A MUTATION IN THE D-DE LOOP OF D-1 MODIFIES THE STABILITY OF THE S(2)Q(A)(-) AND S(2)Q(B)(-) STATES IN PHOTOSYSTEM-II

Photosystem II electron transfer, charge stabilization, and photoinhib ition were studied in three site-specific mutants of the D-1 polypepti de of Synechocystis PCC 6803: E243K, E229D, and CA1 (deletion of three glutamates 242-244 and a substitution, glutamine-241 to histidine). T he phenotypes of the E229D and E243K mutants were similar to that of t he control strain (AR) in all of the studied aspects. The characterist ics of CA1 were very different. Formate, which inhibits the Q(A)(-) to Q(B)(-) reaction, was severalfold less effective in CA1 than in AR. T he S(2)Q(A)(-) and S(2)Q(B)(-) states were stabilized in CA1. It was p reviously shown that the electron transfer between Q(A)(-) and Q(B) wa s modified in CA1 (P Maenpaa, T. Kallio, P. Mule, G. Salih, E.-M. Aro, E. Tyystjarvi, C. Jansson [1993] Plant Mol Biol 22: 1-12). A change i n the redox potential of the Q(A)/Q(A)(-) couple, which renders the re oxidation of Q(A)(-) by back or forward reactions more difficult, coul d explain the phenotype of CA(1). Although the rates of photoinhibitio n measured as inhibition of oxygen evolution, Chl fluorescence quenchi ng, and decrease of thermoluminescence B and Q bands were similar in A R and CA1, the CA1 strain more quickly reached a state from which the cells were unable to recover their activity. The results described in this paper suggest that a modification in the structure of the D-de lo op of D-1 could influence the properties of the couple Q(A)/Q(A)(-) in D-2 and the mechanism of recovery from photoinhibition.