PSAL SUBUNIT IS REQUIRED FOR THE FORMATION OF PHOTOSYSTEM-I TRIMERS IN THE CYANOBACTERIUM SYNECHOCYSTIS SP PCC-6803

When membranes of the wild type strain of the cyanobacterium Synechocy stis sp. PCC 6803 were solubilized with detergents and fractionated by sucrose-gradient ultracentrifugation, photosystem I could be obtained as trimers and monomers. We could not obtain trimers from the membran es of any mutant strain that lacked PsaL subunit. In contrast, absence of PsaE, PsaD, PsaF, or PsaJ did not completely abolish the ability o f photosystem I to form trimers. Furthermore, PsaL is accessible to di gestion by thermolysin in the monomers but not in the trimers of photo system I purified from wild type membranes. Therefore, PsaL is necessa ry for trimerization of photosystem I and may constitute the trimer-fo rming domain in the structure of photosystem I.