Genetically essential and nonessential alpha-tubulin genes specify functionally interchangeable proteins.

Microtubules in yeast are essential components of the mitotic and meiotic spindles and are essential for nuclear movement during cell division and mating. The relative importance in these processes of the two divergent alpha-tubulin genes of the budding yeast Saccharomyces cerevisiae, TUB1 and TUB3, was examined through the construction of null mutations and by increasing their copy number on chromosomes and on plasmids. Experiments with null alleles of TUB3 showed that TUB3 was not essential for mitosis, meiosis, or mating. Null alleles of TUB3, however, did cause several phenotypes, including hypersensitivity to the antimicrotubule drug benomyl and poor spore viability. On the other hand, the TUB1 gene was essential for growth of normal haploid cells. Even in diploids heterozygous for a TUB1 null allele, several dominant phenotypes were evident, including slow growth and poor sporulation. This functional difference between the two genes is apparently due to different levels of expression, because extra copies of either gene could suppress the defects caused by a null mutation in the other. We conclude that in spite of the 10% divergence between the products of the two genes, there is no essential qualitative functional difference between them.