Abstract
DNA Double-Strand-Breaks (DSBs) are exceedingly deleterious chromosomal lesions. The failure to repair can lead to mutations, which could eventually result in genomic instability (lr cancer in humans. A DSB can be repaired by gene conversion, and DS.B·induced gene conversion in MAT switching in Saccharomyces cerevisiae allows us to elucidate the origins of break-induced mutations and to study the DNA repair mechanisms. When a site specific DSB at HO cut site within MAT locus is induced, the broken DNA can be repaired by homologous recombination as a dominant repair mechanism. For this experiment, genes that are involved in DNA synthesis and post-replication repairs were deleted in the yeast strain, Spontaneous mutation rates and mutation rates of MAT switching in mlhl A, msh6d, rev 3d and pol32d were closely observed to learn the rate by which DNA fails to repair correctly. We examined types of mutation that arose during gene conversion in order to understand the DNA repair mechanisms. We observed a 241-fold increase in the MAT switching mutation rate in a wild type strain compared to its spontaneous mutation rate. We found that mlhld and msh6d did not significantly increase the mutation rate of MAT switching; additionally, rev3!.l and pol32d did not decrease the mutation rate of MAT switching as we originally hypothesized. We concluded that MLHJ, MSH6, REV3 and POL32 play non-essential roles in DSB-induced gene conversion. To further our study of DNA repair mechanisms, we are continuing our experiment with 3 PCNA mutants: pol30· KJ27R, pol30-KI64R and pol30·KI27RI KI64R.