In the Phaeodactylum tricornutum alga, as in most diatoms, cytochrome c6 is the only electron donor to photosystem I, and thus they lack plastocyanin as an alternative electron carrier. We have investigated, by using laser-flash absorption spectroscopy, the electron transfer to Phaeodactylum photosystem I from plastocyanins from cyanobacteria, green algae and plants, as compared with its own cytochrome c6. Diatom photosystem I is able to effectively react with eukaryotic acidic plastocyanins, although with less efficiency than with Phaeodactylum cytochrome c6. This efficiency, however, increases in some green alga plastocyanin mutants mimicking the electrostatics of the interaction site on the diatom cytochrome. In addition, the structure of the transient electron transfer complex between cytochrome c6 and photosystem I from Phaeodactylum has been analyzed by computational docking and compared to that of green lineage and mixed systems. Taking together, the results explain why the Phaeodactylum system shows a lower efficiency than the green systems, both in the formation of the properly arranged [cytochrome c6-photosystem I] complex and in the electron transfer itself.

Research work was supported by FEDER and the Spanish Ministry of Economy and Competitiveness (MINECO, Grants BIO2012-35271 to JAN, and BIO2013-48213-R to JFR) and the Andalusian Government (PAIDI BIO-022, to JAN). The authors thank Prof. Marcellus Ubbink (Leiden Institute of Chemistry, The Netherlands) for the supply of fern plastocyanin, and to Rocío Rodríguez (Proteomic Service, IBVF) for technical assistance.

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