Plant Methyltransferases and Their Targets in the Plant Genome

Abstract

In addition to the methylation of cytosine in some CG dinucleotides, plant genomes contain 5-methylcytosine (a cytosine with a methyl group in place of the hydrogen at position 5) in the trinucleotide sequence mCNG, where N is reportedly any of the four common DNA bases (Gruenbaum et al. 1981). 5-Methylcytosine has also been found in non-symmetrical sequences in transgenes (Meyer et al. 1994), but there is no evidence in plants for the N-4 methylcytosine that is found in some prokaryotes. We have shown that CG and CNG methylation are carried out by different methyltransferases (Pradhan and Adams 1995), and one possible consequence of this is that the two reactions could be independently regulated. This would allow the methylation of CG and CNG sequences to have unrelated functions in the plant cell, although to date there are no data to support this suggestion.

A typical base composition for angiosperm DNA is 40% G+C. If all CG and CNG sequences were to be methylated, then the percentage of cytosines methylated would be 36% (20% in mCG and 20% in mCNG, of which 4% would also be in mCG). Some angiosperms approach this value, and cleavage of pea DNA with methyl-sensitive restriction enzymes indicates that most CG and CWG sequences are methylated (Pradhan et al. 1995). (W stands for A or T and hence the CWG trinucleotide contains CAG on one strand of DNA and CTG on the other.) Yet in the DNA from Arabidopsis, only 5% of the cytosines are methylated (Leutwiler...