Slm9, a novel nuclear protein involved in mitotic control in fission yeast

In the fission yeast Schizosaccharomyces pombe as in other eukaryotic cells , Cdc2/cyclin B complex is the key regulator of mitosis. Perhaps the most i mportant regulation of Cdc2 is the inhibitory phosphorylation of tyrosine-1 5 that is catalyzed by Wee1 and Mik1. Cdc25 and Pyp3 phosphatases dephospho rylate tyrosine-15 and activate Cdc2. To isolate novel activators of Cdc2 k inase, we screened synthetic lethal mutants in a cdc25-22 background at the permissive temperature (25 degrees). One of the genes, slm9, encodes a nov el protein of 807 amino acids. Slm9 is most similar to Hir2, the histone ge ne regulator in budding yeast. Slm9 protein level is constant and Slm9 is l ocalized to the nucleus throughout the cell cycle. The slm9 disrupstant is delayed at the G(2)-M transition as indicated by cell elongation and analys is of DNA content. Inactivation of Wee1 fully suppressed the cell elongatio n phenotype caused by the slm9 mutation. The slm9 mutant is defective in re covery from G(1) arrest after nitrogen starvation. The slm9 mutant is also UV sensitive, showing a defect in recovery from the cell cycle arrest after UV irradiation.