Postreplication Repair Inhibits CAG · CTG Repeat Expansions in Saccharomyces cerevisiae

Trinucleotide repeats (TNRs) are unique DNA microsatellites that can expand to cause human disease. Recently, Srs2 was identified as a protein that inhibits TNR expansions inSaccharomyces cerevisiae . Here, we demonstrate that Srs2 inhibits CAG · CTG expansions in conjunction with the error-free branch of postreplication repair (PRR). Likesrs2 mutants, expansions are elevated inrad18 andrad5 mutants, as well as the PRR-specific PCNA allelespol30-K164R andpol30-K127/164R . Epistasis analysis indicates that Srs2 acts upstream of these PRR proteins. Also, likesrs2 mutants, thepol30-K127/164R phenotype is specific for expansions, as this allele does not alter mutation rates at dinucleotide repeats, at nonrepeating sequences, or for CAG · CTG repeat contractions. Our results suggest that Srs2 action and PRR processing inhibit TNR expansions. We also investigated the relationship between PRR and Rad27 (Fen1), a well-established inhibitor of TNR expansions that acts at 5′ flaps. Our results indicate that PRR protects against expansions arising from the 3′ terminus, presumably replication slippage events. This work provides the first evidence that CAG · CTG expansions can occur by 3′ slippage, and our results help define PRR as a key cellular mechanism that protects against expansions.