Testing Excision Models for Responses of Mismatch-Repair Systems to UV Photoproducts in DNA

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Abstract

Mismatch-repair (MMR) systems correct DNA replication errors and respond to a variety of DNA lesions. Previous observations that MMR antagonizes UV mutagenesis, and that the mismatch-recognition protein heterodimer MSH2 • MSH6 (MutSα) selectively binds DNA containing “mismatched” photoproducts (T[CPD]T/AG, T[6-4]T/AG) but not “matched” photoproducts (T[CPD]T/AA, T[6-4]T/AA), suggested that mismatched photoproducts would provoke MMR excision similar to mismatched bases. Excision of incorrect nucleotides inserted opposite template photoproducts might then prevent UV-induced mutation. We tested T[CPD]T/AG DNA, in a sequence context in which it is bound substantially by hMutSα and in three other contexts, for stimulation of 3′ MMR excision in mammalian nuclear extracts. T[CPD]T/AG was inactive in HeLa extracts, or in extracts deficient in the photoproduct-binding proteins DDB or XPC • hHR23B, arguing against interference from the nucleotide excision repair pathway. Prior incubation with hMutSα and MLH2 · PMS2 (hMutLα) did not increase excision relative to homoduplex controls. T[6-4]T/AG also failed to provoke excision. T/G, C/A, and T/T substrates, even though bound by hMutSα no better than T[CPD]T/AG substrates, efficiently provoked excision. Even a substrate containing three T[CPD]T/AG photoproducts (in different contexts) did not significantly provoke excision. Thus, MMR may suppress UV mutagenesis by non-excisive mechanisms.

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