Regulation of theSaccharomyces cerevisiae Srs2 helicase during the mitotic cell cycle,meiosis and after irradiation

The expression of theSRS2 gene, which encodes a DNA helicase involved in DNA repair inSaccharomyces cerevisiae, was studied using anSRS2-lacZ fusion integrated at the chromosomalSRS2 locus. It is shown here that this gene is expressed at a low level and is tightly regulated. It is cell-cycle regulated, with induction probably being coordinated with that of the DNA-synthesis genes, which are transcribed at the G1-S boundary. It is also induced by DNA-damaging agents, but only during the G2 phase of the cell cycle; this distinguishes it from a number of other repair genes, which are inducible throughout the cycle. During meiosis, the expression ofSRS2 rises at a time nearly coincident with commitment to recombination. Sincesrs2 null mutants are radiation sensitive essentially when treated in G1, the mitotic regulation pattern described here leads us to postulate that either secondary regulatory events limit Srs2 activity to G1 cells or Srs2 functions in a repair mechanism associated with replication.