Molecular mechanism of NAD(P)H -cyclic electron flow

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1. Electron donation from stromal components to P700^+ via the intersystem chain was demonstrated in spinach chloroplasts and the pool size of electrons that can be donated to P700^+ was estimated to be in the range of 12 and 28 electrons per p700 under aerobic conditions.2. Electron flow to the intersystem chain from stromal components and cyclic elctron flow in maize were estimated by monitoring redox changes of P700 and chlorophyll fluorescence. Storomal pool size of electrons that can be donated to P700^+ was much larger in maize than in c3 plants, giving a maximum value of 225 elctrons per P700.3. Using cyanobacteria, Synechococcus and Synechocystis, it has been demonstrated that electron donation from cyclic and respiratory flows to the photosynthetic intersystem chain is mediated by pyridine nucleotide dehydrogenase (NDH), which is analogous to mitochondrial complex I.The NDH in cyanobacteria mediates not only NADPH- dependent cyclic flow but also ferredoxin-dependent one.