title: Site-directed mutagenesis identifies a tyrosine radical involved in the photosynthetic oxygen-evolving system.
abstract: Photosynthetic oxygen evolution takes place in the thylakoid protein complex known as photosystem II. The reaction center core of this photosystem, where photochemistry occurs, is a heterodimer of homologous polypeptides called D1 and D2. Besides chlorophyll and quinone, photosystem II contains other organic cofactors, including two known as Z and D. Z transfers electrons from the site of water oxidation to the oxidized reaction center primary donor, P+.680, while D+. gives rise to the dark-stable EPR spectrum known as signal II. D+. has recently been shown to be a tyrosine radical. Z is probably a second tyrosine located in a similar environment. Indirect evidence indicates that Z and D are associated with the D1 and D2 polypeptides, respectively. To identify the specific tyrosine residue corresponding to D, we have changed Tyr-160 of the D2 polypeptide to phenylalanine by site-directed mutagenesis of a psbD gene in the cyanobacterium Synechocystis 6803. The resulting mutant grows photosynthetically, but it lacks the EPR signal of D+.. We conclude that D is Tyr-160 of the D2 polypeptide. We suggest that the C2 symmetry in photosystem II extends beyond P680 to its immediate electron donor and conclude that Z is Tyr-161 of the D1 polypeptide.
authors.affiliation: Michigan State University-Department of Energy, East Lansing 48824.
authors.forename: R J
authors.initials: RJ
authors.lastname: Debus
authors.forename: B A
authors.initials: BA
authors.lastname: Barry
authors.forename: G T
authors.initials: GT
authors.lastname: Babcock
authors.forename: L
authors.initials: L
authors.lastname: McIntosh
mesh_terms: D002734:Chlorophyll / Q000235:genetics* / Q000378:metabolism; D000458:Cyanobacteria / Q000235:genetics* / Q000378:metabolism; D004262:DNA Restriction Enzymes; D004578:Electron Spin Resonance Spectroscopy; D005796:Genes; D045342:Light-Harvesting Protein Complexes; D009154:Mutation; D010100:Oxygen / Q000378:metabolism*; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins / Q000235:genetics* / Q000378:metabolism
chemical_list: D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D002734:Chlorophyll; D004262:DNA Restriction Enzymes; D010100:Oxygen