The interactions of cyanobacterial cytochrome c₆ and cytochrome f, characterized by NMR

During oxygenic photosynthesis, cytochrome c₆ shuttles electrons between the membrane-bound complexes cytochrome bf and photosystem I. Complex formation between Phormidium laminosum cytochrome f and cytochrome c₆ from both Anabaena sp. PCC 7119 and Synechococcus elongatus has been investigated by nuclear magnetic resonance spectroscopy. Chemical-shift perturbation analysis reveals a binding site on Anabaena cytochrome c₆, which consists of a predominantly hydrophobic patch surrounding the heme substituent, methyl 5. This region of the protein was implicated previously in the formation of the reactive complex with photosytem I. In contrast to the results obtained for Anabaena cytochrome c₆, there is no evidence for specific complex formation with the acidic cytochrome c₆ from Synechococcus. This remarkable variability between analogous cytochromes c₆ supports the idea that different organisms utilize distinct mechanisms of photosynthetic intermolecular electron transfer.