Trimeric forms of the photosystem I reaction center complex pre-exist in the membranes of the cyanobacterium Spirulina platensis.

Oligomeric and monomeric forms of chlorophyll-protein complexes of photosystem I (PSI) have been isolated from the mesophilic cyanobacterium Spirulina [(1992) FEBS Lett. 309, 340-342]. Electron microscopic analysis of the complexes showed that the oligomeric form is a trimer of the shape and dimensions similar to those of the trimer from thermophilic cyanobacteria. The chlorophyl ratio in the isolated trimer and monomer 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 used as an intrinsic probe for the trimeric complex. The 77K circular dichroism spectrum of the trimeric form is much more similar to that of Spirulina membranes than the spectrum of the monomer. Thus, the trimeric PSI complexes exist and dominate in the Spirulina membranes.