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Mol Microbiol. 1996 May;20(3):467-73.

Very short patch repair: reducing the cost of cytosine methylation.

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Department of Molecular Microbiology and Immunology, University of Southern California School of Medicine, Los Angeles 90033, USA.


In Escherichia coli and related bacteria, the product of gene dcm methylates the second cytosine of 5'-CCWGG sequences (where W is A or T). Deamination of 5-methylcytosine (5meC) results in C to T mutations. The mutagenic potential of 5meC is reduced by a system called very short patch (VSP) repair, which replaces T with C. T:G and U:G mispairs in the methylatable sequence and in related sequences are recognized by the product of vsr, a gene adjacent to dcm. Vsr creates a nick just 5' of the mispaired pyrimidine to initiate the repair. Additional products known to be required for VSP repair are DNA polymerase I and DNA ligase. MutS and MutL have a stimulatory role but are not required. The ability of Vsr to recognize T:G mispairs in sequences related to CCWGG is probably responsible for over- and under-representation of certain tetranucleotides in the E. coli genome. Although VSP repair reduces spontaneous mutations at 5meCs in replicating bacteria, mutation hot-spots persist at these sites. Under conditions that more accurately mimic the natural environment of E. coli, VSP repair appears to be effective in preventing mutation at 5meC.

[Indexed for MEDLINE]

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