Eukaryotically encoded and chloroplast-located rubredoxin is associated with photosystem II

We analyzed a eukaryotically encoded rubredoxin from the cryptomonad Guilla rdia theta and identified additional domains at the N- and C-termini in com parison to known prokaryotic paralogous molecules. The cryptophytic N-termi nal extension was shown to be a transit peptide for intracellular targeting of the protein to the plastid, whereas a C-terminal domain represents a me mbrane anchor. Rubredoxin was identified in all tested phototrophic eukaryo tes. Presumably facilitated by its C-terminal extension, nucleomorph-encode d rubredoxin (nmRub) is associated with the thylakoid membrane. Association with photosystem II (PSII) was demonstrated by co-localization of nmRub an d PSII membrane particles and PSII core complexes and confirmed by comparat ive electron paramagnetic resonance measurements. The midpoint potential of nmRub was determined as +125 mV, which is the highest redox potential of a ll known rubredoxins. Therefore, nmRub provides a striking example of the a bility of the protein environment to tune the redox potentials of metal sit es, allowing for evolutionary adaption in specific electron transport syste ms, as for example that coupled to the PSII pathway.