Organization and function of photosystem I trimers and monomers of the cyanobacterium Spirulina platensis

Trimeric complexes of photosystem I (PSI) of Spirulina contain the extreme longwave chlorophyll (Chl) form absorbing at 735 nm (Chl735) and emitting a t 760 mn (F760). The intensity of F760 is maximal when P700 is reduced and decreases when P700 is oxidized (P700(+)). Chl735 originates from interacti on of Chi molecules, located on the surface of the monomeric subunits formi ng trimer: (i) deconvolution of 77 K absorption spectrum of PSI trimers sho ws that each monomer contains two molecules of Chl735; (ii) transfer of PSI trimers into monomers in cyanobacterial membranes at "high salt" is accomp anied by decrease of F760 (Chl735 disaggregation), while trimerization of m onomers at,,low salt" results the recovery of F760 (Chl735 formation); (iii ) incubation of PSI monomers in liposomes causes reconstitution of trimers containing Chl735. Illumination of PSI complexes with intense light bleache s bulk Chls but does not effect P700. The non-linear relationship between t he kinetics of P700 photooxidation and F760 quenching in PSI trimers at 77 K is interpreted as energy exchange (via Chl735) between antennae of monome rs and quenching of the excess energy by P700(+). The energy exchange betwe en monomers promotes dissipation of the overexcitation into heat. Thus both PSI trimers and monomers contribute in electron transport whereas PSI trim ers in addition protect complex against photodestruction.