Spontaneous Mutation at a 5-Methylcytosine Hotspot Is Prevented by Very Short Patch (Vsp) Mismatch Repair

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In many strains of Escherichia coli, the product of gene dcm methylates the internal cytosines in the sequence 5'CC(A or T)GG. Spontaneous deamination of 5-methylcytosine produces thymine which, if not corrected, can result in a transition mutation. 5-Methylcytosines in the lacI gene are hotspots for spontaneous C to T mutations. dcm is linked to vsr, a gene required for very short patch (VSP) repair. VSP repair corrects T.G mispairs in the following contexts: (GGTCC)(CTAGG), (GGACC)(CTTGG), (GTCC)(TAGG) and (GGTC)(CTAG). I have investigated the relationships between cytosine methylation, mutation, and VSP repair. Spontaneous mutations in the repressor (cI) gene of lambda prophage were isolated in wild-type and mutant lysogens. A hotspot for spontaneous mutation that corresponds with a 5-methylcytosine was observed in wild-type lysogens but was not present in bacteria lacking both methylase and VSP repair activity. Introduction of a plasmid containing dcm(+) and vsr(+) restored the mutation hotspot. If the added plasmid carried only dcm(+), the frequency of spontaneous mutations at the 5-methylcytosine was over 10-fold higher than in Dcm(+)Vsr(+) lysogens. The addition of vsr on a plasmid to a wild-type lysogen resulted in a 4-fold reduction in mutation at the hotspot. These findings support the previously untested hypothesis that VSP repair prevents mutations resulting from deamination of 5-methylcytosine.

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