Homologous recombination is required for the viability of rad27 mutants.

The RAD27/RTH1 gene of Saccharomyces cerevisiae encodes a structural and functional homolog of the 5'-3' exonuclease function of Escherichia coli DNA polymerase I. Four alleles of RAD27 were recovered in a screen for hyper-recombination, a phenotype also displayed by polA mutants of E.coli. All four rad27 mutants showed similar high levels of mitotic recombination, but varied in their growth rate at various temperatures, and sensitivity to the DNA damaging agent methyl methane sulfonate. Dependence of viability of rad27 strains on recombination was determined by crossing a strain containing a null allele of RAD27 to strains containing a mutation in either the RAD1, RAD50, RAD51, RAD52, RAD54, RAD55, RAD57, MRE11, XRS2 or RAD59 gene. In no case were viable spore products recovered that contained both mutations. Elimination of the non-homologous end-joining pathway did not affect the viability of a rad27 strain. This suggests that lesions generated in the absence of RAD27 must be processed by homologous recombination.