Site-directed mutagenesis identifies a tyrosine radical involved in the photosynthetic oxygen-evolving system

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, P680+·, 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 sitedirected 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.