Differences in the promutagenic nature of 3-methylcytosine as revealed by DNA and RNA polymerising enzymes.

Poly(dC,3- MedC ) has been synthesised and used as a template to compare the miscoding properties of 3-methylcytosine (3-MeC) during DNA and RNA synthesis. Although 3-MeC was promutagenic with the RNA polymerase incorporating both AMP and UMP in the ratio of approximately 5:1 (agreeing with results reported by earlier workers) no non-complementary nucleotide incorporation was observed with DNA polymerase I. The results show that 3-MeC, which is a strong inhibitor of DNA synthesis, is only promutagenic with the less accurate RNA polymerase and that the reported differences in promutagenicity for this modified base with the two nucleotide polymerising enzymes arise from different specificities for the two enzymes.