity. In this review, we describe the fungal source of laccases and their application in environment protection.

Biocatalytic potential of laccase-like multicopper oxidases from Aspergillus niger

BACKGROUND

Laccase-like multicopper oxidases have been reported in several Aspergillus species but they remain uncharacterized. The biocatalytic potential of the Aspergillus niger fungal pigment multicopper oxidases McoA and McoB and ascomycete laccase McoG was investigated.

RESULTS

The laccase-like multicopper oxidases McoA, McoB and McoG from the commonly used cell factory Aspergillus niger were homologously expressed, purified and analyzed for their biocatalytic potential. All three recombinant enzymes were monomers with apparent molecular masses ranging from 80 to 110 kDa. McoA and McoG resulted to be blue, whereas McoB was yellow. The newly obtained oxidases displayed strongly different activities towards aromatic compounds and synthetic dyes. McoB exhibited high catalytic efficiency with N,N-dimethyl-p-phenylenediamine (DMPPDA) and 2,2-azino-di(3-ethylbenzthiazoline) sulfonic acid (ABTS), and appeared to be a promising biocatalyst. Besides oxidizing a variety of phenolic compounds, McoB catalyzed successfully the decolorization and detoxification of the widely used textile dye malachite green.

CONCLUSIONS

The A. niger McoA, McoB, and McoG enzymes showed clearly different catalytic properties. Yellow McoB showed broad substrate specificity, catalyzing the oxidation of several phenolic compounds commonly present in different industrial effluents. It also harbored high decolorization and detoxification activity with the synthetic dye malachite green, showing to have an interesting potential as a new industrial biocatalyst.

Molecular characterization of pathogenic members of the genus Fonsecaea using multilocus analysis

Members of the fungal genus Fonsecaea causing human chromoblastomycosis show substantial geographic structuring. Genetic identity of clinical and environmental strains suggests transmission from plant debris, while the evolutionary processes that have led to spatially separated populations have remained unexplained. Sequences of ITS, BT2, ACT1, Cdc42, Lac and HmgA were analyzed, either by direct sequencing or by cloning. Thirty-seven clinical and environmental Fonsecaea strains from Central and South America, Asia, Africa and Europe were sequenced and possible recombination events were calculated. Phylogenetic trees of Cdc42, Lac and HmgA were statistically supported, but ITS, BT2 and ACT1 trees were not. The Standardized Index of Association (I_A^S) did not detect recombination (I_A^S = 0.4778), neither did the Phi-test for separate genes. In Fonsecaea nubica non-synonymous mutations causing functional changes were observed in Lac gene, even though no selection pressures were detected with the neutrality test (Tajima D test, p>0.05). Genetic differentiation of populations for each gene showed separation of American, African and Asian populations. Strains of clinical vs. environmental origin showed genetic distances that were comparable or lower than found in geographic differentiation. In conclusion, here we demonstrated clonality of sibling species using multilocus data, geographic structuring of populations, and a low functional and structural selective constraint during evolution of the genus Fonsecaea.

Expression of novel β-glucanase Cel12A from Stachybotrys atra in bacterial and fungal hosts

β-glucanase Cel12A from Stachybotrys atra has been cloned and expressed in Aspergillus niger. The purified enzyme showed high activity of β-1,3-1,4-mixed glucans, was also active on carboxymethylcellulose (CMC), while it did not hydrolyze crystalline cellulose or β-1,3 glucans as laminarin. Cel12A showed a marked substrate preference for β-1,3-1,4 glucans, showing maximum activity on barley β-glucans (27.69 U mg(-1)) while the activity on CMC was much lower (0.51 U mg(-1)). Analysis by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), isoelectric focussing (IEF), and zymography showed the recombinant enzyme has apparent molecular weight of 24 kDa and a pI of 8.2. Optimal temperature and pH for enzyme activity were 50°C and pH 6.5. Thin layer chromatography analysis showed that major hydrolysis products from barley β-glucan and lichean were 3-O-β-cellotriosyl-D-glucose and 3-O-β-cellobiosyl-D-glucose, while glucose and cellobiose were released in smaller amounts. The amino acid sequence deduced from cel12A revealed that it is a single domain enzyme belonging to the GH12 family, a family that contains several endoglucanases with substrate preference for β-1,3-1,4 glucans. We believe that S. atra Cel12A should be considered as a lichenase-like or nontypical endoglucanase.

Evolution of multicopper oxidase genes in coprophilous and non-coprophilous members of the order sordariales

Multicopper oxidases (MCO) catalyze the biological oxidation of various aromatic substrates and have been identified in plants, insects, bacteria, and wood rotting fungi. In nature, they are involved in biodegradation of biopolymers such as lignin and humic compounds, but have also been tested for various industrial applications. In fungi, MCOs have been shown to play important roles during their life cycles, such as in fruiting body formation, pigment formation and pathogenicity. Coprophilous fungi, which grow on the dung of herbivores, appear to encode an unexpectedly high number of enzymes capable of at least partly degrading lignin. This study compared the MCO-coding capacity of the coprophilous filamentous ascomycetes Podospora anserina and Sordaria macrospora with closely related non-coprophilous members of the order Sordariales. An increase of MCO genes in coprophilic members of the Sordariales most probably occurred by gene duplication and horizontal gene transfer events.

The Aspergillus niger multicopper oxidase family: analysis and overexpression of laccase-like encoding genes

BACKGROUND

Many filamentous fungal genomes contain complex groups of multicopper oxidase (MCO) coding genes that makes them a good source for new laccases with potential biotechnological interest. A bioinformatics analysis of the Aspergillus niger ATCC 1015 genome resulted in the identification of thirteen MCO genes. Ten of them were cloned and homologously overexpressed.

RESULTS

A bioinformatic analysis of the A. niger ATCC 1015 genome revealed the presence of 13 MCO genes belonging to three different subfamilies on the basis of their phylogenetic relationships: ascomycete laccases, fungal pigment MCOs and fungal ferroxidases. According to in silico amino acid sequence analysis, the putative genes encoding for functional extracellular laccases (mcoA, mcoB, mcoC, mcoD, mcoE, mcoF, mcoG, mcoI, mcoJ and mcoM) were placed under the control of the glaA promoter and overexpressed in A. niger N593. Enzyme activity plate assays with several common laccase substrates showed that all genes are actually expressed and code for active MCOs. Interestingly, expressed enzymes show different substrate specificities. In addition, optimization of fungal pigment MCOs extracellular production was investigated. The performance of the widely used glucoamylase signal sequence (ssGlaA) in McoA secretion was studied. Results obtained suggest that ssGlaA do not yield higher levels of secreted McoA when compared to its native secretion signal. Also, McoB synthesis was investigated using different nitrogen sources in minimal medium liquid cultures. Higher yields of extracellular McoB were achieved with (NH4)2 tartrate.

CONCLUSIONS

Aspergillus niger is a good source of new laccases. The different substrate specificity observed in plate assays makes them interesting to be purified and biochemically compared. The homologous signal sequence of McoA has been shown to be a good choice for its extracellular overexpression. From the nitrogen sources tested (NH4)2 tartrate has been found to be the most appropriate for McoB production in A. niger.

Heterologous laccase production and its role in industrial applications

Laccases are blue multicopper oxidases, catalyzing the oxidation of an array of aromatic substrates concomitantly with the reduction of molecular oxygen to water. These enzymes are implicated in a variety of biological activities. Most of the laccases studied thus far are of fungal origin. The large range of substrates oxidized by laccases has raised interest in using them within different industrial fields, such as pulp delignification, textile dye bleaching, and bioremediation. Laccases secreted from native sources are usually not suitable for large-scale purposes, mainly due to low production yields and high cost of preparation/purification procedures. Heterologous expression may provide higher enzyme yields and may permit to produce laccases with desired properties (such as different substrate specificities, or improved stabilities) for industrial applications. This review surveys researches on heterologous laccase expression focusing on the pivotal role played by recombinant systems towards the development of robust tools for greening modern industry.

LccA, an archaeal laccase secreted as a highly stable glycoprotein into the extracellular medium by Haloferax volcanii

Laccases couple the oxidation of phenolic compounds to the reduction of molecular oxygen and thus span a wide variety of applications. While laccases of eukaryotes and bacteria are well characterized, these enzymes have not been described in archaea. Here, we report the purification and characterization of a laccase (LccA) from the halophilic archaeon Haloferax volcanii. LccA was secreted at high levels into the culture supernatant of a recombinant H. volcanii strain, with peak activity (170 +/- 10 mU.ml(-)(1)) at stationary phase (72 to 80 h). LccA was purified 13-fold to an overall yield of 72% and a specific activity of 29.4 U.mg(-)(1) with an absorbance spectrum typical of blue multicopper oxidases. The mature LccA was processed to expose an N-terminal Ala after the removal of 31 amino acid residues and was glycosylated to 6.9% carbohydrate content. Purified LccA oxidized a variety of organic substrates, including bilirubin, syringaldazine (SGZ), 2,2,-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and dimethoxyphenol (DMP), with DMP oxidation requiring the addition of CuSO(4). Optimal oxidation of ABTS and SGZ was at 45 degrees C and pH 6 and pH 8.4, respectively. The apparent K(m) values for SGZ, bilirubin, and ABTS were 35, 236, and 670 muM, with corresponding k(cat) values of 22, 29, and 10 s(-)(1), respectively. The purified LccA was tolerant of high salt, mixed organosolvents, and high temperatures, with a half-life of inactivation at 50 degrees C of 31.5 h.

Stachybotrys atra BP-A produces alkali-resistant and thermostable cellulases

A cellulose-degrading fungal strain has been isolated from a rotten rag. Morphological characterization and ITS1, 5.8S and ITS2 rDNA sequencing showed that the strain is a new isolate of Stachybotrys atra. The strain secreted high cellulase activity in media supplemented with rice straw. However, cellulases were not produced in glucose-supplemented media. The crude cellulase showed the highest activity on amorphous celluloses such as carboxymethyl cellulose, while activity on crystalline celluloses such as Avicel was lower. The optimal temperature and pH for CMCase activity were 70 degrees C and pH 5 respectively, although a second peak of activity was found at pH 8. Activity was strongly inhibited by Cu(2+), Mn(2+) and Hg(2+). Analysis by SDS-PAGE, isoelectric focusing and zymography showed that the strain secretes a complex cellulase system comprising several enzymes. Most of these enzymes are alkali-resistant CMCases that remained stable at pH 9 and 65 degrees C for at least 1 h. Cellulose binding assays showed notable differences among the CMCases. While some CMCase bands did not bind Avicel, other bands bound to this polymer and were eluted either with NaCl or by boiling with SDS. Analysis by two-dimensional electrophoresis showed that the band eluted by SDS boiling contained at least 4 different polypeptides. The complex set of cellulases produced by the strain, and their activity and stability at alkaline pH and a high temperature indicate that both the isolated strain and the cellulases identified are good candidates for biotechnological applications involving cellulose modification.

Wooden art objects and the museum environment: identification and biodegradative characteristics of isolated microflora

AIMS

The identification of culturable microbial communities on wooden art objects and from indoor air, and the analysis of their biodegradative properties.

METHODS AND RESULTS

Common and newly-developed agar media were used for the isolation of fungal and bacterial microflora. The identification was carried out by traditional methods and by the sequencing of 16S or 18S rDNA PCR products. Different plate assays were employed to screen the lignolytic and cellulolytic activities of the isolated microflora. Interesting bacteria were isolated from art objects even though the fungi were the principal contaminants of art works. Various fungal and bacterial species exhibited their lignolytic and cellulolytic activity by the decolorization of Remazol Brilliant Blue R, Phenol Red, Azure B and Ostazin Brilliant Red H-3B.

CONCLUSIONS

The microbial communities on wooden art objects exposed in an indoor environment were identified. The study showed the biodegradative power of many microorganisms, and new data were added to this field barely investigated.

SIGNIFICANCE AND IMPACT OF THE STUDY

By the development of new culture media and the evaluation of different biodegradative plate assays, a strategy for the analysis of microflora in wooden art objects was established. Several aspects of the study could be also exploited for biotechnology applications.