ISOLATION AND CHARACTERIZATION OF FISSION YEAST SNS MUTANTS DEFECTIVEAT THE MITOSIS-TO-INTERPHASE TRANSITION

pim1-dl(ts) was previously identified in a visual screen for fission l east mutants unable to complete the mitosis-to-interphase transition. pim1(+) encodes the guanine nucleotide exchange factor (GEF) for the s pi1 GTPase. Perturbations of this GTPase system by either mutation or overproduction of its regulatory proteins cause cells to arrest with p ostmitotic condensed chromosomes, an unreplicated genome, and a wide m edial septum. The septation phenotype of pim1-dl(ts) was used as the b asis for a more extensive screen for this novel class of sns (septated , not in S-phase) mutants. Seventeen mutants representing 14 complemen tation groups were isolated. Three strains, sns-A3, sns-A5, and sns-X6 , representing two different alleles, are mutated in the pim1(+) gene. Of the 13 non-pim1(ts) sns complementation groups, 11 showed genetic interactions with the spil GTPase system. The genes mutated in 10 sns strains were synthetically lethal with pim1-d1, and sis sns strains we re hypersensitive to overexpression of one or more of the known compon ents of the spil GTPase system. Epistasis analysis places the action o f the genes mutated in nine of these strains downstream of pim1(+) and the action of one gene upstream of pim1(+). Three strains, sns-A2, sn s-B1, and sns-B9, shelved genetic interaction with the spil GTPase sys tem in every test performed. sns-B1 and sns-B9 are likely to identify downstream targets, whereas sns-A2 is likely to identify upstream regu lators of the spil GTPase system that are required for the mitosis-to- interphase transition.