A NIMA homologue promotes chromatin condensation in fission yeast

Entry into mitosis requires p34(cdc2), which activates downstream mitotic events through phosphorylation of key target proteins. In Aspergillus nidulans, the NIMA protein kinase has been identified as a potential downstream target and plays a role in regulating chromatin condensation at mitosis. nimA- mutants arrest in a state that physically resembles interphase even though p34(cdc2) is fully active. Despite evidence for the existence of NIMA-like activities in a variety of cell types, the only bona fide NIMA homologue that has been identified is the nim-1 gene of Neurospora crassa. We report here the isolation of a fission yeast NIMA homologue, and have designated this gene fin1 and the 83 kDa predicted protein p83(fin1). Overexpression of fin1 promotes premature chromatin condensation from any point in the cell cycle independently of p34(cdc2) function. Like NIMA, p83(fin1) levels fluctuate through the cell cycle, peaking in mitosis and levels are greatly elevated by removal of C-terminal PEST sequences. Deletion of fin1 results in viable but elongated cells, indicative of a cell cycle delay. Genetic analysis has placed this delay in G2 but, unlike in nimA mutants of Aspergillus, p34(cdc2) activation appears to be delayed. Interaction of fin1 mutants with other strains defective in chromatin organisation also support the hypothesis of p83(fin1) playing a role in this process at the onset of mitosis. These data indicate that NIMA-related kinases may be a general feature of the cell cycle and chromatin organisation at mitosis.