DELETION MUTATIONS IN A LONG HYDROPHILIC LOOP IN THE PHOTOSYSTEM-II CHLOROPHYLL-BINDING PROTEIN CP43 IN THE CYANOBACTERIUM SYNECHOCYSTIS SPPCC6803

In order to investigate the role and function of the hydrophilic regio n between transmembrane regions V and CI in the photosystem II core an tenna protein CP43, we introduced eight different deletions in psbC of Synechocystis sp; PCC 6803 resulting in a loss of 7-11 codons in evol utionary conserved domains in this region. All deletions resulted in a n obligate photoheterotrophic phenotype (requirement of glucose for ce ll growth) and the absence of any detectable oxygen evolution activity . The various deletion mutations showed a different impact on the amou nt of CP43 in the thylakoid, ranging from wild-type levels of (a now s lightly smaller) CP43 to no detectable CP43 at all. All deletions led to a decrease in the amount of the D1 and D2 proteins in the thylakoid s with a larger effect on D2 than on D1. CP47, the other major chlorop hyll-binding protein, was present in reduced but significant amounts i n the thylakoid. Herbicide binding (diuron) was lost in all but one mu tant indicating the PSII components are not assembled into functionall y intact complexes. Fluorescence-emission spectra confirmed this notio n. This indicates that the large hydrophilic loop of CP43 plays an imp ortant role in photosystem II, and even though a shortened CP43 is pre sent in thylakoids of most mutants, functional characteristics resembl e that of a mutant with interrupted psbC.