1. 1. Previous studies have shown that polycytidylic acid templates which have been treated with methyl methanesulfonate lead to the anomalous incorporation of UTP into a product polynucleotide with RNA polymerase. To prove that this anomalous incorporation is caused by 3-methylcytosine units in the treated templates, 3-methylcytidine diphosphate has been synthesized and used with polynucleotide phosphorylase to prepare methylcytosine-containing copolymers with cytidylic acid. These copolymers show the same anomalous UTP incorporation as was observed previously 2. 2. Copolymers of cytidylic acid and 3-ethylcytidylic acid have been produced by copolymerization of the appropriate monomers or by ethylation of polycytidylic acid. These templates also lead to the nonspecific incorporation of UTP. 3. 3. The misincorporation of UTP which is directed by methyl- or ethylcytosine can be distinguished from that which is directed by 7-methylguanine by the addition of CTP to the polymerization mixture. The presence of CTP prevents incorporation of UTP in the later case only, evidently because cytosine forms a stable hydrogen bond with 7-methylguanine. © 1970.
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