Two phylogenetically highly distinct beta-tubulin genes of the basidiomycete Suillus bovinus

Genes tubb1 and tubb2 which encode beta-tubulins 1 and 2, respectively, were characterised from the ectomycorrhizal basidiomycete Suillus bovinus. The two beta-tubulins are surprisingly divergent, with the lowest known sequence identity (60%) in any single fungal species. Comparative analysis showed that beta-tubulin 1 and the intron distribution within the tubb1 gene resemble the other beta-tubulins. beta-Tubulin 2, in contrast, is the most divergent fully described fungal beta-tubulin and the gene contains at least 21 introns, which is the largest amount known for any beta-tubulin gene. Despite this divergence, both genes are constitutively expressed in the functional compartments of the mycorrhizosphere and in pure cultures. Transcription of tubb1 is about 2.4 times higher than that of tubb2; and this difference is also seen at the translation level. Evidence suggested that phosphorylation may be the main post-translational modification of both beta-tubulins. The putative GTP-binding site residues of beta-tubulin 1 match crystallised pig beta-tubulin residues, while five of the nine differences in beta-tubulin 2 match the pig alpha-tubulin GTP-site, suggesting the presence of adaptive sequence evolution. In a Bayesian analysis, beta-tubulin 1 joins the other basidiomycete sequences, while beta-tubulin 2 loosely associates with the group of divergent ascomycete sequences without any clear relative among the known full-length fungal beta-tubulin sequences.