Multiple Multi-Copper Oxidase Gene Families in Basidiomycetes – What for?

Abstract

Genome analyses revealed in various basidiomycetes the existence of multiple genes for blue multi-copper oxidases (MCOs). Whole genomes are now available from saprotrophs, white rot and brown rot species, plant and animal pathogens and ectomycorrhizal species. Total numbers (from 1 to 17) and types of mco genes differ between analyzed species with no easy to recognize connection of gene distribution to fungal life styles. Types of mco genes might be present in one and absent in another fungus. Distinct types of genes have been multiplied at speciation in different organisms. Phylogenetic analysis defined different subfamilies of laccases sensu stricto (specific to Agaricomycetes), classical Fe2+-oxidizing Fet3-like ferroxidases, potential ferroxidases/laccases exhibiting either one or both of these enzymatic functions, enzymes clustering with pigment MCOs and putative ascorbate oxidases. Biochemically best described are laccases sensu stricto due to their proposed roles in degradation of wood, straw and plant litter and due to the large interest in these enzymes in biotechnology. However, biological functions of laccases and other MCOs are generally little addressed. Functions in substrate degradation, symbiontic and pathogenic intercations, development, pigmentation and copper homeostasis have been put forward. Evidences for biological functions are in most instances rather circumstantial by correlations of expression. Multiple factors impede research on biological functions such as difficulties of defining suitable biological systems for molecular research, the broad and overlapping substrate spectrum multi-copper oxidases usually possess, the low existent knowledge on their natural substrates, difficulties imposed by low expression or expression of multiple enzymes, and difficulties in expressing enzymes heterologously.

Differential Regulation by Organic Compounds and Heavy Metals of Multiple Laccase Genes in the Aquatic Hyphomycete Clavariopsis aquatica

“…The use of this primer pair and cloning and sequencing of the resulting PCR products allowed an extension of the gene fragments lcc1 and lcc2 already identified in reference 49, as well as the identification of 3 additional putative laccase gene fragments (lcc3, lcc4, and lcc5). The deduced partial amino acid sequences of lcc1, lcc2, lcc3, lcc4, and lcc5 cover a span of 334 (lcc4) to 382 amino acids (lcc2) and perfectly match the fungal laccase signature sequences L1 (H-W-H-G-X 9 -D-G-X 5 (32), hence indicating that C. aquatica lcc1 to lcc5 represent laccases sensu stricto (13,31). The identities between the amino acid sequences corresponding to cbr I to III of the 5 putative laccase genes are rather low and range from 21 to 44% for pairwise comparisons of lcc2 and lcc3 and of lcc1 and lcc5 (Table 2).…”

“…The occurrence of multiple laccase genes in one organism and their differential regulation in response to numerous external factors and the developmental stage are widely known from terrestrial asco-and basidiomycetes (10,18,31,33). The present study on C. aquatica thus expands the knowledge to ascomycete-related freshwater fungi.…”

“…2C and D (18). Maximal laccase expression during the earlier growth stages of basidiomycetes has been attributed to a role in lignin bioconversion expected to be required during the colonization of lignocellulosic substrates, whereas maximal laccase expression during the stationary phase has been linked to laccase functions in fungal morphogenesis of, e.g., fruiting bodies (13,18,31). Different functions of the C. aquatica laccases are also indicated by the observation that lcc4 obviously accounts for the extracellular laccase activity recorded in the culture media during the trophophase to a major extent (see Table S3 in the supplemental material).…”

“…Natural functions of ascomycete as well as basidiomycete laccases are not fully understood and potentially include, among others, lignocellulose degradation and oxidation of toxic phenolic compounds (31,56). Lignin solubilization has been reported for diverse freshwater ascomycetes, but there is only scarce information regarding the abilities of AQHs to act on lignin, and extensive lignin degradation comparable to that caused by terrestrial white rot fungi is not known from AQHs (9,30).…”

“…They couple the one-electron oxidation of numerous substrates to the reduction of molecular oxygen to form water (4,18). Various functions have been attributed to fungal laccases, for example, the degradation of lignin and many xenobiotic compounds, morphogenesis, stress defense, and host-pathogen interactions (4,18,31,37).…”