Many fungi produce several isoenzymes endowed with slightly different catalytic properties, being the physiological significance of this multiplicity still unknown. Likewise, this feature is shared by fungi belonging to Pleurotus genera concerning the laccase gene family (1). As a fact, four laccase gene members have already been isolated in Pleurotus sajior-caju (2), two in Pleurotus eryngii (3), and seven in Pleurotus ostreatus (4). Moreover the existence of a “laccase subfamily” consisting of three out of seven members has been postulated in the latter fungus, on the basis of sequence similarity, and intron-exon structure. Five P. ostreatus encoded isoenzymes, representing a variegated group of laccases endowed with peculiar properties, have been purified from culture broth and fully characterized (4). More recently, the identification of a new laccase isoenzyme from its fruiting body has been achieved.
A more complex multicopper oxidase family has been disclosed since the release of the white-rot fungus Pleurotus ostreatus genome (http://www.jgi.doe.gov/sequencing/why/50009.html). The automatic annotation of the retrieved gene models was analyzed and improved, thanks to the information available on the structure-function of laccases. Clustering of laccase genes was evaluated along with their belonging to the already defined subfamilies (4). Genes encoding heterodimeric laccase, the large (5) and the small subunit (6), lie in the same chromosome. A cluster of seven out of twelve laccase genes in sub-telomeric region of chromosome 6 has been recently mapped by Perez and colleagues (7), persuading the authors that this location could have an evolutionary significance permitting the fungus to adapt rapidly to new lignocellulosic substrates. The presence of all the conserved residues characterizing laccases sequences (8) was checked in all the deduced proteins. These analyses allowed to understand that ten out of twelve deduced proteins correspond to laccases in sensu stricto
