Sequence specificity of methylation in higher plant DNA

Although plant DNA has a high content of 5-methylcytosine (5mC1)very little is known about the distribution of this modification. Many of these methylations are found in the dinucleotide sequenceC-G (refs 2-5)which is also the major modified site in animal cell DNA. It is clear, however, that C-G methylation cannot account for all the 5mC in DNA, in which this modified base may represent over 30 per cent of the cytosine residues (as in certain varieties of plant1). It was this observation that prompted the search for other methylated sites in the DNA. The results presented here show that methylated cytosine is indeed present at a variety of cytosine-containing dinucleotides, all of which, however, are part of the basic trinucleotide C-X-G. This sequence is probably essential for modification as it provides the symmetrical cytosines necessary for ensuring the inheritance of methylation at these sites.