DIVISION-SITE SELECTION, CELL-SEPARATION, AND FORMATION OF ANUCLEATE MINICELLS IN SCHIZOSACCHAROMYCES-POMBE MUTANTS RESISTANT TO CELL-WALL LYTIC ENZYMES

In most eukaryotic organisms that have cell walls, cell separation or cytokinesis is a degradative enzymatic process. Zn the fission yeast S chizosaccharomyces pombe, it is a post-M-phase event that includes the degradation of part of the cell wall and the primary septum. We descr ibe the isolation of mutants partially defective in cytokinesis by enr ichment of clones resistant to cell-wall lytic enzymes. The mutations confer mycelial morphology (chains of nonseparated cells) and define f our genes. Sep2-SA2 was subjected to detailed genetic and cytological analysis. Its cells frequently form complex septa composed of multiple layers, which appear as twin septa separated by anucleate minicells i f the cell length is extended. This suggests that a polar signal-like mechanism may also operate in S. pombe during division-site selection and sep2(+) takes part in it. Sep2(+) seems to be involved in several cell cycle functions because its mutation can transiently block cell-c ycle progression after nuclear division and provoke a transition from haploidy to diploidy in the double mutant sep2-SA2 cex1-SA2. Cex1-SA2 is another novel mutation which causes cell-length extension.