SUPPRESSION OF A NEW ALLELE OF THE YEAST RAD52 GENE BY OVEREXPRESSIONOF RAD51, MUTATIONS IN SRS2 AND CCR4, OR MATING-TYPE HETEROZYGOSITY

The RAD52 gene of Saccharomyces cerevisiae is involved both in the rec ombinational repair of DNA damage and in mitotic and meiotic recombina tion. A new allele of rad52 has been isolated that has unusual propert ies. Unlike other alleles of rad52, this allele (rad52-20) is partiall y suppressed by an srs2 deletion; srs2 mutations normally act to suppr ess only rad6 and rad18 mutations. In addition, although haploid rad52 -20 strains are very X-ray sensitive, diploids homozygous for this all ele are only slightly X-ray sensitive and undergo normal meiosis and m eiotic recombination. Because rad52-20 diploids homozygous for mating type are very X-ray sensitive, mating-type heterozygosity is acting to suppress rad52-20. Mating-type heterozygosity suppresses this allele even in haploids, because sir mutations, which result in expression of the normally silent mating-type cassettes, were identified among the extragenic revertants of rad52-20. A new allele of srs2 and alleles of the transcriptional regulatory genes ccr4 and caf1 were among the oth er extragenic revertants of rad52-20. Because other researchers have s hown that the RAD51 and RAD52 proteins interact, RAD51 on a high copy number plasmid was tested and found to suppress the rad52-20 allele, b ut RAD54, 55 and 57 did not suppress. The RAD51 plasmid did not suppre ss rad52-1. The rad52-20 allele may encode a protein that has low affi nity binding to the RAD51 protein. To test whether the selected revert ants suppressed rad52-20 by elevating the expression of RAD51, an inte grated RAD51-lacZ fusion was genetically crossed into each revertant. Because none of the revertants increased the level of RAD51-lacZ, the revertants must exert their effect by one or more mechanisms that are not mediated by RAD51.