Characterization of null mutants of the RAD55 gene of Saccharomyces cerevisiae: effects of temperature, osmotic strength and mating type.

RAD55 belongs to a group of genes required for resistance to ionizing radiation, RAD50-RAD57, which are thought to define a pathway of recombinational repair. Since all four alleles of RAD55 are temperature conditional (cold sensitive) for their radiation phenotype, we investigated the phenotype produced by null mutations in the RAD55 gene, constructed in vitro and transplaced to the yeast chromosome. The X-ray sensitivity of these null mutant strains was surprisingly suppressed by increased temperature, osmotic strength of the growth medium and heterozygosity at the mating-type locus. These first two properties, temperature conditionality and osmotic remediability, are commonly associated with missense mutations; these rad55 null mutants are unique in that they exhibit these properties although the mutant gene cannot be expressed. X-ray-induced mitotic recombination was also cold sensitive in rad55 mutant diploids. Although mitotic growth was unaffected in these strains, meiosis was a lethal event at both high and low temperatures. Whereas the phenotype of rad55 null mutants is consistent with a role of RAD55 in recombination and recombinational repair, there is evidence for considerable RAD55-independent recombination, at least in mitotic cells, which is influenced by temperature and MAT. We discuss models for the role of RAD55 in recombination to explain the unusual properties of rad55 mutants.