CHARACTERIZATION OF THE HYPERRECOMBINATION PHENOTYPE OF THE pol3-t MUTATION OF Saccharomyces cerevisiae

The DNA polymerase (Pol3p/Cdc2p) allele pol3-t of Saccharomyces cerevisiae has previously been shown to increase the frequency of deletions between short repeats (several base pairs), between homeologous DNA sequences separated by long inverted repeats, and between distant short repeats, increasing the frequency of genomic deletions. We found that the pol3-t mutation increased intrachromosomal recombination events between direct DNA repeats up to 36-fold and interchromosomal recombination 14-fold. The hyperrecombination phenotype of pol3-t was partially dependent on the Rad52p function but much more so on Rad1p. However, in the double-mutant rad1 rad52 , the pol3-t mutation still increased spontaneous intrachromosomal recombination frequencies, suggesting that a Rad1p Rad52p-independent single-strand annealing pathway is involved. UV and -rays were less potent inducers of recombination in the pol3-t mutant, indicating that Pol3p is partly involved in DNA-damage-induced recombination. In contrast, while UV- and -ray-induced intrachromosomal recombination was almost completely abolished in the rad52 or the rad1 rad52 mutant, there was still good induction in those mutants in the pol3-t background, indicating channeling of lesions into the above-mentioned Rad1p Rad52p-independent pathway. Finally, a heterozygous pol3-t/POL3 mutant also showed an increased frequency of deletions and MMS sensitivity at the restrictive temperature, indicating that even a heterozygous polymerase mutation might increase the frequency of genetic instability.