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 size of 20 nm were identified on thylakoid membranes of the cyanobacterium Synechocystis sp. strain BO 9201 by freeze-fracture analysis. Monomeric and trimeric photosystem I complexes were isolated. The size of monomeric photosystem 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 photosystem I of Synechocystis sp. strain BO 9201 are similar with fluorescence maxima at 722 nm. Comparing the room temperature absorption spectra of trimeric and monomeric photosystem I the trimeric form exhibits an additional shoulder at 696 nm, especially obvious by derivative or difference spectroscopy. The absorption band at 696 nm is also present in longterm green light adapted cells of Synechocystis sp. BO 9201 where the photosystem I : photosystem II ratio is increased and can be used as an in vivo indicator for trimeric photosystem I in this species.