Cdc2–cyclin B kinase activity links Crb2 and Rqh1–topoisomerase III

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Abstract
The availability of a sister chromatid, and thus the cell cycle phase in which DNA double-strand breaks (DSBs) occur, influences the choice between homologous recombination (HR) or nonhomologous end joining (NHEJ). The sequential activation and destruction of CDK–cyclin activities controls progression through the cell cycle. Here we provide evidence that the major Schizosaccharomyces pombe CDK, Cdc2–cyclin B, influences recombinational repair of radiation-induced DSBs during the G2 phase at two distinct stages. At an early stage in HR, a defect in Cdc2 kinase activity, which is caused by a single amino acid change in cyclin B, affects the formation of Rhp51 (Rad51sp) foci in response to ionizing radiation in a process that is redundant with the function of Rad50. At a late stage in HR, low Cdc2–cyclin B activity prevents the proper regulation of topoisomerase III (Top3) function, disrupting a recombination step that occurs after the assembly of Rhp51 foci. This effect of Cdc2–cyclin B kinase on Top3 function is mediated by the BRCT-domain-containing checkpoint protein Crb2, thus linking checkpoint proteins directly with recombinational repair in G2. Our data suggest a model in which CDK activity links processing of recombination intermediates to cell cycle progression via checkpoint proteins.