Photosystem I reaction center from the thermophilic cyanobacterium Mastigocladus laminosus

Abstract

Photosystem [PS] I reaction center was isolated from the cyanobacterium M. laminosus. It contained 4 different subunits with MW (as determined by sodium dodecyl sulfate gels) of .apprx. 70,000 (subunit I), 16,000 (subunit II), 11,000 (subunit III) and 10,000 (subunit IV) daltons. The purified reaction center contained .apprx. 100 chlorophyll [Chl] a molecules per P700; however, they could be readily depleted down to .apprx. 50 Chl a per P700 without loss in the photochemical activities. The reaction center was active in cytochrome [Cyt] c photooxidation, but the photooxidation of an acidic Cyt, like the Euglena Cyt 552, required the presence of cations. The purified reaction center was similar in several respects to the PS I reaction centers from higher plants and, especially, to the one isolated from green algae. Subunit I appeared on sodium dodecyl sulfate gels in the same position and possessed the same shape of an apparent double band as the corresponding subunits I of green plants and of algae. Subunits I and II of PS I reaction centers from Mastigocladus, higher plants and green algae showed immunological crossreactivity. This might serve as biochemical evidence for the common evolution of the PS I reaction centers. In higher plants and green algae, subunit II is a product of cytoplasmic ribosomes; and, therefore, a high degree of homology should have been preserved upon transfer of its gene from the prokaryote to the nucleus of the eukaryotes.