title: Spontaneous mutation at a 5-methylcytosine hotspot is prevented by very short patch (VSP) mismatch repair.
abstract: 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:CTAAGGGGTCC, CTTGGGGACC, TAGGGTCC and CTAGGGTC. 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.
authors.affiliation: Department of Microbiology, University of Southern California School of Medicine, Los Angeles 90033.
authors.forename: M
authors.initials: M
authors.lastname: Lieb
mesh_terms: D044503:5-Methylcytosine; D010582:Bacteriophage lambda / Q000235:genetics; D001483:Base Sequence; D003596:Cytosine / Q000031:analogs & derivatives* / Q000737:chemistry; D004252:DNA Mutational Analysis; D004260:DNA Repair; D004269:DNA, Bacterial / Q000235:genetics*; D004279:DNA, Viral / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics*; D008745:Methylation; D009154:Mutation
chemical_list: D004269:DNA, Bacterial; D004279:DNA, Viral; D044503:5-Methylcytosine; D003596:Cytosine