However, no information as to the biochemical functions of the RAD and MRE encoded products is currently available.The human homologs of RAD and MRE genes have been identified. At the times indicated, ml of the reaction mixture was withdrawn, mixed with SDS, and then run in a. C, the gels in A and B were subjected to image analysis to obtain data points for a graphical representation of the results.The gel was dried and subjected to autoradiography.At the times indicated, a ml portion of the reaction mixture was withdrawn and mixed with an equal volume of loading buffer containing SDS.agarose gel, which was stained with ethidium bromide and photographed. For instance, at the reaction end point of min, only about of the supercoiled DNA had been incised, as compared with about incision of the circular ssDNA by min. Interestingly, the nuclease function was dependent on manganese, which could not be at all substituted by magnesium. Also, labeling of the nucleolytic product by polynucleotide kinase was stimulated markedly by prior phosphatase treatment of the digested DNA, indicating the presence of a phosphate. Symington, personal communication.Trujillo, ShyngShiou F.Biol. Chem. doi. jbc. Access the most updated version of this article at http:www.jbc.orgcontent When a correction for this article is posted Click here to choose from all of JBCs email alerts This article cites references, of which can be accessed free at http:www.jbc.orgcontent.full.htmlreflist Downloadedfromhttp: www.jbc.orgbyguestonSeptember, Although BRCA functions to help the cell repair doublestranded DNA breaks, the function of BRCA remains enigmatic.Here, we develop a human genetic system to study the role of BRCA in oxidative DNA damage.We show that human cancer cells containing mutated BRCA are hypersensitive to ionizing radiation.This hypersensitivity can be reversed by the expression of forms of BRCA that are not growth suppressing.Reversal of hypersensitivity requires the ring finger of BRCA, its transactivation domain, and its BRCT domain.Lastly, we show that unlike BRCA, BRCA does not function in the repair of doublestranded DNA breaks.Instead, it functions in transcriptioncoupled DNA repair. TCR ability correlated with radioresistance as cells containing BRCA showed both increased TCR and radioresistance, whereas cells without BRCA showed decreased TCR and radiosensitivity.These findings give physiologic significance to the interaction of BRCA with the basal transcription machinery.BRCA and BRCA, the breast cancer and genes, are responsible for over of hereditary breast cancers. This article must therefore be hereby marked advertisement in accordance with U.The second finding is that cells from BRCA mice have a defect in transcriptioncoupled DNA repair, implying that BRCA may be involved in DNA repair andor the stress response of the cell.Despite these suggestive findings in the mouse system, there are such large differences in mouse and human BRCA biology that it is unclear whether the DNA repair function of mouse BRCA is applicable to human BRCA.Mouse BRCA is only homologous to human BRCA, and BRCA appears to function differently in the two systems.Although BRCA has been shown to be required for cellular proliferation during mouse development, BRCA has been shown to be a powerful growth suppressor in both yeast and human systems, mice carrying homozygous BRCA mutations die early in gestation.