Differential expressions of essential and nonessential alpha-tubulin genes in Schizosaccharomyces pombe.

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The fission yeast Schizosaccharomyces pombe has two alpha-tubulin genes and one beta-tubulin gene. Gene disruption experiments showed that the alpha 1-tubulin gene (NDA2) is essential whereas the alpha 2 gene is dispensable. The alpha 2-disrupted cells missing alpha 2 transcript and alpha 2 polypeptide could grow and sporulate normally. The alpha 2 gene, however, was expressed in the wild type and the alpha 1 mutant. Alpha 2-Tubulin was distinguished as an electrophoretic band and was assembled into microtubules. The alpha 2-disrupted cells had an increased sensitivity to an antimicrotubule drug thiabendazole, and the alpha 1(cold-sensitive [cs]) alpha 2 (disrupted) cells became not only cs but also temperature sensitive. Northern blot experiments indicated that alpha 2 transcription was minor and constitutive whereas alpha 1 transcription was major and modulated, depending on the gene copy number of the alpha 2 gene. The amounts of alpha 1 and alpha 2 polypeptides estimated by beta-galactosidase activities of the lacZ-fused genes integrated on the chromosome and by intensities of the electrophoretic bands in crude tubulin fractions, however, were comparable, indicating that alpha 2 tubulin is not a minor subtype. We assume that the cells of Schizosaccharomyces pombe have no excess tubulin pool. alpha 1 mutants would then be blocked in the cell cycle because only half the amount of functional alpha-tubulin required for growth can be produced by the alpha 2 gene. On the other hand, the alpha 2-disrupted cells became viable because the synthesis of alpha 1 tubulin was increased by transcriptional or translational modulation or both. The real cause for essential alpha 1 and dispensable alpha 2 genes seems to be in their regulatory sequences instead of the coding sequences.

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