MutSα but not UNG distinguishes U•G and U•A in duplex DNAs. (A) Electrophoretic mobility shift assay of purified hMutSα (16, 32, 65 or 130 nM) binding to labeled DNA duplexes containing U•G mispairs (left) or U•A pairs (right). Arrows indicate bound and free DNA. The percentage of DNA bound is shown below. (B) Quantitation of binding of purified MutSα to radiolabeled U•G duplexes in the presence of indicated levels of unlabeled U•G or U•A duplex competitor. (C) Products of deglycosylation of DNA duplex substrates containing U•G mispairs or U•A pairs by 0, 0.2, 0.4, 0.8, or 1.6 nM of purified hUNG, resolved by denaturing gel electrophoresis. Substrates were 5' end labeled on the uracil-containing strand, and following incubation with hUNG were treated with alkali to hydrolyze the backbone at abasic sites. The fraction of cleaved molecules, quantified by phosphorimager, is shown below each lane.