GENERATION OF ELECTRIC-POTENTIAL DIFFERENCE BY PHOTOSYSTEM-II FROM THERMOPHILIC CYANOBACTERIA

The generation of electric potential difference by proteoliposomes con taining reaction center Photosystem II from the thermophilic cyanobact erium Synechococcus elongatus was measured by the direct electrometric method. The fast phase of membrane potential generation was found to be mainly due to the charge separation in reaction centers oriented wi th donor side facing outside and acceptor side inside. A dark relaxati on phase of membrane potential with characteristic time of 50 mu sec i s probably associated with charge recombination between the primary qu inone acceptor Q(A)(-) and the primary electron donor P-680(+) in Phot osystem II reaction centers with inactive oxygen evolving complex. The disappearance of a similar to 50 mu sec relaxation phase and increasi ng amplitude of a fast phase of membrane potential in the presence of MnCl2 as well as reversion of these effects on addition of hydroxylami ne indicate that the reaction between tyrosine Y-Z and photooxidized P -680(+) is electrogenic. The amplitude of this phase is similar to 18% of the photoelectric response induced by charge separation between P- 680 and Q(A). Redox titration of the fast phase of membrane potential showed that the midpoint potential of the primary quinone acceptor in the proteoliposomes containing Photosystem II reaction center is -0.1 V at pH 7.5. There is no low-potential Q(A) in cyanobacteria.