Photosynthetic water oxidation in cytochrome b(559) mutants containing a disrupted heme-binding pocket.
The role of cytochrome b(559) 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reinhardtii, in which one of the two histidine axial ligands to the heme, provided by the alpha subunit, has been replaced by the residues methionine, tyrosine, and glutamine.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 approximately 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 alpha 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 b(559) is not required for photosynthetic oxygen evolution. Instead, our data provide new evidence of a role for cytochrome b(559) in the protection of the photosystem two complex in vivo.
Morais
F
F
Department of Biochemistry, Imperial College of Science, Technology, and Medicine, London, SW7 2AY, United Kingdom.
Kühn
K
K
Stewart
D H
DH
Barber
J
J
Brudvig
G W
GW
Nixon
P J
PJ
D000818:Animals; D001483:Base Sequence; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D016825:Chlamydomonas reinhardtii / Q000201:enzymology / Q000378:metabolism / Q000502:physiology; D003573:Cytochrome b Group / Q000235:genetics*; D017931:DNA Primers; D004578:Electron Spin Resonance Spectroscopy; D000081062:Heme-Binding Proteins; D006420:Hemeproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D010084:Oxidation-Reduction; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism / Q000528:radiation effects; D045332:Photosystem II Protein Complex; D014867:Water / Q000737:chemistry*
D002352:Carrier Proteins; D003573:Cytochrome b Group; D017931:DNA Primers; D000081062:Heme-Binding Proteins; D006420:Hemeproteins; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D014867:Water; C019313:cytochrome b559