ISOLATION AND CHARACTERIZATION OF NEW FISSION YEAST CYTOKINESIS MUTANTS

Schizosaccharomyces pombe is an excellent organism in which to study c ytokinesis as it divides by medial fission using an F-actin contractil e ring. To enhance our understanding of the cell division process, a l arge genetic screen was carried out in which 17 genetic loci essential for cytokinesis were identified, 5 of which are novel. Mutants identi fying three genes, rng3(+), rng4(+), and rng5(+), Mere defective in or ganizing an actin contractile ring. Four mutants defective in septum d eposition, septum initiation defective (sid) 1, sid2, sid3, and sid4, were also identified and characterized. Genetic analyses revealed that the sid mutants display strong negative interactions with the previou sly described septation mutants cdc7-24, cdc11-123, and cdc14-118. The rng5(+), sid2(+), and sid3(+) genes were cloned and shown to encode M yo2p (a myosin heavy chain), a protein kinase related to budding yeast Dbf2p, and Spg1p, a GTP binding protein that is a member of the ras s uperfamily of GTPases, respectively. The ability of Spg1p to promote s eptum formation from any point in the cell cycle depends on the activi ty of Sid4p. In addition, we have characterized a phenotype that has n ot been described previously in cytokinesis mutants, namely the failur e to reorganize actin patches to the medial region of the cell in prep aration for septum formation.