5-methylcytidylic acid: absence from mitochondrial DNA of frogs and HeLa cells.

The content of 5-methylcytidylic acid in nuclear DNA and mito chondrial DNA of Xenopus laevis and HeLa cells has been determined. Both nuclear DNA's contain 5-methylcytidylic acid. The 5-methylcytidylic acid content of X. laevis DNA is 1.7 mole percent of total nucleotides, and that of HeLc cell DNA is 0.7 mole percent. In neither mitochondrial DNA could any 5-methylcytidylic acid be detected; the limit of sensitivity was judged at below 0.1 mole percent for X. laevis DNA and below 0.05 mole percent for HeLa cell DNA.