Links between replication and recombination in Saccharomyces cerevisiae: a hypersensitive requirement for homologous recombination in the absence of Rad27 activity.

The RAD27 gene of Saccharomyces cerevisiae encodes a 5'-3' flap exo/endonuclease, which plays an important role during DNA replication for Okazaki fragment maturation. Genetic studies have shown that RAD27 is not essential for growth, although rad27 Delta mutants are temperature sensitive. Moreover, they exhibit increased sensitivity to alkylating agents, enhanced spontaneous recombination, and repetitive DNA instability. The conditional lethality conferred by the rad27 Delta mutation indicates that other nuclease(s) can compensate for the absence of Rad27. Indeed, biochemical and genetical analyses indicate that Okazaki fragment processing can be assured by other enzymatic activities or by alternative pathways such as homologous recombination. Here we present the results of a screen that makes use of a synthetic lethality assay to identify functions required for the survival of rad27 Delta strains. Altogether, we confirm that all genes of the Rad52 recombinational repair pathway are required for the survival of rad27 Delta strains at both permissive (23 degrees C) and semipermissive (30 degrees C) temperatures for growth. We also find that several point mutations that confer weaker phenotypes in mitotic than in meiotic cells (rad50S, mre11s) and additional gene deletions (com1/sae2, srs2) exhibit synthetic lethality with rad27 Delta and that rad59 Delta exhibits synergistic effects with rad27 Delta. This and previous studies indicate that homologous recombination is the primary, but not only, pathway that functions to bypass the replication defects that arise in the absence of the Rad27 protein.