stf1: a new suppressor of the mitotic control gene, cdc25, in Schizosaccharomyces pombe.

A novel element in the mitotic control, stf1, has been identified genetically by its ability to rescue cdc25-22 as well as a gene disruption of cdc25. This is the first phenotypically non-wee mutation shown to do so. stf1-1 functions additively with cdc2-1w, cdc2-3w, or wee1-6 to rescue cdc25. The available data are consistent with the wild-type gene product operating either on the same pathway as cdc25 or to stimulate cdc2 by a pathway independent of cdc25 or wee1. The stf1 gene has been cloned and sequenced and encodes a putative protein of 50-65 kD, depending on whether a potential intron is present. It is a novel protein with no homology detected in the current data bases. When challenged with hydroxyurea, stf1-1 acts additively with cdc2-3w in rescuing cdc25 mutants and in allowing mitosis to occur without DNA synthesis. It does not appear to play a role in the nutritional sensing pathway nor in the pathway mediating radiation-induced G2 delay.