DNA-substrate sequence specificity of human G:T mismatch repair activity

Abstract

G:T mispairs in DNA originate spontaneously via deamlnation of 5-methylcytoslne. Such mispairs are restored to normal G:C pairs by both E.coli K strains and human cells. In this study we have analyzed the repair by human cell extracts of G:T mismatches in various DNA contexts. We performed two sets of experiments. In the first, repair was sequence specific in that G:T mispairs at CpG sites at four different CpG sites were repaired, but a G:T mismatch at a GpG site was not. Cytosine hemlmethylatlon did not block repair of a substrate containing a CpG/GpT mismatch. In the second set of experiments, substrates with a G:T mismatch at a fixed position were constructed with an A, T, G, or C 5′ to the mismatched G, and alterations in the complementary strand to allow otherwise perfect Watson - Crick pairing. All were incised just 5′ to the mismatched T and competed for repair incision with a G:T substrate in which a C was 5′ to the mismatched G. Thus human G:T mismatch activity shows sequence specificity, Incising G:T mismatched pairs at some DNA sites, but not at others. At an incisable site, however, incision is little influenced by the base 5′ to the mismatched G. © 1993 Oxford University Press.