Photosystem II-Dependent Electron Transfer Reactions in a Sub-Thylakoid Membrane Preparation of Spirulina Platensis.

Abstract

Thylakoid membrane vesicles prepared by sonic oscillation of intact trichomes of the cyanobacterium Spirulina platensis exhibit unusual photosynthetic electron transfer reactions. The electron acceptor ferricyanide is generally employed as an unambiguous, PS I-dependent acceptor in measurements of whole chains (PS I and PS II) electron transfer activity in isolated chloroplast thylakoids. However, reduction of this acceptor by the cyanobacterial membrane vesicles is more complex. Steady-state kinetic analyses show that three distinct ferricyanide reduction sites are operative in these membranes. One site, inhibited by DBMIB addition, functions at the reducing side of PS I. A second site occurs in the interphotosystem chain prior to the point where DBMIB inhibits electron transfer. The presence of Ca('2+) or La('3+) stimulates this reaction by screening negative surface charges and facilitating the access of anionic ferricyanide to this interphotosystem site. The third site of ferricyanide photoreduction is the most complex and functions in the region of the PS II primary acceptor, Q, prior to the site where DCMU blocks electron transfer. Ferricyanide reduction at this site is dependent on the presence of cations and the pH of the external aqueous medium. At pH values near neutrality, a trivalent cation, La('3+), mediates this reaction, while divalent cations are effective, but to a lesser extent, only under more acidic conditions (pH < 6.5). Divalent cations mediate DCMU-insensitive ferricyanide photoreduction in membranes subjected to mild trypsin proteolysis, which appears to eliminate polypeptide(s) carrying negatively charged groups from the vicinity of Q, thus enabling a divalent cation to mediate ferricyanide photoreduction. It is unlikely that these multiple ferricyanide reduction sites are artifactual. Permeabilized intact cells also demonstrate multiple ferricyanide reduction reactions. Furthermore, the membrane vesicles actively evolve oxygen, phosphorylate, and retain complete sensitivity to inhibition by DCMU. These results not only reveal ferricyanide access to reduction at more than one site on the membrane; they also indicate that the membrane surface topology in this cyanobacterium is quite different from that of higher plants, specifically with respect to the primary acceptor region where Q exists in a negatively-charged, proteinaceous environment and is accessible to direct oxidation by ferricyanide.