Potent induction of the adaptive response by a weak mutagen, methyl iodide, in Escherichia coli

Methyl iodide (MeI), a weakly mutagenic and highly chemoselective chemical, was tested for its abilities to induce the adaptive and SOS responses in E. coli CSH26/pMCP1000 (alkA'-lacZ') and CSH26/pSK1002 (umuC'-lacZ'). MeI induced the adaptive response effectively but gave a very weak SOS response. Its potent ability in inducing the adaptive response was also demonstrated by adaptation to both the mutagenic and killing effects of N-methyl-N-nitrosourea (MNU) in E. coli WP2 cells. Simultaneous treatment with MeI in a non-growth medium slightly increased the mutagenicity of MNU, probably as a result of depletion of the repair enzyme, O6-methylguanine-DNA methyltransferase, which is constitutively present in the cells. As MeI itself proved to be only weakly mutagenic, a small part of the adaptive response which we have observed may involve indirect methylation of the repair enzyme by methyl transfer from MeI-induced O6-methylguanine residues in DNA. But the extent of the induced adaptive response seems to be much higher than would be expected from the observed weak mutagenicity of MeI. It is therefore suggested that the mechanism of induction of the adaptive response may involve direct methylation of the O6-methylguanine-DNA methyltransferase itself.