Evidence for extensive methylation of ribosomal RNA genes in a rat XC cell line

XC cell line was established from a rat fibrosarcoma induced by avian sarcoma virus. Analysis of ribosomal RNA genes in the DNA of this cell line indicated that there was a 3- to 4-fold increase in the number of the rDNA repeating units. [methyl-14C]Methionine labeling experiments as well as Southern blot analysis of the restriction endonuclease fragments showed extensive methylation of the cytosine residues in the rDNA. Further analysis by HpaII, HhaI and MspI suggested that the entire rDNA repeat unit was uniformly methylated in the recognition sequence for these enzymes. Parallel experiments with two other rat cell lines (rat embryo fibroblasts and adenovirus-transformed rat embryo cells) indicated no evidence of methylation in the rDNA.