New Insights Into the Evolution of the Electron Transfer from Cytochrome f to Photosystem I in the Green and Red Branches of Photosynthetic Eukaryotes.

In cyanobacteria and most green algae of the eukaryotic green lineage, the copper-protein plastocyanin alternatively replaces the heme-protein cytochrome c  6 as the soluble electron carrier from cytochrome f to photosystem I. The functional and structural equivalence of "green" plastocyanin and cytochrome c  6 has been well established, representing an example of convergent evolution of two unrelated proteins. However, plants only produce plastocyanin, despite having evolved from green algae. On the other hand, cytochrome c  6 is the only soluble donor available in most species of the red lineage of photosynthetic organisms, which includes, among others, red algae and diatoms. Interestingly, plastocyanin genes have been identified in oceanic diatoms, probably acquired by horizontal gene transfer from green algae. However, the mechanisms that regulate the expression of a functional plastocyanin in diatoms are still unclear. In the green eukaryotic lineage, the transfer of electrons from cytochrome f to photosystem I has been characterized in depth. The conclusion is that in the green line this process involves strong electrostatic interactions between partners, which ensures a high affinity and an efficient electron transfer, at the cost of limiting the turnover of the process. In the red lineage, recent kinetic and structural modelling data suggest a different strategy, based on weaker electrostatic interactions between partners, with lower affinity and less efficient electron transfer, but favouring instead the protein exchange and the turnover of the process. Finally, in diatoms the interaction of the acquired green-type plastocyanin with both cytochrome f and photosystem I may not yet be optimized.