Effect of the direction of DNA replication on mutagenesis by N-methyl-N'-nitro-N-nitrosoguanidine in adapted cells of Escherichia coli.

The methylating agent N-methyl-N'-nitro-N-nitrosoguanidine preferentially induces G:C to A:T transitions at DNA base pairs with the G in one particular strand of the cI gene in a lambda prophage, in this case the nontranscribed strand, in Escherichia coli cells in which the adaptive response is induced. The same preference is found for the cI gene inserted in the genome in the inverse orientation, so the differential effect is not caused by the direction of motion of the DNA replicating fork.