MUTATION OF CHLOROPHYLL LIGANDS IN THE CHLOROPHYLL-BINDING CP47-PROTEIN AS STUDIED IN A SYNECHOCYSTIS SP PCC-6803 PHOTOSYSTEM-I-LESS BACKGROUND

Site-directed mutations have been introduced to replace conserved hist idine residues in the chlorophyll-binding protein CP47 of photosystem II (PS II) in a PS I-less/apcE(-) background strain of the cyanobacter ium Synechocystis sp. PCC 6803. In thylakoids isolated from such a sys tem, the degree of loss of the 695-nm fluorescence emission maximum at 77 K compared to that at 685 nm generally was consistent with the dec rease in oxygen evolution rates measured at saturating light intensity . Taking into account that in the absence of CP47 and PS I some chloro phyll remains detectable in cells, the relative 695-nm fluorescence em ission and the rate of oxygen evolution also correlate with the relati ve amount of chlorophyll per cell and with the number of PS II reactio n centers on a chlorophyll basis. Interestingly, the 77 K fluorescence excitation spectra monitoring 695-nm emission of thylakoids from the CP47 His-to-Tyr mutants in a photosystem I-less/apcE(-) background sho wed increases in the 413- and 531-nm absorption regions, compared to s pectra of thylakoids from the background strain. These wavelengths coi ncide with absorption maxima of pheophytin. No increase in the 531-nm excitation band was observed in thylakoids from mutants lacking PS II or with a His-to-Asn mutation. These results are interpreted to indica te that replacement of conserved histidine residues by tyrosine in CP4 7 leads to the loss of Mg2+ from chlorophyll, resulting in the formati on of pheophytin, or to the binding of pheophytin (rather than chlorop hyll) at a particular pigment-binding site of CP47 during biogenesis a nd assembly of the protein. It was observed that the light-harvesting efficiency of CP47 His mutants was lower judging from the light intens ity dependence of electron transport and analysis of fluorescence deca y kinetics. This suggests that the presence of pheophytin in the anten na decreases antenna efficiency.