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 sequence C-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% 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.