tRNA chemical methylation. In vitro and in vivo formation of 1,7-dimethylguanosine at high concentrations of methylating agents

The methylation patterns produced in Escherichia coli B tRNA by a range of concentrations of the weak carcinogen dimethyl sulphate were examined with the following results: 1. 1,7-Dimethylguanosine was found to be formed in high amounts in the tRNA methylation reaction at high concentrations of methylating agent. 2. The dialkylated compound was recovered mainly in the form of derivatives, the spectral and chromatographic behaviour of which varied according to the procedures used for their isolation. Similar results were obtained for the in vivo methylation of rat-liver tRNA: after administration of a very high dose of the powerful carcinogen dimethylnitrosamine, 1,7-dimethylguanosine was found in rat-liver tRNA. Moreover, the analysis of the time-course of nucleic acid methylation indicated that this dialkylated product was still present in rat-liver tRNA when the major product of alkylation, 7-methylguanine, had almost completely disappeared.