HbpR, a new member of the XylR/DmpR subclass within the NtrC family of bacterial transcriptional activators, regulates expression of 2-hydroxybiphenyl metabolism in Pseudomonas azelaica HBP1

The regulation of 2-hydroxybiphenyl and 2,2'-dihydroxybiphenyl degradation in Pseudomonas azelaica is mediated by the regulatory gene, hbpR, The hbpR gene encodes a 63-kDa protein belonging to the NtrC family of prokaryotic t ranscriptional activators and having the highest homology to members of the XglR/DmpR subclass. Disruption of the hbpR gene in P. azelaica and complem entation in trans showed that the HbpR protein was the key regulator for 2- hydroxybiphenyl metabolism. Induction experiments with P, azelaica and Esch erichia coli containing luxAB-based transcriptional fusions revealed that H bpR activates transcription from a promoter (P-hbpC) in front of the first gene for 2-hydroxybiphenyl degradation, hbpC, and that 2-hydroxybiphenyl it self is the direct effector for HbpR-mediated activation. Of several compou nds tested, only the pathway substrates 2-hydroxybiphenyl and 2,2'-dihydrox ybiphenyl and structural analogs like 2-aminobiphenyl and 2-hydroxybiphenyl methane were effecters for HbpR activation, HbpR is therefore, to our knowl edge, the first regulator of the XylR/DmpR class that recognizes biaromatic but not monoaromatic structures, Analysis of a spontaneously occurring mut ant, P, azelaica HBP1 Pro, which can grow with the non-wild-type effector 2 -propylphenol, revealed a single mutation in the hbpR gene (T613C) leading to a Trp-->Arg substitution at amino acid residue 205. P. azelaica HBP1 der ivative strains without a functional hbpR gene constitutively expressed the genes for 2-hydroxybiphenyl degradation when complemented in trans with th e hbpR-T613C gene. This suggests the importance of this residue, which is c onserved among all members of the XylR/DmpR subclass, for interdomain repre ssion.