Different Mating-Type-Regulated Genes Affect the DNA Repair Defects of Saccharomyces RAD51 RAD52 and RAD55 Mutants

摘要

Saccharomyces cerevisiae cells expressing both >a- and α-mating-type (MAT) genes (termed mating-type heterozygosity) exhibit higher rates of spontaneous recombination and greater radiation resistance than cells expressing only MAT>a or MATα. MAT heterozygosity suppresses recombination defects of four mutations involved in homologous recombination: complete deletions of RAD55 or RAD57, an ATPase-defective Rad51 mutation (rad51-K191R), and a C-terminal truncation of Rad52, rad52-Δ327. We investigated the genetic basis of MAT-dependent suppression of these mutants by deleting genes whose expression is controlled by the Mat>a1-Matα2 repressor and scoring resistance to both campothecin (CPT) and phleomycin. Haploid rad55Δ strains became more damage resistant after deleting genes required for nonhomologous end-joining (NHEJ), a process that is repressed in MAT>a/MATα cells. Surprisingly, NHEJ mutations do not suppress CPT sensitivity of rad51-K191R or rad52-Δ327. However, rad51-K191R is uniquely suppressed by deleting the RME1 gene encoding a repressor of meiosis or its coregulator SIN4; this effect is independent of the meiosis-specific homolog, Dmc1. Sensitivity of rad52-Δ327 to CPT was unexpectedly increased by the MAT>a/MATα-repressed gene YGL193C, emphasizing the complex ways in which MAT regulates homologous recombination. The rad52-Δ327 mutation is suppressed by deleting the prolyl isomerase Fpr3, which is not MAT regulated. rad55Δ is also suppressed by deletion of PST2 and/or YBR052C (RFS1, rad55 suppressor), two members of a three-gene family of flavodoxin-fold proteins that associate in a nonrandom fashion with chromatin. All three recombination-defective mutations are made more sensitive by deletions of Rad6 and of the histone deacetylases Rpd3 and Ume6, although these mutations are not themselves CPT or phleomycin sensitive.