Specificity in the methylation of DNA by N-nitroso compounds.

A sequencing assay was used to determine the reactivity of N-nitroso compounds that are simple methylating agents with individual nucleotides in a defined DNA sequence. The maximal difference in reactivity between guanines is about five fold. DNA in the Z, cruciform and H conformations was shown to be methylated by N-methyl-N-nitrosourea in a manner which was indistinguishable from the reactivity of B-DNA. Electronic factors rather than steric factors appear to dominate the methylation reaction. Transcriptionally active genes were shown to be methylated by N-nitroso compounds in vivo more extensively than untranscribed genes. The results suggest that local sequence, secondary conformation and transcriptional activity may all influence the carcinogenic potential of N-nitroso compounds.