RPD3 (REC3) mutations affect mitotic recombination in Saccharomyces cerevisiae

Prior research identified the recessive rec3-1ts mutation in Saccharomyces cerevisiae which, in homozygous diploid cells, confers a conditional phenotype resulting in reduced levels of spontaneous mitotic recombination and loss of sporulation at the restrictive temperature of 36 degrees C. We found that a 3.4-kb genomic fragment that complements the rec3-1ts/rec3-1ts mutation and which maps to chromosome XIV, is identical to RPD3, a gene encoding a histone de-acetylase. Sporulation is reduced in homozygous diploid strains containing the rec3-1ts allele at 24 degrees C, suggesting that this allele of RPD3 encodes a gene product with a reduced function. Sporulation is abolished in diploid strains homozygous for the rpd3Delta or rec3-1ts alleles, as well as in rpd3Delta/rec3-1ts heteroallelic diploids, at the non-permissive temperature. Acid-phosphatase expression has been shown to be RPD3 dependent. We found that acid-phosphatase activity is greater in diploid strains homozygous for the temperature-sensitive rec3-1ts allele than in RPD3/RPD3 strains and increased further when mutant strains are grown at 36 degrees C. We also tested the rpd3Delta/rpd3Delta strains for their effects on spontaneous mitotic recombination. By assaying a variety of intra- and inter-genic recombination events distributed over three chromosomes, we found that in the majority of cases spontaneous mitotic recombination was reduced in diploid rpd3Delta/rpd3Delta cells (relative to a RPD3/RPD3 control). Finally, although 90% of mitotic recombinant events are initiated in the G1 phase of the growth cycle (i.e., before DNA synthesis) we show that RPD3 is not regulated in a cell-cycle-dependent manner. These data suggest that mitotic recombination, in addition to gene expression, is affected by changes in chromatin architecture mediated by RPD3.