A Defined Human System That Supports Bidirectional Mismatch-Provoked Excision

Mismatch-provoked excision directed by a strand break located 3′ or 5′ to the mispair has been reconstituted using purified human proteins. While MutSα, EXOI, and RPA are sufficient to support hydrolysis directed by a 5′ strand break, 3′ directed excision also requires MutLα, PCNA, and RFC. EXOI interacts with PCNA. RFC and PCNA suppress EXOI-mediated 5′ to 3′ hydrolysis when the nick that directs excision is located 3′ to the mispair and activate 3′ to 5′ excision, which is dependent on loaded PCNA and apparently mediated by a cryptic EXOI 3′ to 5′ hydrolytic function. By contrast, RFC and PCNA have only a limited effect on 5′ to 3′ excision directed by a 5′ strand break.