Gene replacement in the lignin-degrading basidiomycete Phanerochaete chrysosporium

Novel promoter sequence required for manganese regulation of manganese peroxidase isozyme 1 gene expression in Phanerochaete chrysosporium

Manganese peroxidase (MnP) is a major, extracellular component of the lignin-degrading system produced by the wood-rotting basidiomycetous fungus Phanerochaete chrysosporium. The transcription of MnP-encoding genes (mnps) in P. chrysosporium occurs as a secondary metabolic event, triggered by nutrient-nitrogen limitation. In addition, mnp expression occurs only under Mn2+ supplementation. Using a reporter system based on the enhanced green fluorescent protein gene (egfp), we have characterized the P. chrysosporium mnp1 promoter by examining the effects of deletion, replacement, and translocation mutations on mnp1 promoter-directed egfp expression. The 1,528-bp mnp1 promoter fragment drives egfp expression only under Mn2+-sufficient, nitrogen-limiting conditions, as required for endogenous MnP production. However, deletion of a 48-bp fragment, residing 521 bp upstream of the translation start codon in the mnp1 promoter, or replacement of this fragment with an unrelated sequence resulted in egfp expression under nitrogen limitation, both in the absence and presence of exogenous Mn2+. Translocation of the 48-bp fragment to a site 120 bp downstream of its original location resulted in Mn2+-dependent egfp expression under conditions similar to those observed with the wild-type mnp1 promoter. These results suggest that the 48-bp fragment contains at least one Mn2+-responsive cis element. Additional promoter-deletion experiments suggested that the Mn2+ element(s) is located within the 33-bp sequence at the 3' end of the 48-bp fragment. This is the first promoter sequence containing a Mn2+-responsive element(s) to be characterized in any eukaryotic organism.

Hebeloma cylindrosporum- a model species to study ectomycorrhizal symbiosis from gene to ecosystem

The basidiomycete Hebeloma cylindrosporum has been extensively studied with respect to mycorrhiza differentiation and metabolism and also to population dynamics. Its life cycle can be reproduced in vitro and it can be genetically transformed. Combined biochemical, cytological, genetical and molecular approaches led to the characterisation of mutant strains affected in mycorrhiza formation. These studies demonstrated the role of fungal auxin as a signal molecule in mycorrhiza formation and should allow the characterisation of essential fungal genes necessary to achieve a compatible symbiotic interaction. Random sequencing of cDNAs has identified numerous key functional genes which allowed dissection of essential nitrogen assimilation pathways. H. cylindrosporum also proved to be a remarkable model species to uncover the dynamics of natural populations of ectomycorrhizal fungi and the way in which they respond and adapt to anthropogenic disturbance of the forest ecosystem. Although studies on mycorrhiza differentiation and functioning and those on the population dynamics of H. cylindrosporum have been carried out independently, they are likely to converge in a renewed molecular ecophysiology which will envisage how ectomycorrhizal symbiosis functions under varying field conditions. Contents Summary 481 I. Introduction 482 II. Taxonomy, distribution, autecology, and host range of H. cylindrosporum 482 III. The Hebeloma cylindrosporum toolbox 483 IV. Mycorrhiza differentiation 486 V. Nutritional interactions 488 VI. Genetic diversity and dynamics of H. cylindrosporum populations in P. pinaster forest ecosystems 491 VII. Future directions 494 Acknowledgements 494 References 494.

Cloning of Phanerochaete chrysosporium leu2 by complementation of bacterial auxotrophs and transformation of fungal auxotrophs

A Phanerochaete chrysosporium cDNA library was constructed in an expression vector that allows expression in both Escherichia coli and Saccharomyces cerevisiae. This expression vector, lambda YES, contains the lacZ promoter for expression in E. coli and the GAL1 promoter for expression in yeast. A number of genes were cloned by complementation of bacterial amino acid auxotrophs. The cDNA encoding the beta-isopropylmalate dehydrogenase from P. chrysosporium was characterized further. The genomic clone (gleu2) was subsequently isolated and was used successfully as a selectable marker to transform P. chrysosporium auxotrophs for LEU2. Protoplasts for transformation were prepared with readily obtained conidiospores rather than with basidiospores, which were used in previous P. chrysosporium transformation procedures. The method described here allows other genes to be isolated from P. chrysosporium for use as selectable markers.

Recent advances on the molecular genetics of ligninolytic fungi

This review highlights significant recent advances in the molecular genetics of white-rot fungi and identifies areas where information remains sketchy. The development of critical experimental tools such as genetic mapping techniques is described. A major portion of the text focuses on the structure, genomic organization and transcriptional regulation of the genes encoding peroxidases, laccases and glyoxal oxidase. Finally, recent efforts on heterologous expression of lignin-degrading enzymes are discussed.

Quantitation of fungal mRNAs in complex substrates by reverse transcription PCR and its application to Phanerochaete chrysosporium-colonized soil

Thorough analysis of fungi in complex substrates has been hampered by inadequate experimental tools for assessing physiological activity and estimating biomass. We report a method for the quantitative assessment of specific fungal mRNAs in soil. The method was applied to complex gene families of Phanerochaete chrysosporium, a white-rot fungus widely used in studies of organopollutant degradation. Among the genes implicated in pollutant degradation, two closely related lignin peroxidase transcripts were detected in soil. The pattern of lignin peroxidase gene expression was unexpected; certain transcripts abundant in defined cultures were not detected in soil cultures. Transcripts encoding cellobiohydrolases and beta-tubulin were also detected. The method will aid in defining the roles of specific genes in complex biological processes such as organopollutant degradation, developing strategies for strain improvement, and identifying specific fungi in environmental samples.

A novel strategy for the isolation of defined pyrG mutants and the development of a site-specific integration system for Aspergillus awamori

A homologous gene transfer system for Aspergillus awamori for site-specific integration is described, based on two components. First, a defined A. awamori pyrG mutant strain constructed by a selection strategy for gene-replacement in fungi. Second, a vector with a homologous pyrG selection marker containing a defined mutation at a site different from that of the mutations in the pyrG gene of the defined mutant strain. Defined mutation in the A. awamori pyrG gene, isolated from a genomic library by heterologous hybridisation with the A. niger pyrG gene as a probe, were introduced by specifically altering sequences at restriction sites in the coding region of the gene. After transformation of the A. awamori wild-type strain with vectors containing these mutated pyrG genes, and selection for 5-fluoro-orotic acid resistance (5-FOAR), on the average 60% of the 5-FOAR colonies originated from replacement of the wild-type pyrG gene by the mutated pyrG allele. After transformation of a mutant strain, carrying a mutation near the 5' end of the pyrG gene with vectors containing a mutation near the 3' end of the pyrG gene, 35% of the resulting transformants contained one copy of the vector at the pyrG locus.