TRIMERIC FORMS OF THE PHOTOSYSTEM-I REACTION-CENTER COMPLEX PRE-EXISTIN THE MEMBRANES OF THE CYANOBACTERIUM SPIRULINA-PLATENSIS

Oligomeric and monomeric forms of chlorophyll-protein complexes of pho tosystem I (PSI) have been isolated from the mesophilic cyanobacterium Spirulina [(1992) FEBS Lett. 309, 340-342]. Electron microscopic anal ysis of the complexes showed that the oligomeric form is a trimer of t he shape and dimensions similar to those of the trimer from thermophil ic cyanobacteria. The chlorophyl ratio in the isolated trimer and mono mer was found to be 7:3. The trimeric form of PSI complex in contrast to the monomeric one contains the chlorophyll emitting at 760 nm (77K) , which is also found in Spirulina membranes and therefore could be us ed as an intrinsic probe for the trimeric complex. The 77K circular di chroism spectrum of the trimeric form is much more similar to that of Spirulina membranes than the spectrum of the monomer. Thus, the trimer ic PSI complexes exist and dominate in the Spirulina membranes.