DOMAINS DETERMINING THE FUNCTIONAL DISTINCTION OF THE FISSION YEAST-CELL CYCLE START MOLECULES RES1 AND RES2

In Schizosaccharomyces pombe the ''start'' of the cell cycle is regula ted by two parallel, functionally overlapping complexes composed of Re s1-Cdc10 and Res2-Cdc10. Res1 and Res2 are structurally very homologou s and are required for the start of the mitotic and meiotic cycle, res pectively. We have addressed the question which parts of the proteins are essential for function and determine the functional specificity. S everal discrete domains in the nonconserved C-terminal region are esse ntial for the mitotic and meiotic start function and determine the fun ctional specificity independently of the structurally conserved motifs at the N-terminal end and in the center. One of these domains in Res2 restricts Res2 to interact only with Rep2. Res2 without this domain b ehaves like a functional chimera having the properties of Res2 and Res 1. Likewise, internally truncated forms of Res1 lacking the centrally located ankyrin repeats and adjacent sequences can partially suppress the meiotic defect in res2(-) cells. These truncated Res1 molecules be have like functional chimeras with the properties of Res1 and Res2.