Investigating the photoprotective role of cytochrome b-559 in photosystem II in a mutant with altered ligation of the haem.
Despite many years of study, the physiological role of cytochrome b-559 (Cyt b-559) within the photosystem II (PSII) complex still remains unclear. Here we describe the analysis of a mutant of the green alga Chlamydomonas reinhardtii in which the His ligand to the haem, provided by the alpha subunit, has been replaced by a Cys residue. The mutant is unable to grow photoautotrophically but can assemble oxygen-evolving PSII supercomplexes to 15-20% of the levels found in the wild-type control. Haem is still detected in the isolated PSII supercomplexes but at sub-stoichiometric levels consistent with weaker binding to the mutated cytochrome. Analysis of PSII activity in cells indicates slowed electron transfer in the mutant between plastoquinones QA and QB. We show that PSII activity in the mutant is more sensitive to chronic photoinhibition than the WT control because of two effects: a faster rate of damage and an impaired PSII repair cycle at the level of synthesis and/or incorporation of D1 into PSII. We also demonstrate that Cyt b-559 plays a role during the critical stage of assembling the Mn4CaO5 cluster. Overall we conclude that Cyt b-559 optimises electron transfer on the acceptor side of PSII and plays physiologically important roles in the assembly, repair and maintenance of the complex.
Department of Life Sciences, Sir Ernst Chain Building-Wolfson Laboratories, Imperial College London, S. Kensington campus, London, SW7 2AZ, UKPresent address: Rothamsted Research, West Common, Harpenden, Hertfordshire, AL5 2JQ, UK.
Mary L
ML
Hamilton
Department of Life Sciences, Sir Ernst Chain Building-Wolfson Laboratories, Imperial College London, S. Kensington campus, London, SW7 2AZ, UK.
Emanuel
E
Franco
Institute of Plant Biology, Biological Research Center of the Hungarian Academy of Sciences, H-6701 Szeged, Hungary.
Zsuzsanna
Z
Deák
Max-Volmer-Institut für Biophysikalische Chemie und Biochemie, Technische Universität Berlin, D-10623 Berlin, Germany.
Eberhard
E
Schlodder
Institute of Plant Biology, Biological Research Center of the Hungarian Academy of Sciences, H-6701 Szeged, Hungary.
Imre
I
Vass
Department of Life Sciences, Sir Ernst Chain Building-Wolfson Laboratories, Imperial College London, S. Kensington campus, London, SW7 2AZ, UK p.nixon@imperial.ac.uk.
Peter J
PJ
Nixon
D016825:Chlamydomonas reinhardtii / Q000201:enzymology* / Q000235:genetics / Q000528:radiation effects; D002736:Chloroplasts / Q000378:metabolism; D003573:Cytochrome b Group / Q000235:genetics / Q000378:metabolism*; D004579:Electron Transport; D008027:Light; D009154:Mutation; D010100:Oxygen / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000235:genetics / Q000378:metabolism*; D021122:Protein Subunits; D011993:Recombinant Fusion Proteins
D003573:Cytochrome b Group; D045332:Photosystem II Protein Complex; D021122:Protein Subunits; D011993:Recombinant Fusion Proteins; C019313:cytochrome b559; D010100:Oxygen
Chloroplast mutant; PSII repair; Photoinhibition