ILHigher plant DNA is extensively methylated, but the two methylated sequences (CpG and CpNpG) show different characteristics. Using sequence analysis techniques, we demonstrate that while CpG methylation follows the existing models for cytosine methylation in animals, CpNpG methylation does not. Although there is evidence to support the suggestion that the low CpG frequency has arisen from deaminational conversion of 5-methylcytosine to thymidine, there appears to be no comparable conversion of 5-methylcytosine in the CpNpG configuration. It therefore appears that between the evolution of CpG and CpNpG cytosine methylation systems, a mechanism evolved for the correction of C + T conversion, probably using the methylated strand to direct the repair in the correct direction.

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