very short patch (VSP) repair

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VSP repair corrects T: G mismatches to C: G when found in certain sequence contexts. Two of the substrate mismatches (5'-CTWGG/3'-GGW'CC; W = A or T) occur in the context of cytosine methylation in DNA and reduce the mutagenic effects of 5-methylcytosine deamination to thymine. However, VSP repair is also known to repair T: G mismatches that are not expected to arise from 5-methylcytosine deamination (example–CTAG/GGT-C). In these cases, if the original base pair were a T: A, VSP repair would cause a T to C transition.

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 protein 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 5meC!; 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.
In Escherichia coli and related enteric bacteria, repair of base-base mismatches is performed by two overlapping biochemical processes, methyl-directed mismatch repair (MMR) and very short-patch (VSP) repair. While MMR repairs replication errors, VSP repair corrects to C*G mispairs created by 5-methylcytosine deamination to T. The efficiency of the two pathways changes during the bacterial life cycle; MMR is more efficient during exponential growth and VSP repair is more efficient during the stationary phase. VSP repair and MMR share two proteins, MutS and MutL, and although the two repair pathways are not equally dependent on these proteins, their dual use creates a competition within the cells between the repair processes. The structural and biochemical data on the endonuclease that initiates VSP repair, Vsr, suggest that this protein plays a role similar to MutH (also an endonuclease) in MMR. Biochemical and genetic studies of the two repair pathways have helped eliminate certain models for MMR and put restrictions on models that can be developed regarding either repair process. 
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ORTHOLOGY CLASS Homo sapiens L. (human) [HSA] Mus musculus L. (mouse) [MMU] Caenorhabditis elegans Maupas (nematode) [CEL] Drosophila melanogaster Meigen (fruit fly) [DME] Saccharomyces cerevisiae Meyen ex E.C. Hansen (budding yeast) [SCE] Schizo-saccharomyces pombe Lindner (fission yeast) [SPO] Escherichia coli Migula (bacterium) K-12 MG1655 [ECO] Arabidopsis thaliana (L.) Heynh. (mouse-ear cress) [ATH]
ko:K07458 (DNA mismatch endonuclease, patch repair protein [EC:3.1.-.-]) Vsr

Last modification date: Dec. 27, 2011