ISOLATION OF A SCHIZOSACCHAROMYCES-POMBE RAD21(TS) MUTANT THAT IS ABERRANT IN CHROMOSOME SEGREGATION, MICROTUBULE FUNCTION, DNA-REPAIR AND SENSITIVE TO HYDROXYUREA - POSSIBLE INVOLVEMENT OF RAD21 IN UBIQUITIN-MEDIATED PROTEOLYSIS

The fission yeast DNA repair gene rad21(+) is essential for cell growt h. To investigate the function essential for cell proliferation, we ha ve isolated a temperature-sensitive mutant of the rad21(+) gene. The m utant, rad21-K1, showed abnormal mitosis at the nonpermissive temperat ure. Some cells contained abnormal nuclear structures, such as condens ed chromosomes with short spindles, or chromosomes stretched or unequa lly separated by elongating spindles. Other sells exhibited the displa ced nucleus or a cut-like phenotype. Similar abnormalities were observ ed when the Rad21 protein was depleted from cells. We therefore conclu ded that Rad21 is essential for proper segregation of chromosomes. Mor eover, the rad21-K1 mutant is sensitive not only to UV and gamma-ray i rradiation but to thiabendazole and hydroxyurea, indicating that Rad21 plays important roles in microtubule function, DNA repair, and S phas e function. Tile relation to the microtubule function was further conf irmed by the fact that rad21(+) genetically interacts with tubulin gen es, nda2(+) and nda3(+). Finally the growth of the rad21-K1 mutant was inhibited at the permissive temperature by introduction of another-mu tation in the cut9(+) gene, coding for a component of the 20S cyclosom e/anaphase promoting complex, which is involved in ubiquitin-mediated proteolysis. The results suggest that these diverse functions of Rad2i may br facilitated through ubiquitin-mediated proteolysis.