Purification of a sixth ferredoxin from Rhodobacter capsulatus. Primary structure and biochemical properties

A new ferredoxin has been purified from the photosynthetic bacterium Rhodobacter capsulatus. It is the sixth ferredoxin to be isolated from this bacterium and it was called FdVI. Its primary structure was established based on amino acid sequence analysis of the protein and of peptides derived from it. It is composed of 106 residues including five cysteines. The calculated mass of the polypeptide is 11,402.6 Da which matches the experimental value determined by electrospray mass spectrometry. Amino acid sequence comparison revealed that ferredoxin VI (FdVI) is strikingly similar to a ferredoxin from Caulobacter crescentus and to the putidaredoxin from Pseudomonas putida. FdVI exhibited an ultraviolet-visible absorption spectrum typical for a [2Fe-2S] ferredoxin. EPR spectroscopy of the reduced protein showed a nearly axial signal similar to that of mitochondrial and P. putida ferredoxins. FdVI is biosynthesized in cells growing anaerobically under either nitrogen-sufficient or nitrogen-deficient conditions. Although the function of FdVI is unknown, its structural resemblance to [2Fe-2S] ferredoxins known to transfer electrons to oxygenases such as P-450 cytochromes, suggests that FdVI may have a similar role in R. capsulatus.