ORGANIZATION AND FUNCTION OF PHOTOSYSTEM-I PIGMENT-PROTEIN COMPLEXES OF THE CYANOBACTERIUM SPIRULINA

The data on the organization and function of the photosystem I pigment -protein complexes of the cyanobacterium Spirulina platensis and the c haracteristics of pigment antenna of the photosystem I monomeric and t rimeric core complexes are presented and discussed. We proved that the photosystem I complexes in the cyanobacterial membrane preexist mainl y as trimes, though both types of complexes contribute in the photosyn thetic electron transport. In contrast to the monomers, the antenna of the photosystem I trimeric complexes of Spirulina contain the extreme longwave chlorophyll form absorbing at 735 nm and emitting at 760 nm (77 K). The intensity of fluorescence at 760 nm depends strongly on th e P700 redox state: it is maximum with the reduced P700 and strongly d ecreased with the oxidized P700 which is the most efficient quencher o f fluorescence at 760 nm. The energy absorbed by the extra longwave ch lorophyll form is active in the photooxidation of P700 in the trimeric complex. The data obtained indicate that the longwave form of chlorop hyll originates from interaction of the chlorophyll molecules localize d on different monomeric subunits forming photosystem I rimer. Kinetic analysis of the P700 photooxidation and light-induced quenching of fl uorescence at 760 nm (77 K) allows the suggestion that the excess ener gy absorbed by the antenna of monomeric subunits within the trimer mig rates via the extreme longwave chlorophyll to the P700 cation radical and is quenched, which prevents the complex photodestruction.