EVIDENCE FOR THE EXISTENCE OF TRIMERIC AND MONOMERIC PHOTOSYSTEM-I COMPLEXES IN THYLAKOID MEMBRANES FROM CYANOBACTERIA

In cyanobacteria, solubilization of thylakoid membranes by detergents yields both monomeric and trimeric Photosystem I (PS I) complexes in variable amounts. We present evidence for the existence of both monomeric and trimeric PS I in cyanobacterial thylakoid membranes with the oligomeric state depending 'in vitro' on the ion concentration. At low salt concentrations (i.e. less than or equal to 10 mM MgSO4) PS I is mainly extracted as a trimer from these membranes and at high salt concentrations (i.e. greater than or equal to 150 mM MgSO4) nearly exclusively as a monomer, irrespective of the type of salt used (i.e. mono- or bivalent ions) and the temperature (i.e. 4 degrees C or 20 degrees C). Once solubilized, the PS I trimer is stable over a wide range of ion concentrations (i.e. beyond 0.5 M). A model is presented which suggests a monomer-oligomer equilibrium of PS I, but also of PS II and the cyt. b6/f-complex in the cyanobacterial thylakoid membrane. The possible physiological role of this equilibrium in the regulation of state transitions is discussed.