A FISSION YEAST RCC1-RELATED PROTEIN IS REQUIRED FOR THE MITOSIS TO INTERPHASE TRANSITION

The isolation and characterization of the mutant dcd(ts) (defect in chromatin decondensation) has led to the identification of two conserved proteins required for the re-establishment of the interphase state following the completion of mitosis. The gene that rescues the dcd(ts) mutant encodes a protein similar to the human chromatin binding protein, RCC1. A suppressor of dcd(ts) encodes a protein nearly identical to the human GTP-binding protein, RAN, encoded by the TC4 gene. These results indicate that completion of mitosis is regulated at least in part by a GTPase molecular switch. The gene and suppressor of dcd(ts) are identical to the previously described Schizosaccharomyces pombe genes pim1 (premature initiation of mitosis) and spi1 (suppressor of pim), but the dcd(ts) mutant does not enter mitosis prematurely, a phenotype that has been reported for the pim1-46ts mutant. Based on our studies we propose that the pim1 gene product is required for regulating chromatin condensation with a primary role at the end of mitosis and pleiotropic effects on other aspects of cell behavior.