No abstract

Double-strand DNA break (DSB) repair by homologous recombination occurs through the RAD52 pathway in Saccharomyces cerevisiae. Its biological importance is underscored by the conservation of many RAD52 pathway genes, including RAD54, from fungi to humans. We have analyzed the phenotype of mouse RAD54-/- (mRAD54-/-) cells. Consistent with a DSB repair defect, these cells are sensitive to ionizing radiation, mitomycin C, and methyl methanesulfonate, but not to ultraviolet light. Gene targeting experiments demonstrate that homologous recombination in mRAD54-/- cells is reduced compared to wild-type cells. These results imply that, besides DNA end-joining mediated by DNA-dependent protein kinase, homologous recombination contributes to the repair of DSBs in mammalian cells. Furthermore, we show that mRAD54-/- mice are viable and exhibit apparently normal V(D)J and immunoglobulin class-switch recombination. Thus, mRAD54 is not required for the recombination processes that generate functional immunoglobulin and T cell receptor genes.

Transcription‐coupled repair (TCR) is a universal sub‐pathway of the nucleotide excision repair (NER) system that is limited to the transcribed strand of active structural genes. It accomplishes the preferential elimination of transcription‐blocking DNA lesions and permits rapid resumption of the vital process of transcription. A defect in TCR is responsible for the rare hereditary disorder Cockayne syndrome (CS). Recently we found that mutations in the ERCC6 repair gene, encoding a putative helicase, underly the repair defect of CS complementation group B. Here we report the cloning and characterization of the Saccharomyces cerevisiae homolog of CSB/ERCC6, which we designate RAD26. A rad26 disruption mutant appears viable and grows normally, indicating that the gene does not have an essential function. In analogy with CS, preferential repair of UV‐induced cyclobutane pyrimidine dimers in the transcribed strand of the active RBP2 gene is severely impaired. Surprisingly, in contrast to the human CS mutant, yeast RAD26 disruption does not induce any UV‐, cisPt‐ or X‐ray sensitivity, explaining why it was not isolated as a mutant before. Recovery of growth after UV exposure was somewhat delayed in rad26. These findings suggest that TCR in lower eukaryotes is not very important for cell survival and that the global genome repair pathway of NER is the major determinant of cellular resistance to genotoxicity.