Pg. Egland et Cs. Harwood, BadR, a new MarR family member, regulates anaerobic benzoate degradation by Rhodopseudomonas palustris in concert with AadR, an Fnr family member, J BACT, 181(7), 1999, pp. 2102-2109
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.