Trimeric photosystem I complexes exist in vivo in thylakoid membranes of the Synechocystis strain BO 9201 and differ in absorption characteristics from monomeric photosystem I complexes

Particles of trimeric photosystem I complexes with triple symmetry and a si ze of 20 nm were identified on thylakoid membranes of the cyanobacterium Sy nechocystis sp. strain BO 9201 by freeze-fracture analysis. Monomeric and t rimeric photosystem I complexes were isolated. The size of monomeric photos ystem I complexes is 15 x 7 nm, that of trimeric 20 x 7 nm as determined by electron microscopy after negative staining. Freeze fracture of liposomes with reconstituted mono- or trimeric photosystem I complexes showed a size of 19 nm for trimeric- and 15 nm for monomeric complexes respectively. The 77 degrees K fluorescence characteristics of monomeric and trimeric photosy stem I of Synechocystis sp. strain BO 9201 are similar with fluorescence ma xima at 722 nm. Comparing the room temperature absorption spectra of trimer ic and monomeric photosystem I the trimeric form exhibits an additional sho ulder at 696 nm, especially obvious by derivative or difference spectroscop y. The absorption band at 696 nm is also present in longterm green light ad apted cells of Synechocystis sp. BO 9201 where the photosystem I : photosys tem II ratio is increased and can be used as an in vivo indicator for trime ric photosystem I in this species.