Identification of the transcriptional activator pobR and characterization of its role in the expression of pobA, the structural gene for p-hydroxybenzoate hydroxylase in Acinetobacter calcoaceticus.

We have identified pobR, a gene encoding a transcriptional activator that regulates expression of pobA, the structural gene for p-hydroxybenzoate hydroxylase (PobA) in Acinetobacter calcoaceticus ADP1. Inducible expression of cloned pobA in Escherichia coli depended upon the presence of a functional pobR gene, and mutations within pobR prevented pobA expression in A. calcoaceticus. A pobA-lacZ operon fusion was used to demonstrate that pobA expression in A. calcoaceticus is enhanced up to 400-fold by the inducer p-hydroxybenzoate. Inducer concentrations as low as 10(-7) M were sufficient to elicit partial induction. Some structurally related analogs of p-hydroxybenzoate, unable to cause induction by themselves, were effective anti-inducers. The nucleotide sequence of pobR was determined, and the activator gene was shown to be transcribed divergently from pobA; the genes are separated by 134 DNA base pairs. The deduced amino acid sequence yielded a polypeptide of M(r) = 30,764. Analysis of this sequence revealed at the NH2 terminus a stretch of residues with high potential for forming a helix-turn-helix structure that could serve as a DNA-binding domain. A conservative amino acid substitution (Arg-61-->His-61) in this region inactivated PobR. The primary structure of PobR appears to be evolutionarily distinct from the four major families of NH2-terminal helix-turn-helix containing bacterial regulatory proteins that have been identified thus far.