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 t hat regulates expression of pobA, the structural gene for p-hydroxyben zoate hydroxylase (PobA) in Acinetobacter calcoaceticus ADP1. Inducibl e expression of cloned pobA in Escherichia coli depended upon the pres ence of a functional pobR gene, and mutations within pobR prevented po bA 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 s tructurally related analogs of p-hydroxybenzoate, unable to cause indu ction by themselves, were effective anti-inducers. The nucleotide sequ ence of pobR was determined, and the activator gene was shown to be tr anscribed divergently from pobA; the genes are separated by 134 DNA ba se pairs. The deduced amino acid sequence yielded a polypeptide of M(r ) = 30,764. Analysis of this sequence revealed at the NH2 terminus a s tretch of residues with high potential for forming a helix-turn-helix structure that could serve as a DNA-binding domain. A conservative ami no acid substitution (Arg-61 --> His-61) in this region inactivated Po bR. The primary structure of PobR appears to be evolutionarily distinc t from the four major families of NH2-terminal helix-turn-helix contai ning bacterial regulatory proteins that have been identified thus far.