The role of an extrinsic 9 kDa polypeptide in oxygen evolution by Photosystem II particles from Phormidium laminosum

Abstract

Photosystem II particles from the thermophilic cyanobacterium Phormidium laminosum have been reported to contain an extrinsic 9 kDa polypeptide that can be dissociated by exposure to a glycerol-free medium, with concomitant loss of activity (Stewart, A.C., Ljungberg, U., kerlund, H.E. and Andersson, B. (1985) Biochim. Biophys. Acta 808, 353–362; Stewart, A.C., Siczkowski, M. and Ljungberg, U. (1985) FEBS Lett. 193, 175–179). In the present paper it is shown that a partial recovery of oxygen-evolution activity could be achieved by reconcentrating the particles in the presence of glycerol and an excess of the purified polypeptide. This was shown to be a specific effect of the 9 kDa polypeptide which rebound stoichiometrically. Rapid lowering of the glycerol concentration by dilution gave less inactivation but nearly complete reversibility. When diphenylcarbazide was substituted for water as electron donor no loss of activity was observed. It was concluded that dissociation of the 9 kDa polypeptide caused a reversible inactivation of the oxygen-evolution machinery. Evidence was presented that this could be explained by a slowing, but not complete inhibition, of a dark step in the S-statetransitions. No evidence could be obtained for Cl− or Ca2+ concentrating functions of the kind proposed for certain extrinsic polypeptides associated with Photosystem II in chloroplasts.