Active site and complete sequence of the suicidal methyltransferase that counters alkylation mutagenesis.

Abstract

The inducible resistance to alkylation mutagenesis and killing in Escherichia coli (the adaptive response) is controlled by the ada gene. The Ada protein acts both as a positive regulator of the response and as a DNA repair enzyme, correcting premutagenic O6-alkylguanine in DNA by suicidal transfer of the alkyl group to one of its own Cys residues. The DNA sequence of the cloned ada+ gene and its regulatory region were determined. The data reveal potential sites of ada autoregulation. Amino acid sequence determinations show that the active center for the O6-methylguanine-DNA methyltransferase is located close to the polypeptide COOH terminus and has the unusual sequence -Pro-Cys-His, preceded by a very hydrophobic region. These same structural features are presented at the active site of thymidylate synthase, suggesting a common chemical mechanism for activation of the Cys.