CLONING, SEQUENCE AND EXPRESSION OF A BETA-TUBULIN-ENCODING GENE IN THE HOMOBASIDIOMYCETE SCHIZOPHYLLUM-COMMUNE

The beta-tubulin (betaTub)-encoding gene (tub-2) of Schizophyllum commune is the first tubulin gene isolated, cloned and sequenced from higher filamentous fungi (homobasidiomycetes). The S. commune tub-2 gene is organized into nine exons and eight introns. The introns vary from 48 to 107 nt in length, and are distributed throughout the gene. The tub-2 exons code for a protein of 445 amino acids (aa), which shows great homology with betaTubs of filamentous ascomycetes, plants, and animals, but less homology with yeasts. The codon usage of tub-2 from S. commune is biased, as it is in most betaTub-encoding genes of filamentous fungi. The S. commune betaTub shows a conserved aa sequence in the C-terminal domain, which is suggested to interact with microtubule-associated proteins in animals. In contrast, the S. commune betaTub deviates from most known betaTubs by having a Cys165 residue, which might be significant for the insensitivity of S. commune haploid strains to the antimicrotubule drug, benomyl. In tub-2 of different haploid strains, sequence polymorphisms occur in the 5' and 3' flanking regions. The expression of tub-2 is high in young mycelium, which has a high number of extending apical cells, but decreases with the aging of the mycelium. No significant difference in the hybridization signal intensity for the tub-2 transcripts was recorded either during intercellular nuclear migration at early mating, or in mycelia with a mutation in the B mating-type gene. The weak signal obtained with the 3' end of tub-2 as a probe in Southern hybridizations, under conditions of low stringency, warrants attention to the possibility that S. commune might have another betaTub-encoding gene highly divergent from tub-2.