Fig. 9
Mismatch repair in E. coli. (a) The products of the mutH, L, and S genes, along with ATP, recognize a base mismatch (center), identify the newly synthesized strand by the absence of methyl groups on GATC sequences, and introduce a nick into that new strand, across from a methylated GATC and upstream from the incorrect nucleotide. (b) Exonuclease I, along with MutL, MutS, DNA helicase, and ATP, removes DNA downstream from the nick, including the incorrect nucleotide. (c) DNA polymerase III holoenzyme, with help from single-stranded binding protein (SSB), fills in the gap left by the exonuclease, and DNA ligase seals the remaining nick. (d) A methyl transferase methylates GATC sequences in the progeny strand across from methylated GATC sequences in the parental strand. Once this happens, mismatch repair cannot occur because the progeny and parental strands are indistinguishable.
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