N-Methyl-N′-nitro-N-nitrosoguanidine induced DNA sequence alteration; non-random components in alkylation mutagenesis

Our approach to the study of how the molecular nature of DNA modulates the behavior of mutational sites involves the characterisation of distributions of mutations. The Escherichia coli lacI genetic/M13 cloning system allows the comparison of base substitution frequencies at a large number of sites. The observed distribution of N-methyl-N′-nitro-N-nitrosoguanidine (MNNG)-induced G:C → A:T transition (the predominant event), and A:T → G:C transition (a relatively rare event), is strikingly non-random. Some sites of G:C → A:T mutation are almost 100 times more often mutated by MNNG than the least susceptible sites. Sites of mutation, however, do not display a continuum of mutability, but rather can be strictly demarcated by their 5′ flanking base. Sites with a high frequency of occurrence share a common sequence motif, namely 5′-R-G-N-3′, which is the sole apparent feature that distinguishes them from sites less commonly mutated (i.e. 5′-Y-G-N-3′). A corollary of this defined site specificity is the absence of a strand bias in MNNG-induced lacI^−d mutation. The availability of specific or non-specific alkylation-repair systems does not appear to alter the distribution of mutation, which suggests that the observed mutational distribution is a direct reflection of the initial damage distribution. MNNG does not belong to that class of compounds typified by ultraviolet light or 4-nitroquinoline-N-oxide which exhibit both random and non-random components of mutagenesis.