BadR, a new MarR family member, regulates anaerobic benzoate degradation by Rhodopseudomonas palustris in concert with AadR, an Fnr family member

A cluster of genes for the anaerobic degradation of benzoate has been descr ibed for the phototrophic bacterium Rhodopseudomonas palustris. Here ne pro vide an initial analysis of the regulation of anaerobic benzoate degradatio n by examining the contributions of two regulators: a new regulator, BadR, encoded by the benzoate degradation gene cluster, and a previously describe d regulator, AadR, whose gene lies outside the cluster. Strains with single mutations in either badR or aadR grew slowly on benzoate but were relative ly unimpaired in growth on succinate and several intermediates of benzoate degradation. A badR aadR double mutant was completely defective in anaerobi c growth on benzoate, Effects of the regulators on transcriptional activati on were monitored with an R. palustris strain carrying a chromosomal fusion of 'lacZ to the badE gene of the badDEFG operon, This operon encodes benzo yl-coenzyme A (benzoyl-CoA) reductase, an unusual oxygen-sensitive enzyme t hat catalyzes the benzene ring reduction reaction that is the rate-limiting step in anaerobic benzoate degradation. Expression of badE::'lacZ was indu ced 100-fold when cells grown aerobically on succinate were shifted to anae robic growth on succinate plus benzoate, The aadR gene was required for a 2 0-fold increase in expression that occurred in response to anaerobiosis, an d badR was responsible for a further 5-fold increase in expression that occ urred in response to benzoate, Further studies with the badE::'lacZ fusion strain grown with various kinds of aromatic acids indicated that BadR proba bly responds to benzoyl-CoA acting as an effector molecule. Sequence inform ation indicates that BadR is a member of the MarR family of transcriptional regulators. These studies expand the range of functions regulated by MarR family members to include anaerobic aromatic acid degradation and provide a n example of a MarR-type protein that acts as a positive regulator rather t han as a negative regulator, as do most MarR family members, AadR resembles the Escherichia coli Fnr regulator in sequence and contains cysteine resid ues that are spaced appropriately to serve in the capacity of a redox-sensi ng protein.