Evolution, Stability and Regulation of Methylation Patterns
Benzer (1961) demonstrated large variations in the mutability of certain sites in phage T 4; he called the highly mutable sites “hotspots”. Coulondre et al. (1978) studied the effects of 5-methylcytosine on mutation rates in the lacl gene of E. coli, and showed that several hotspots for base substitutions involve the transition from a G:C base pair to A:T in the lacl gene. The hotspot occurred at the second cytosine in the sequence CCAGG which is known to be methylated at this position by the product of the mec gene in the strain of E. coli K 12 used in the study. To test the hypothesis that the hotspots were the result of the methylation rather than some other characteristic of the sites a new CCAGG sequence was selected. From an amber mutant with a sequence CTAGG at the mutant site they obtained a reversion to a sense codon with the new sequence CCAGG. The new base sequence was now a hot spot with a factor 10 higher mutation rate than the unmethylated site. When the gene was transferred to E. coli B, which does not methylate the CCAGG sequences, the difference in mutation rate disappeared.
KeywordsMethylation Pattern Globin Gene Thymidine Kinase Activity Hemoglobin Gene Disperse Repeat
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