Fungal laccases (p-diphenol:oxygen oxidoreductase; EC 1.10.3.2) are multi-copper-containing oxidases that catalyse the oxidation of a great variety of phenolic compounds and aromatic amines through simultaneous reduction of molecular oxygen to water. Fungi generally produce several laccase isoenzymes encoded by complex multi-gene families. The Pleurotus ostreatus genome encodes 11 putative laccase coding genes, and only six different laccase isoenzymes have been isolated and characterised so far. Laccase expression was found to be regulated by culture conditions and developmental stages even if the redundancy of these genes still raises the question about their respective functions in vivo. In this context, laccase transcript profiling analysis has been used to unravel the physiological role played by the different isoforms produced by P. ostreatus. Even if reported results depict a complex picture of the transcriptional responses exhibited by the analysed laccase genes, they were allowed to speculate on the isoform role in vivo. Among the produced laccases, LACC10 (POXC) seems to play a major role during vegetative growth, since its transcription is downregulated when the fungus starts the fructification process. Furthermore, a new tessera has been added to the puzzling mosaic of the heterodimeric laccase LACC2 (POXA3). LACC2 small subunit seems to play an additional physiological role during fructification, beside that of LACC2 complex activation/stabilisation.

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