Methylation of DNA by three N-nitroso compounds: Evidence for sequence specific methylation by a common intermediate

Abstract Methylation in vitro of calf thymus DNA, a supercoiled plasmid, poly(dG) · poly(dC), and poly(dGdC) · poly(dGdC) by N -nitroso(acetoxymethyl)methylamine and N -nitroso(acetoxybenzyl)methylamine in the presence of esterase, and by N -nitrosomethylurea was investigated. Although there were differences in the amounts of 7-methylguanine and O 6 -methylguanine formed in the various DNA substrates, the methylation pattern was the same for each of these methylating agents. The three compounds reacted identically when methylation of a portion of a 345 bp restriction fragment of the plasmid pBR322 was examined at nucleotide resolution by a sequencing assay. They also showed a tendency to react preferentially with particular guanines. These data suggest that the three N -nitroso compounds methylate DNA via a common intermediate such as the methyl diazonium ion, which exhibits some sequence specificity.