HbaR, a 4-hydroxybenzoate sensor and FNR-CRP superfamily member, regulatesanaerobic 4-hydroxybenzoate degradation by Rhodopseudomonas palustris

Under anaerobic conditions, structurally diverse aromatic compounds are cat abolized by bacteria to form benzoyl-coenzyme A (benzoyl-CoA), the starting compound for a central reductive pathway for aromatic ring degradation. Th e structural genes required for the conversion of g-hydroxybenzoate (4-HBA) to benzoyl-CoA by Rhodopseudomonas palustris have been identified. Here we describe a regulatory gene, hbaR, that is part of the 4-HBA degradation ge ne cluster, An hbaR mutant that was constructed was unable to grow anaerobi cally on 4-HBA, However, the mutant retained the ability to grow aerobicall y on 4-HBA by an oxygen-requiring pathway distinct from the anaerobic route of 4-HBA degradation. The effect of the HbaR protein on expression of hbaA encoding 4-HBA-CoA ligase, the first enzyme for 4-HBA degradation, was inv estigated by using hbaA::'lacZ transcriptional fusions, HbaR was required f or a 20-fold induction of beta-galactosidase activity that was observed wit h a chromosomal hbaA::'lacZ fusion when cells grown anaerobically on succin ate were switched to anaerobic growth on succinate and 4-HBA. HbaR also act ivated expression from a plasmid-borne hbaA-'lacZ fusion when it was expres sed in aerobically grown Pseudomonas aeruginosa cells, indicating that the activity of this regulator is not sensitive to oxygen, The deduced amino ac id sequence of HbaR indicates that it is a member of the FNR-CRP superfamil y of regulatory proteins, It is most closely related to transcriptional act ivators that are involved in regulating nitrate reduction. Previously, it h as been shown that R, palustris has an FNR homologue, called AadR, that is also required for 4-HBA degradation. Our evidence indicates that AadR activ ates expression of hbaR in response to anaerobiosis and that HbaR, in turn, activates expression of 4-HBA degradation in response to 4-HBA as an effec tor molecule.