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5-Methylcytosine is not a Mutation Hot Spot in Nondividing Escherichia coli

Margaret Lieb and Shehnaz Rehmat
Proceedings of the National Academy of Sciences of the United States of America
Vol. 94, No. 3 (Feb. 4, 1997), pp. 940-945
Stable URL: http://www.jstor.org/stable/41414
Page Count: 6
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
5-Methylcytosine is not a Mutation Hot Spot in Nondividing Escherichia coli
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Abstract

Spontaneous deamination of 5-methylcytosine (5meC) causes hot spots of C· G → T· A mutations in Escherichia coli and in human cells. In E. coli, the resulting T· G mispairs can be corrected to C· G by very short patch (VSP) repair, which requires the product of gene vsr. Mutation hot spots in genes of replicating vsr+ bacteria are attributable to low Vsr activity. To determine the rate of deamination of 5meC and the efficiency of VSP repair in nondividing bacteria, we used kanamycin-sensitive (KanS) lysogens containing a λ kan- prophage. Deamination of a 5meC in the kan- gene resulted in mutation to kanamycin resistance (KanR). Lysogens containing a single λ kan- prophage per bacterial genome were grown in synthetic medium with limiting amino acids and stored at 15 degrees C or 37 degrees C. In the absence of VSP repair, KanR mutants accumulated at the rate of approximately 1.3 × 10-7 per bacterium per day at 37 degrees C. This is similar to the 5meC → T mutation rate reported for DNA in solution. In vsr+ bacteria, the KanR accumulation rate was 3 × 10-9 per bacterium per day, which is not significantly higher than the rate observed when the target cytosine was unmethylated. The increase in KanR mutants was barely detectable in vsr+ cultures stored at 15 degrees C for 4 months. It is likely that mutation hot spots at 5meC in rapidly dividing cells are attributable to insufficient time for T· G correction in the interval between deamination of 5meC and subsequent DNA replication. DNA synthesis occurred in bacteria starved for amino acids and this synthesis was not highly mutagenic.

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