Sequence polymorphism and molecular characterization of laccase genes of the conifer pathogen Heterobasidion annosum.

The oxidizing enzyme laccase produced by many fungi is generally considered to be active in the biodegradation of lignin, a major plant cell wall component highly resistant to microbial attack. The enzyme is secreted at high levels by the P-type of the highly aggressive pathogen Heterobasidion annosum, but at much lower levels by the S-type which correlated with their varying wood decay capability. To investigate the evolutionary relationship between laccase genes of the different H. annosum types from several geographical regions we have compared the nucleotide sequence of the laccase gene from 32 different isolates of the fungus together with two other Asian isolates (H. araucariae, H. insulare). In addition to nucleotide sequence assessment, we have also cloned, characterized and analysed the partial sequences of the laccase gene from the homokaryotic (FSE-7: S-type; Sä16-4: P-type) and heterokaryotic (Faf-8: F-type) strains of H. annosum. Using degenerate primers, two distinct laccase gene fragments of 1.64 and 2 kb were amplified from the genomic DNA of this fungus. DNA sequence analyses showed that the 1.64 kb laccase fragment in all three H. annosum types shared significant homology (86-96%). But comparative analyses of the 1.64 and 2 kb laccase gene fragment revealed only 48% nucleotide sequence similarity. Using the cDNA sequence information, exon regions were predicted and this revealed that about nine small introns interrupted the genomic DNA. Southern hybridization analysis indicated a single copy of the gene in the homokaryotic S-type (FSE-7) examined but presence of double bands in the homokaryotic and heterokaryotic P-type strains of the fungus suggest the existence of two laccase genes. Northern analyses revealed that the gene is constitutively expressed but appear to be enhanced several fold with the addition of ferulic acid or oxalic acid. Alignments of the nucleotide sequences and phylogenetic analyses are presented allowing estimations of evolutionary relationships to be made. These comparisons indicate that laccase gene of P-type is distinct from other Heterobasidion sequences, including the outgroups H. araucariae and H. insulare, while the relationship between the S- and F-groups could not be resolved. Comparative phylogenetic analyses using predicted amino acid sequence also showed strong similarity to the laccases from other basidiomycetes (Pleurotus ostreatus, Phlebia radiata and Trametes versicolor) but least similar to laccases from ascomycete fungi. In addition, the results of McDonald-Kreitman test for possible evidence of selection based on analyses of two exon regions of the H. annosum laccase gene are presented and discussed.