DESTABILIZATION OF YEAST MICRO-SATELLITE AND MINISATELLITE DNA-SEQUENCES BY MUTATIONS AFFECTING A NUCLEASE INVOLVED IN OKAZAKI FRAGMENT PROCESSING (RAD27) AND DNA-POLYMERASE-DELTA (POL3-T)

We examined the effects of mutations in the Saccharomyces cerevisiae R AD27 (encoding a nuclease involved in the processing of Okazaki fragme nts) and POL3 (encoding DNA polymerase delta) genes on the stability o f a minisatellite sequence (20-bp repeats) and microsatellites (1- to 8-bp repeat units). Both the rad27 and pol3-t mutations destabilized b oth classes of repeats, although the types of tract alterations observ ed in the two mutant strains were different. The tract alterations obs erved in rad27 strains were primarily additions, and those observed in pol3-t strains were primarily deletions. Measurements of the rates of repetitive tract alterations in strains with both rad27 and pol3-t in dicated that the stimulation of microsatellite instability by rad27 wa s reduced by the effects of the pol3-t mutation. We also found that ra d27 and pol3-01 (an allele carrying a mutation in the ''proofreading'' exonuclease domain of DNA polymerase delta) mutations were synthetica lly lethal.