The reaction of N-methyl-N-nitrosourea with DNA in B and non-B conformations.

Reaction of N-methyl-N-nitrosourea (MNU) with DNA in B and non-B conformations was investigated using plasmids containing inserts in which the non-B topological isomers could readily be prepared by topoisomerase I catalyzed relaxation in the presence of ethidium. DNA sequences in the Z, cruciform and H conformations were shown to be methylated by MNU at the N7 position of guanines or the N3 position of adenines in a manner that was indistinguishable from the reaction of MNU with the same sequences in the B conformation. Electronic factors rather than steric factors, therefore, appear to dominate methylation of DNA by MNU.