The Photoactive Photosynthetic Reaction Center of a Rhodobacter sphaeroides Mutant Lacking 3-Vinyl (Bacterio)Chlorophyllide Hydratase Contains 3-Vinyl Bacteriochlorophyll .

Rhodobacter sphaeroides mutant BF-lacking 3-vinyl (bacterio)chlorophyllide hydratase (BchF)-accumulates chlorophyllide (Chlide ) and 3-vinyl bacteriochlorophyllide (3V-Bchlide ). BF synthesizes 3-vinyl bacteriochlorophyll (3V-Bchl ) through prenylation of 3V-Bchlide and assembles a novel reaction center (V-RC) using 3V-Bchl and Mg-free 3-vinyl bacteriopheophytin (3V-Bpheo ) at a molar ratio of 2:1. We aimed to verify whether a -deleted R. sphaeroides mutant produces a photochemically active RC that facilitates photoheterotrophic growth. The mutant grew photoheterotrophically-implying a functional V-RC-as confirmed by the emergence of growth-competent suppressors of -deleted mutant (BC) under irradiation. Suppressor mutations in BC were localized to , which diminished BchF activity and caused 3V-Bchlide accumulation. expression carrying the suppressor mutations in resulted in the coproduction of V-RC and wild-type RC (WT-RC) in BF. The V-RC had a time constant (τ) for electron transfer from the primary electron donor P (a dimer of 3V-Bchl ) to the A-side containing 3V-Bpheo (H) similar to that of the WT-RC and a 60% higher τ for electron transfer from H to quinone A (Q). Thus, the electron transfer from H to Q in the V-RC should be slower than that in the WT-RC. Furthermore, the midpoint redox potential of P/P of the V-RC was 33 mV more positive than that of the WT-RC. R. sphaeroides, thus, synthesizes the V-RC when 3V-Bchlide accumulates. The V-RC can support photoheterotrophic growth; however, its photochemical activity is inferior to that of the WT-RC. 3V-Bchlide is an intermediate in the bacteriochlorophyll (Bchl )-specific biosynthetic branch and prenylated by bacteriochlorophyll synthase. R. sphaeroides synthesizes V-RC that absorbs light at short wavelengths. The V-RC was not previously discovered because 3V-Bchlide does not accumulate during the growth of WT cells synthesizing Bchl . The levels of reactive oxygen species increased with the onset of photoheterotrophic growth in BF, resulting in a long lag period. Although the inhibitor of BchF is unknown, the V-RC may act as a substitute for the WT-RC when BchF is completely inhibited. Alternatively, it may act synergistically with WT-RC at low levels of BchF activity. The V-RC may broaden the absorption spectra of R. sphaeroides and supplement its photosynthetic ability at various wavelengths of visible light to a greater extent than that by the WT-RC alone.