In vivo electron donation from plastocyanin and cytochrome c6 to PSI in Synechocystis sp. PCC6803

Many cyanobacteria species can use both plastocyanin and cytochrome c(6) as lumenal electron carriers to shuttle electrons from the cytochrome b(6)f to either photosystem I or the respiratory cytochmme c oxidase. In Synechocystis sp. PCC6803 placed in darkness, about 60% of the active PSI centres are bound to a reduced electron donor which is responsible for the fast re-reduction of P-700 in vivo after a single charge separation. Here, we show that both cytochmme c(6) and plastocyanin can bind to PSI in the dark and participate to the fast phase of P-700 reduction, but the fraction of pre-bound PSI is smaller in the case of cytochrome c(6) than with plastocyanin. Because of the inter-connection of respiration and photosynthesis in cyanobacteria, the inhibition of the cytochrome c oxidase results in the over-reduction of the photosynthetic electron transfer chain in the dark that translates into a lag in the kinetics of P-700 oxidation at the onset of light. We show that this is true both with plastocyanin and cytochrome c(6), indicating that the partitioning of electron transport between respiration and photosynthesis is regulated in the same way independently of which of the two lumenal electron carriers is present, although the mechanisms of such regulation are yet to be understood.