Cyanobacterial redox carriers support photosynthesis in a purple phototrophic bacterium.

In oxygenic and anoxygenic photosynthesis, excitation energy migrates from a surrounding antenna to specialised chlorophyll (Chl) or bacteriochlorophyll (BChl) pigments housed within a reaction centre (RC) complex. Here, a charge-separated state is formed within a few picoseconds, and an electron moves along a series of cofactors until it arrives at a quinone or iron-sulphur centre acceptor. Further photochemical cycles rely on rapid re-reduction in the photo-oxidised RC, usually by small, soluble metalloproteins which vary considerably between different phototrophic clades. In the purple phototrophic bacterium Rhodobacter (Rba.) sphaeroides, the electron carrier cytochrome c2 (cyt c2) shuttles between the periplasmic faces of the cytochrome bc1 complex and the reaction centre-light harvesting 1 (RC-LH1) core complex, the location of the BChl special pair (P865). By contrast, in the model cyanobacterium Synechocystis sp. PCC 6803, electrons are transferred from cytochrome b6f to photosystem I (PSI) via two isofunctional redox carrier proteins, cytochrome c6 (cyt c6) or plastocyanin (Pc). In the present study, we demonstrate that both cyt c6 and Pc can substitute for cyt c2in silico, in vitro and in vivo, even though their electrostatic properties may be counter-productive for binding the RC-LH1 complex. Interestingly, whilst P865+ reduction was highest with cyt c2 and the full physiological RC-LH1 complex, both Synechocystis proteins were more compatible with the RC-only complex lacking the surrounding LH1 antenna. Taken together, this suggests the subunits that constitute the LH1 ring improve both the donor side activity and selectivity of the Rba. sphaeroides RC complex.