A Glutathione Redox Effect on Photosynthetic Membrane Expression in Rhodospirillum rubrum

Abstract

The formation of intracytoplasmic photosynthetic membranes by facultative anoxygenic photosynthetic bacteria has become a prime example for exploring redox control of gene expression in response to oxygen and light. Although a number of redox-responsive sensor proteins and transcription factors have been characterized in several species during the last several years in some detail, the overall understanding of the metabolic events that determine the cellular redox environment and initiate redox signaling is still poor. In the present study we demonstrate that in Rhodospirillum rubrum, the amount of photosynthetic membranes can be drastically elevated by external supplementation of the growth medium with the low-molecular-weight thiol glutathione. Neither the widely used reductant dithiothreitol nor oxidized glutathione caused the same response, suggesting that the effect was specific for reduced glutathione. By determination of the extracellular and intracellular glutathione levels, we correlate the GSH/GSSG redox potential to the expression level of photosynthetic membranes. Possible regulatory interactions with periplasmic, membrane, and cytosolic proteins are discussed. Furthermore, we found that R. rubrum cultures excrete substantial amounts of glutathione to the environment.It is well established that the thiol-containing tripeptide glutathione (␥-Glu-Cys-Gly [GSH]) is the most abundant redox buffer in all eukaryotic and in many prokaryotic cells and that GSH is critically important for the defense of oxidative stress which inevitably is associated with respiratory life. Various functions and mechanisms where GSH is involved in bacteria are summarized in a recent review (22). During many of these reactions, two molecules of GSH are oxidized to glutathione disulfide (GSSG), and the reduced form has to be recycled by the enzyme glutathione reductase at the expense of NADPH. A signaling role for GSH, however, is thus far not readily established. In bacteria, the reversible formation of intramolecular or intermolecular disulfide bonds of cysteines of regulatory proteins appears to be a common motif in molecular redox switches. For example, the ArcB sensor of Escherichia coli or the PpsR-and RegB-type regulators in facultative anoxygenic photosynthetic bacteria undergo reversible thiol/disulfide switches when exposed to different redox conditions in vitro (21, 32). The latter group of bacteria (Rhodospirillaceae) has contributed significantly to the recent paradigms of redox signaling and control of gene expression due to its facultative phototrophic lifestyle, which allows these bacteria to adapt their energy metabolism to the redox conditions of the environment. Under conditions of limiting oxygen, the Rhodospirillaceae induce the biosynthesis of an extensive system of pigmented intracytoplasmic membranes (ICM) which harbor photosynthetic reaction centers and light-harvesting complexes. The major environmental factor that determines the amount of ICM is the availability of oxygen, since aerobic growth conditio...

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Quantitative Proteomic Analysis of the Rice (Oryza sativa L.) Salt Response

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