Numbers and functions of plastoquinone molecules associated with photosystem II preparations from Synechococcus sp.

The contents of plastoquinone and PS II electron carriers (Q_B and Z) of four PS II preparations from the thermophilic cyanobacterium Synechococcus sp. were determined. (1) The oxygen-evolving preparations and the PS II reaction center complexes contained about three and two plastoquinone molecules per PS II, respectively. CP2-b, which represents the functional core of the reaction center complex (Yamagishi, A. and Katoh, S. (1985) Biochim. Biophys. Acta 807, 74-80), had a reduced amount of plastoquinone which is just comparable with the Q_A content, whereas no plastoquinone was detected in CP2-c, the antenna chlorophyll-protein containing only the 40 kDa subunit. (2) The four preparations were essentially free from vitamin K₁. (3) The occurrence of Q_B and Z in the PS II preparations were determined by measuring oxidation kinetics of Q^-_A, as well as effects of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, ferricyanide and benzidine on the Q^-_A oxidation with repetitive flash technique. EPR measurements were also carried out to determine Signal II_s and II_f. Q_B and Z were present in the oxygen-evolving preparations, while the PS II reaction center complexes had Z, but no or only a residual amount of Q_B. CP2-b lacked both Q_B and Z. (4) Q^-_A was oxidized partly by a back electron transport to Z⁺ and partly by ferricyanide added in the reaction center complexes. No back reaction between Q^-_A and P-680⁺ was detected in CP2-b, which instead showed absorption changes indicative of the triplet formation. It is concluded that three plastoquinone molecules (but not vitamin K₁) function as Q_A, Q_B and Z and, besides them, there is no extra bound plastoquinone molecule which serves as an additional electron donor or acceptor in the vicinity of the PS II reaction center. The results also suggest that Q_A, pheophytin and P-680 are located in the interior of the reaction center complexes, but are exposed on removal of the 40 kDa subunit.