Prokaryotic mismatch repair

See also Postreplication Repair, Recombinational Repair, Prokaryotic Mismatch Repair, Eukaryotic Mismatch Repair, SOS Regulon (from Chapter 26)... 

See also DNA Methylation, Cytosine, Prokaryotic Mismatch Repair... 

Postreplicational mismatch repair has been found to correct errors in base substitution occurring during DNA replication in prokaryotes.48 This lowers the error rate for the polymerase from 1 in 106 to 107 to the observed range of values of 1 in 108 to 1010 in E. coli. How does the repair system know in this case which base in a mispair is the incorrect one The answer appears to be that the parent strand is tagged by methylation. A small proportion, some 0.2%, of the cytosine residues are methylated at the 5 position, and a similar proportion of the adenine residues are methylated at the 6 position. As methylation is a postreplicative event, the daughter strand is temporarily undermethylated after replication. 

Mismatched base pairs can arise from errors that occur during recombination repair. A variety of prokaryotic and eukaryotic mismatch repair proteins repair mismatched base pairs by a variety of mechanisms that involve the excision of one of the bases [69, 70]. 

Mismatch repair (prokaryotic or eukaryotic), a process that recognizes DNA mismatches created either by replication errors, non-homologous recombination, or damage to one DNA base, and corrects the error. 

Prokaryotes methylate their DNA soon after replication. This aids the process of mismatch repair. The enzymes that carry out the process can recognize the correct strand by its methyl groups. 

Identification of daughter strand in prokaryotes Since DNA methylation lags behind DNA synthesis, the daughter strand is under methylated in comparison to the parental strand. The mismatch repair system seeks to act on the undermethylated DNA, which may elude the editing function of Pol during DNA replication. 

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