The photosynthetic cytochrome c from the diatom Phaeodactylum tricornutum.

The photosynthetic cytochrome c from the marine diatom Phaeodactylum tricornutum has been purified and characterized. Cytochrome c is mostly obtained from the soluble cell extract in relatively large amounts. In addition, the protein appeared to be truncated in the last hydrophobic residues of the C-terminus, both in the soluble cytochrome c and in the protein extracted from the membrane fraction, as deduced by mass spectrometry analysis and the comparison with the gene sequence. Interestingly, it has been described that the C-terminus of cytochrome c forms a hydrophobic finger involved in the interaction with photosystem II in cyanobacteria. Cytochrome c was almost absent in solubilized photosystem II complex samples, in contrast with the PsbO and Psb31 extrinsic subunits, thus suggesting a lower affinity of cytochrome c for the photosystem II complex. Under iron-limiting conditions the amount of cytochrome c decreases up to about 45% as compared to iron-replete cells, pointing to an iron-regulated synthesis. Oxidized cytochrome c has been characterized using continuous wave EPR and pulse techniques, including HYSCORE, and the obtained results have been interpreted in terms of the electrostatic charge distribution in the surroundings of the heme centre.