Acinetobacter PobR and PcaU are transcriptional activators that closely resemble each other in primary structure, DNA-binding sites, metabolic modulators, and physiological function. PobR responds to the inducer-metabolite p-hydroxybenzoate and activates transcription of pobA, the structural gene for the enzyme that converts p-hydroxybenzoate to protocatechuate. This compound, differing from p-hydroxybenzoate only in that it contains an additional oxygen atom, binds to PcaU and thereby specifically activates transcription of the full set of genes for protocatechuate catabolism. Particular experimental attention has been paid to PobR and PcaU from Acinetobacter strain ADP1, which exhibits exceptional competence for natural transformation. This trait allowed selection of mutant strains in which pobR function had been impaired by nucleotide substitutions introduced by PCR replication errors. Contrary to expectation, the spectrum of amino acids whose substitution led to loss of function in PobR shows no marked similarity to the spectrum of amino acids conserved by the demand for continued function during evolutionary divergence of PobR, PcaU, and related proteins. Surface plasmon resonance was used to determine the ability of mutant PobR proteins to bind to DNA in the pobA-pobR intergenic region. Deleterious mutations that strongly affect DNA binding all cluster in and around the PobR region that contains a helix-turn-helix motif, whereas mutations causing defects in the central portion of the PobR primary sequence do not seem to have a significant effect on operator binding. PCR-generated mutations allowing PobR to mimic PcaU function invariably caused a T57A amino acid substitution, making the helix-turn-helix sequence of PobR more like that of PcaU. The mutant PobR depended on p-hydroxybenzoate for its activity, but this dependence could be relieved by any of six amino acid substitutions in the center of the PobR primary sequence. Independent mutations allowing PcaU to mimic PobR activity were shown to be G222V amino acid substitutions in the C terminus of the 274-residue protein. Together, the analyses suggest that PobR and PcaU possess a linear domain structure similar to that of LysR transcriptional activators which largely differ in primary structure.