Molecular signal for induction of the adaptive response to alkylation damage in Escherichia coli.

Exposure of Escherichia coli to simple alkylating agents, such as methylnitrosourea, causes the induction of at least three DNA repair functions that are under the control of the ada gene. The ada gene product itself repairs several O-methylated lesions in DNA, including methylphosphotriesters and the mutagenic adduct O6-methylguanine. The methyl groups are transferred from these lesions on to two different cysteine residues within the Ada protein resulting in self-methylation. We have found that the Ada protein is converted to an activator of expression of genes involved in the adaptive response after accepting a methyl group from a methylphosphotriester, but not from O6-methylguanine. This was shown using the in vitro techniques of DNA-dependent protein synthesis and run-off transcription. Delayed electrophoretic migration and footprinting experiments have shown that the methylated activator of transcription binds to specific DNA sequences immediately upstream from the RNA polymerase binding sites in the promoter regions of the inducible genes. The Ada protein-binding sites contain the common sequence d(A-A-A-N-N-A-A-A-G-C-G-C-A).