Abstract
Cochliobolus heterostrophus Tub1 described here is the first β-tubulin gene characterized from a naturally occurring benomyl-resistant ascomycete plant pathogen. The gene encodes a protein of 447 amino acids. The coding region of Tub1 is interrupted by three introns, of 116, 55, and 56 nt, situated after codons 4, 12, and 53, respectively. As a result of the preference for pyrimidines in the third position of the codons when a choice exists between purines and pyrimidines, codon usage in the Tub1 gene is biased. Tub1 shows high homology with β-tubulin genes of other ascomycete species. However, Tub1 is exceptional in having Tyr167, compared with Phe167, possessed by β-tubulin genes of other ascomycetes sequenced thus far. The Tyr167 residue has been associated with benomyl resistance in other organisms. In contrast, all other benomyl-implicated residues of Tub1 correspond to sensitivity. Based on these results, we suggest that benomyl resistance in the fungus probably is attributed to Tyr167.
