PREPARATION AND CHARACTERIZATION OF HEAT-STABLE AND VERY ACTIVE OXYGEN-EVOLVING PHOTOSYSTEM-II PARTICLES FROM THE THERMOPHILIC CYANOBACTERIUM, SYNECHOCOCCUS-ELONGATUS

Very active and heat-stable oxygen-evolving photosystem II particles were isolated from the thermophilic cyanobacterium Synechococcus elongatus by treatment of thylakoid membranes with a non-ionic detergent, sucrose monolaurate (SML). The particles were analyzed in a comparison with photosystem II particles prepared with beta-octylglucoside (OG). The two preparations had similar polypeptide compositions, which were caracterized by high levels of polypeptides from phycobilisomes. The ratio of chlorophyll a to Q(A) was 45 and there were four Mn atoms and one tightly bound Ca2+ ion per Q(A) in the particles prepared with SML. The preparations were thermophilic, showing substantial rates of oxygen evolution at temperatures up to 60-degrees-C. The maximum rates attained at 45-degrees-C were as high as 6.0 mmoles O2 mg-1 Chl h-1. PS II particles prepared with OG were similarly thermostable but were less active in oxygen evolution at all temperatures examined. Kinetic analysis of flash-induced absorption transients revealed that about 22% and 28% of photosystem II reaction centers were not associated with the functional Q(B) site in the SML- and OG-particles, respectively. When corrected for the inactive reaction centers, the maximum rates of oxygen evolution by SML- and OG-particles were 7.7 and 7.0 mmoles O2 mg-1 Chl h-1, which correspond to half times of 1.9 and 2.1 ms for the first-order electron transfer, respectively. Comparison of these half times with those of the S-state transition and the release of oxygen indicates that the overall photosystem II electron transport is limited by the reduction of added electron acceptors and not by release of oxygen.