Rad51 accumulation at sites of DNA damage and in postreplicative chromatin

Rad51, a eukaryotic RecA homologue, plays a central role in homologous reco mbinational repair of DNA double-strand breaks (DSBs) in yeast and is conse rved from yeast to human. Rad51 shows punctuate nuclear localization in hum an cells, called Rad51 foci, typically during the S phase (Tashiro, S., N. Kotomura, A. Shinohara, K. Tanaka, K. Ueda, and N. Kamada. 1996. Oncogene. 12:2165-2170). However, the topological relationships that exist in human S phase nuclei between Rad51 foci and damaged chromatin have not been studie d thus far. Here, we report on ultraviolet microirradiation experiments of small nuclear areas and on whole cell ultraviolet C (UVC) irradiation exper iments performed with a human fibroblast cell line. Before UV irradiation, nuclear DNA was sensitized by the incorporation of halogenated thymidine an alogues. These experiments demonstrate the redistribution of Rad51 to the s electively damaged, labeled chromatin. Rad51 recruitment takes place from R ad51 foci scattered throughout the nucleus of nonirradiated cells in S phas e. We also demonstrate the preferential association of Rad51 foci with post replicative chromatin in contrast to replicating chromatin using a double l abeling procedure with halogenated thymidine analogues,This finding support s a role of Rad51 in recombinational repair processes of DNA damage present in postreplicative chromatin.