Small subunits of Photosystem I reaction center complexes from Synechococcus elongatus. II. The psaE gene product has a role to promote interaction between the terminal electron acceptor and ferredoxin

Function of a subunit polypeptide (the psaE gene product) of Photosystem I (PS I) reaction center complexes was investigated by comparing the reactivity of the reduced iron-sulfur centers (F_A/F_B)^- with ferredoxin among Synechococcus PS I complexes which had been variously depleted of this polypeptide. Ferredoxin at or below 1 μM can accept electrons from (F_A/F_B)^- effectively competing with the back reaction between P-700⁺ and (F_A/F_B)^- in the thylakoid membranes and PS I complexes that contained all the eight small subunits. The high reactivity of (F_A/F_B)^- with low concentrations of ferredoxin was observed in PS I complexes which contain only the products of psaC, psaD and psaE genes but not in complexes which carry the psaC, psaD, psaL and psaK gene products but no psaE gene product. Varied amounts of the psaE gene product were extracted by treatment with different concentrations of a cationic detergent, dodecyltrimethylammonium bromide, and 2.5 M NaCl. The solubilized polypeptide was then reconstituted to the depleted complexes. The magnitudes of the back reaction that could be suppressed by addition of ferredoxin at or below 1 μM were well correlated to the amounts of the psaE polypeptide remained bound or rebound to the complexes. It is concluded that the product of the psaE gene has a role to promote the interaction between the terminal bound electron acceptor and ferredoxin. A high autooxidizability of (F_A/F_B)^- and contrasting effects of lipophilic cations and anions on the rate of the back reaction from (F_A/F_B)^- to P-700⁺ were also reported.