Photosynthetic electron transport by Zwittergent-extracted chloroplasts requires solubilized plastocyanin

Abstract

Evidence from numerous laboratories had demonstrated that light plays a regulatory role in photosynthesis through the modulation of key enzymes [1-3]. One system that has been proposed to account for light-dependent enzyme modulation involves membrane-bound cyst(e)ine-containing proteins, designated 'light effect mediators' or 'LEMs'-components proposed to occur in the oxidized (disulfide) state in the dark and in the reduced (sulfhydryl) state in the light [2,4]. It is visualized that chloroplasts contain two LEMs (both on the acceptor side of photosystem I) that, following reduction by non-cyclic electron transport, modulate enzymes of the stroma. One of the proposed LEM components can be solubilized from peas by extracting chloroplast membranes with a Zwittergent detergent [5,6]. In the presence of an artificial electron donor, ascorbate/ 2,6-dichlorophenolindophenol (DCPIP), the Zwittergent-treated chloroplast membranes promoted the photoregulation of NADP-malate dehydrogenase of the stroma. Photoregulation in this system was dependent on the solubilized protein fraction that, based on SDS gel electrophoresis, contained several protein components. In experiments designed to determine the effect of Zwittergent extraction on enzyme photoregulation by systems under investigation in our laboratory (i.e., the ferredoxin/thioredoxin and ferralterin systems