Photosynthetic Water Oxidation in Cytochrome b₅₅₉ Mutants Containing a Disrupted Heme-binding Pocket

The role of cytochrome b₅₅₉ in photosynthetic oxygen evolution has been investigated in three chloroplast mutants of Chlamydomonas reinhardtii, in which one of the two histidine axial ligands to the heme, provided by the α subunit, has been replaced by the residues methionine, tyrosine, and glutamine. Photosystem two complexes functional for oxygen evolution could be assembled in the methionine and tyrosine mutants up to ∼15% of wild type levels, whereas no complexes with oxygen evolution activity could be detected in the glutamine mutant. PSII supercomplexes isolated from the tyrosine and methionine mutants were as active as wild type in terms of light-saturated rates of oxygen evolution but in contrast to wild type contained no bound heme despite the presence of the α subunit. Oxygen evolution in the tyrosine and methionine mutants was, however, more sensitive to photoinactivation than the WT. Overall, these data establish unambiguously that a redox role for the heme of cytochrome b₅₅₉ is not required for photosynthetic oxygen evolution. Instead, our data provide new evidence of a role for cytochrome b₅₅₉ in the protection of the photosystem two complex in vivo.