Isolation and Genetic Analysis of Extragenic Suppressors of the Hyper-Deletion Phenotype of the Saccharomyces Cerevisiae Hpr1δ Mutation

The HPR1 gene of Saccharomyces cerevisae is involved in maintaining low levels of deletions between DNA repeats. To understand how deletions initiate in the absence of the Hpr1 protein and the mechanisms of recombination leading to deletions in S. cerevisiae, we have isolated mutations as suppressors of the hyper-deletion phenotype of the hpr1δ mutation. The mutations defined five different genes called HRS for hyper-recombination suppression. They suppress the hyper-deletion phenotype of hpr1δ strains for three direct repeat systems tested. The mutations eliminated the hyper-deletion phenotype of hpr1δ strains either completely (hrs1-1 and hrs2-1) or significantly (hrs3-1, hrs4-1 and hrs5-1). None of the mutations has a clear effect on the levels of spontaneous and double-strand break-induced deletions. Among other characteristics we have found are the following: (1) one mutation, hrs1-1, reduces the frequency of deletions in rad52-1 strains 20-fold, suggesting that the HRS1 gene is involved in the formation of RAD52-independent deletions; (2) the hrs2-1 hpr1δ mutant is sensitive to methyl-methane-sulfonate and the single mutants hpr1δ and hrs2-1 are resistant, which suggests that the HPR1 and HRS2 proteins may have redundant DNA repair functions; (3) the hrs4-1 mutation confers a hyper-mutator phenotype and (4) the phenotype of lack of activation of gene expression observed in hpr1δ strains is only partially suppressed by the hrs2-1 mutation, which suggests that the possible functions of the Hpr1 protein in gene expression and recombination repair can be separated. We discuss the possible relationship between the HPR1 and the HRS genes and their involvement in initiation of the events responsible for deletion formation.     

hpr1Δ Affects Ribosomal DNA Recombination and Cell Life Span in Saccharomyces cerevisiae

Multiple genetic pathways have been shown to regulate life span and aging in the yeast Saccharomyces cerevisiae. Here we show that loss of a component of the RNA polymerase II complex, Hpr1p, results in a decreased life span. Although hpr1Delta mutants have an increased rate of recombination within the ribosomal DNA (rDNA) array, this is not accompanied by an increase in extrachromosomal rDNA circles (ERCs). Analyses of mutants that affect replication of the rDNA array and suppressors that reverse the phenotypes of the hpr1Delta mutant show that the reduced life span is associated with increased genomic instability but not with increased ERC formation. The hpr1Delta mutant acts in a pathway distinct from previously described mutants that reduce life span.