Effect of B-type cyclin over-expression on radiation-induced mitotic delay in the fission yeast

Exposure to ionizing radiation temporarily blocks eukaryotic cell cycle progression at the G2/M boundary (G2 delay). The delay probably provides time for repair of DNA damage before chromosome segregation and is thus an active response, indicative of a checkpoint control function. Transition from G2 into mitosis is normally controlled by the activity of a cyclin-dependent kinase, cdc2 in the fission yeast (Schizosaccharomyces pombe). Genetic and cell kinetic evidence suggest that irradiation may impose mitotic delay by inactivation of the cdc2 product, p34cdc2. The activity of p34cdc2 in G2 is regulated by phosphorylation and association with a B-type cyclin, the product of the cdc13 gene, p56cdc13. Previous work does not support a major role for changes in phosphorylation of p34cdc2 in the induction of mitotic delay. Alternatively the kinase may be regulated by changes in the activity/availability of p56cdc13. We have therefore tested the effect of high level, episomal expression of the cdc13 gene on the induction of mitotic delay. No influence of this procedure on the duration of delay was detected, either in a wild-type cell cycle background, or the mutants wee1-50 and cdc2-3w, which show abnormal phosphorylation of p34cdc2.