FNRP AND NNR OF PARACOCCUS-DENITRIFICANS ARE BOTH MEMBERS OF THE FNR FAMILY OF TRANSCRIPTIONAL ACTIVATORS BUT HAVE DISTINCT ROLES IN RESPIRATORY ADAPTATION IN RESPONSE TO OXYGEN LIMITATION

The Paracoccus denitrificans fnrP gene encoding a homologue of the Esc herichia coli FNR protein was localized upstream of the gene cluster t hat encodes the high-affinity cbb(3)-type oxidase. FnrP harbours the i nvariant cysteine residues that are supposed to be the ligands of the redox-sensitive [4Fe-4S] cluster in FNR. NNR, another FNR-like transcr iptional regulator in P, denitrificans, does not, Analysis of FnrP and NNR single and double mutants revealed that the two regulators each e xert exclusive control on the expression of a discrete set of target g enes, In FnrP mutants, the expression of cytochrome c peroxidase was b locked, that of membrane-bound nitrate reductase and the cbb(3)-type o xidase was significantly reduced, whilst the activity of the bb(3)-typ e quinol oxidase was increased, The amounts of the nitrite and nitric oxide reductases in these FnrP mutants were the same as in the wild ty pe, NNR mutants, on the other hand, were disturbed exclusively in the concentrations of nitrite reductase and nitric oxide reductase. An Fnr P,NNR double mutant combined the phenotypes of the single mutant strai ns. In all three mutants, the concentrations and/or activities of the aa(3)-type oxidase, cytochrome C-550, cytochrome C-552, and nitrous ox ide reductase equalled those in the wild type. As the FNR boxes in fro nt of the FnrP- and NNR-regulated genes are highly similar to or even identical to each other, the absence of cross-talk between the regulat ion by FnrP and NNR implies that as yet unidentified factors are impor tant in the control, It is proposed that the redox state of an intrace llular redox couple other than the oxygen/water couple is one of the f actors that modulates the activity of FnrP.