Analysis of chimeric spinach/cyanobacterial CP43 mutants of Synechocystis sp. PCC 6803: The chlorophyll-protein CP43 affects the water-splitting system of photosystem II

Mutants of the cyanobacterium Synechocystis sp. PCC 6803 have been generated in which parts of psbC (the gene encoding the Photosystem II chlorophyll-protein CP43) have been replaced with the homologous gene fragment from spinach. Upon the replacement of all but the 3′ 84 bp of the cyanobacterial psbC gene with the homologous fragment from spinach, an obligate photoheterotrophic mutant was generated. Two photoautotrophic derivatives of this mutant were made reincorporating 3′ cyanobacterial sequences back into the spinach psbC gene of the mutant. These two mutants are similar to each other, carrying a chimeric CP43 with the N-terminal half from spinach. These mutants are photosynthetically active at a rate of about half that of wild type, which correlates with a decreased Photosystem II/chlorophyll ratio in these mutants. Thylakoids from the chimeric mutants contain a CP43 protein which migrates slightly more slowly on SDS-polyacrylamide gels than the native Synechocystis CP43. Interestingly, these mutants show significant shifts in thermoluminescence peaks, reflecting altered thermodynamic properties of the back reaction between the acceptor side and the water-splitting system. On the basis of the oscillations of these shifts with number of flashes, we conclude that S₂ is stabilized and S₃ is destabilized in these mutants. This represents evidence for an involvement of CP43 in events associated with water splitting.