CHARACTERIZATION OF RHODOBACTER-CAPSULATUS GENES ENCODING A MOLYBDENUM TRANSPORT-SYSTEM AND PUTATIVE MOLYBDENUM-PTERIN-BINDING PROTEINS

The alternative, heterometal-free nitrogenase of Rhodobacter capsulatu s is repressed by traces of molybdenum in the medium. Strains carrying mutations located downstream of nifB copy II were able to express the alternative nitrogenase even in the presence of high molybdate concen trations. DNA sequence analysis of a 5.5-kb fragment of this region re vealed six open reading frames, designated modABCD, mopA, and mopB. Th e gene products of modB and modC are homologous to ChlJ and ChlD of Es cherichia coli and represent an integral membrane protein and an ATP-b inding protein typical of high-affinity transport systems, respectivel y. ModA and ModD exhibited no homology to known proteins, but a leader peptide characteristic of proteins cleaved during export to the perip lasm is present in ModA, indicating that ModA might be a periplasmic m olybdate-binding protein. The MopA and MopB proteins showed a high deg ree of amino acid sequence homology to each other. Both proteins conta ined a tandem repeat of a domain encompassing 70 amino acid residues, which had significant sequence similarity to low-molecular-weight moly bdenum-pterin-binding proteins from Clostridium pasteurianum. Compared with that for the parental nifHDK deletion strain, the molybdenum con centrations necessary to repress the alternative nitrogenase were incr eased 4-fold in a modD mutant and 500-fold in modA, modB, and modC mut ants. No significant inhibition of the heterometal-free nitrogenase by molybdate was observed for mopA mopB double mutants. The uptake of mo lybdenum by mod and mop mutants was estimated by measuring the activit y of the conventional molybdenum-containing nitrogenase. Molybdenum tr ansport was not affected in a mopA mopB double mutant, whereas strains carrying lesions in the binding-protein-dependent transport system we re impaired in molybdenum uptake.