Mapping of 5-methylcytosine residues in Nicotiana tabacum 5S rRNA genes by genomic sequencing.

Genomic sequencing was used to localise 5-methylcytosine residues in individual DNA strands of 5S rRNA genes in tobacco. The density of methylation along the sequence was high in both strands, exceeding the average methylation density of the tobacco genome. Besides methylation of CG and CNG sequences, considerable amounts of mC were found in non-symmetrical sites. Among 69 sequenced clones obtained from leaf DNA we did not detect any non-methylated clone, and Southern blot hybridisation analysis also failed to suggest the presence of methylation-free 5S rDNA units in the tobacco genome. Differences were observed among methylation patterns of individual sequenced clones. This heterogeneity reflects either heterogeneity among individual members of 5S rRNA gene cluster or differences among individual cells. Methylation of CNG and non-symmetrical sites can be efficiently reduced by treatment with dihydroxypropyladenine, an inhibitor of S-adenosylhomocysteine hydrolase.