Isolation of a novel type of mutation in the mitotic control of Schizosaccharomyces pombe whose phenotypic expression is dependent on the genetic background and nutritional environment.

The major cell cycle control in the fission yeast Schizosaccharomyces pombe acts at entry to mitosis, and involves three previously identified genes cdc2, cdc25 and wee1. The presence of a wee1 mutation phenotypically suppresses cdc25 mutations. This paper describes the isolation and subsequent analysis of a strain in which the suppression is reversed by the presence of a new mutation, designated win1.1. The mutation causes a slight increase in cell size at division in most genetic backgrounds. However, when combined with a wee1 mutation and cdc25.22, the win1.1 mutation interacts strongly to generate a novel phenotype: cells are phenotypically cdc during growth on minimal medium but cdc+ when cultured on complex medium. The win1 locus is unlinked to previously identified genes involved in mitosis.