maoB, a gene that encodes a positive regulator of the monoamine oxidase gene (maoA) in Escherichia coli

The structural gene for copper- and topa quinone-containing monoamine oxidase (maoA) and an unknown amine oxidase gene have been located at 30.9 min on the Escherichia coli chromosome. Deletion analysis showed that the unknown gene was located within a 1.1-kb cloned fragment adjacent to the maoA gene. The nucleotide sequence of this fragment was determined, and a single open reading frame (maoB) consisting of 903 bp was found. The gene encoded a polypeptide with a predicted molecular mass of 34,619 Da which was correlated with the migration on a sodium dodecyl sulfate-polyacrylamide gel. The predicted amino acid sequence of the MaoB protein was identical to the NH₂- terminal amino acid sequence derived by Edman degradation of the protein synthesized under the self-promoter. No homology of the nucleotide sequence of maoB to the sequences of any reported genes was found. However, the amino acid sequence of MaoB showed a high level of homology with respect to the helix-turn-helix motif of the AraC family in its C terminus. The homology search and disruption of maoA on the chromosome led to the conclusion that MaoB is a transcriptional activator of maoA but not an amine oxidase. The consensus sequence of the cyclic AMP-cyclic AMP receptor protein complex binding domain was adjacent to the putative promoter for the maoB gene. By use of lac gene fusions with the maoA and maoB genes, we showed that the maoA gene is regulated by tyramine and MaoB and that the expression of the maoB gene is subject to catabolite repression. Thus, it seems likely that tyramine and the MaoB protein activate the transcription of maoA by binding to the regulatory region of the maoA gene.