Methylation of rRNA genes in some higher plants

Cytosine residues were more heavily methylated in rDNA isolated from fruit nuclei of marrow, melon, cucumber and tap root nuclei of turnip, than in total DNA preparations. The rRNA genes showed varying degrees of heterogeneity for both length and methylation as assessed by digestion with restriction endonucleases. Marrow rDNA was not digested by the methylation sensitive restriction endonuclease HpaII and while none of the other rDNAs was extensively digested, cucumber rDNA showed a different pattern of heterogeneity following HpaII digestion to that shown following EcoRI digestion. Cloned (unmethylated) rDNA sequences were extensively digested by HpaII. The relatively high level of methylation of the rRNA genes may be related to the large number of these genes in higher plants.