INTERACTIONS AMONG THE PHOTOSYSTEM-II OXYGEN-FORMING COMPLEXES - A NOVEL MODEL FOR DAMPING OF O2 OSCILLATIONS

It is commonly assumed that the oxygen-evolving centers (OEC's) of PS II operate independently, meaning that there is no transfer of charge among adjacent units, and therefore, that in order to evolve an oxygen molecule each such OEC has to accumulate four charges, as it progress es through the 'S(n)' states. This assumption predicts that when a dar k-adapted photosynthetic system is exposed to a series of single turno ver flashes, the oxygen yield following the third flash will be the pr oduct of the fractions of the population of OEC's hit by each of these flashes, and of their initial distribution of S-states. Indeed, measu rements with the chlorophyte Chlorella vulgaris and the diatom Phaeoda ctylum tricornutum show that their oxygen centers are independent, the former as expected from previous measurements, and predicted by stand ard theory. However, by this criterion, the centers of the cyanobacter ium Synechococcus leopoliensis are not independent. Moreover, in S. le opoliensis the apparent cross-sections derived from the saturation cur ves for the individual flashes differ from each other, whereas in the former two species, they are constant. Another criterion of independen ce is the extent of coherency in the Joliot-Kok (period four) oscillat ions of the flash yield of oxygen. The more independent the OEC's are, the more these oscillations will persist. If the charge flows through a common pool, as in hydrogen formation, the (period two) oscillation s are rapidly damped out. By this criterion S. leopoliensis again diff ers from the classic Chlorella pattern, since the oscillations in this cyanobacterium are rapidly damped. S. leopoliensis differs from Chlor ella in additional parameters: oxygen is maximal on the fourth, not th e third, flash and oxygen is formed on the very first flash irrespecti ve of the length of dark adaptation. We propose a new model integratin g and quantitatively explaining these observations, by allowing 30% of the oxygen-forming centers in S. leopoliensis to exchange charge. A d ifferent dark-state distribution of the S-states, maximal at S0, is al so required. The possibility of charge exchange in other systems is di scussed.