THE DMF1 MID1 GENE IS ESSENTIAL FOR CORRECT POSITIONING OF THE DIVISION SEPTUM IN FISSION YEAST/

Little is known about the mechanisms that establish the position of th e division plane in eukaryotic cells. Wild-type fission yeast cells di vide by forming a septum in the middle of the cell at the end of mitos is. Dmf1 mutants complete mitosis and initiate septum formation, but t he septa that form are positioned at random locations and angles in th e cell, rather than in the middle. We have cloned the dmf1 gene as a s uppressor of the cdc7-24 mutant. The dmf1 mutant is allelic with midi. The gene encodes a novel protein containing a putative nuclear locali zation signal, and a carboxy-terminal PH domain. In wild-type cells, D mf1p is nuclear during interphase, and relocates to form a medial ring at the cell cortex coincident with the onset of mitosis. This relocal ization occurs before formation of the actin ring and is associated wi th increased phosphorylation of Dmf1p. The Dmf1p ring can be formed in the absence of an actin ring, but depends on some of the genes requir ed for actin ring formation. When the septum is completed and the cell s separate, Dmf1p staining is once again nuclear. These data implicate Dmf1p as an important element in assuring correct placement of the di vision septum in Schizosaccharomyces pombe cells.