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 G 2/M boundary (G 2 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 G 2 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, p34 cdc2. The activity of p34 cdc2 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 p56 cdc13. We have therefore tested the effect of high level, episomal expression of the cdc13 gene...