IDENTIFICATION OF NOVEL TEMPERATURE-SENSITIVE LETHAL ALLELES IN ESSENTIAL BETA-TUBULIN AND NONESSENTIAL ALPHA-2-TUBULIN GENES AS FISSION YEAST POLARITY MUTANTS

We have screened for temperature-sensitive (ts) fission yeast mutants with altered polarity (alp1-15). Genetic analysis indicates that alp2 is allelic to atb2 (one of two alpha-tubulin genes) and alp12 to nda3 (the single beta-tubulin gene), atb2(+) is nonessential, and the ts at b2 mutations we have isolated are dominant as expected. We sequenced t wo alleles of ts atb2 and one allele of ts nda3. In the ts atb2 mutant s, the mutated residues (G246D and C356Y) are found at the longitudina l interface between alpha/beta-heterodimers, whereas in ts nda3 the mu tated residue (Y422H) is situated in the domain located on the outer s urface of the microtubule. The ts nda3 mutant is highly sensitive to a ltered gene dosage of atb2(+); overexpression of atb2(+) lowers the re strictive temperature, and, conversely, deletion rescues ts. Phenotypi c analysis shows that contrary to undergoing mitotic arrest with high viability via the spindle assembly checkpoint as expected, ts nda3 mut ants execute cytokinesis and septation and lose viability. Therefore, it appears that the ts nda3 mutant becomes temperature lethal because of irreversible progression through the cell cycle in the absence of a ctivating the spindle assembly checkpoint pathway.