Organization and function of photosystem i trimers and monomers of the cyanobacterium spirulina platensis

摘要

Trimeric complexes of photosystem I (PSI) of Spirulina contain the extreme longwave chlorophyll (Chl) form absorbing at 735 nm (Ch1735) and emitting at 760 nm (F760). The intensity of F760 is maximal when P700 is reduced and decreases when P700 is oxidized (P700~+). Ch1735 originates from interaction of Chl molecules, located on the surface of the monomeric subunits forming trimer: (i) deconvolution of 77 K absorption spectrum of PSI trimers shows that each nonomer contains two molecules of Ch1735; (ii) transfer of PSI trimers into monomer in cyanobacterial membranes at "high salt" is accompanied by decrease of F760 (Chl735 disaggregation), while trimerization of monomers 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 bleaches bulk Chls but does not effect P700. The non-linear relationship between the kinetics of P700 photooxidation and F760 quenching in PSI trimers at 77 K is interpreted as energy exchange (via Ch1735) between antennae of monomers and quenching of the excess energy by P700~+. The energy exchange between monomers promotes dissipation of the overexcitation into heat. Thus both PSI trimers and monomers contribute in electron transport whereas PSI trimers in addition protect complex against photodestruction.