Structure and expression of the alkB gene of Escherichia coli related to the repair of alkylated DNA

When the alkB gene of Escherichia coli that controls sensitivity of bacteria to methyl methanesulfonate was placed under the control of the lac regulatory region on a multicopy plasmid, the gene product, AlkB protein, was overproduced. By monitoring the band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the protein was purified to near physical homogeneity. An amino-terminal sequence and total amino acid composition of the purified AlkB protein were in accord with the amino acid sequence deduced from the nucleotide sequence of the alkB gene, determined by the phage M13 dideoxy method. It was concluded that the AlkB protein is comprised of 216 amino acids and has a molecular weight of 23,900. The nucleotide sequence analysis also revealed that the ada and alkB genes are adjacent on the E. coli chromosome and that the first initiation codon for AlkB protein overlaps with the termination codon for Ada protein. We constructed hybrid plasmids carrying an alkB'-lacZ' fusion, with or without the ada control region, and investigated expression of the alkB gene in response to the alkylating agent. We obtained evidence that the ada and alkB genes constitute an operon.