Differential repair of UV damage in rad mutants of Saccharomyces cerevisiae: a possible function of G2 arrest upon UV irradiation.

After UV irradiation, the transcriptionally active MAT alpha locus in Saccharomyces cerevisiae is preferentially repaired compared with the inactive HML alpha locus. The effect of rad mutations from three different epistasis groups on differential repair was investigated. Three mutants, rad9, rad16, and rad24, were impaired in the removal of UV dimers from the inactive HML alpha locus, whereas they had generally normal repair of the active MAT alpha locus. Since RAD9 is necessary for G2 arrest after UV irradiation, we propose that the G2 stage plays a role in making the dimers accessible for repair, at least in the repressed HML alpha locus.