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...
The PsbO protein is an essential extrinsic subunit of photosystem II, the pigment–protein complex responsible for light‐driven water splitting. Water oxidation in photosystem II supplies electrons to the photosynthetic electron transfer chain and is accompanied by proton release and oxygen evolution. While the electron transfer steps in this process are well defined and characterized, the driving forces acting on the liberated protons, their dynamics and their destiny are all largely unknown. It was suggested that PsbO undergoes proton‐induced conformational changes and forms hydrogen bond networks that ensure prompt proton removal from the catalytic site of water oxidation, i.e. the Mn4CaO5 cluster. This work reports the purification and characterization of heterologously expressed PsbO from green algae Chlamydomonas reinhardtii and two isoforms from the higher plant Solanum tuberosum (PsbO1 and PsbO2). A comparison to the spinach PsbO reveals striking similarities in intrinsic protein fluorescence and CD spectra, reflecting the near‐identical secondary structure of the proteins from algae and higher plants. Titration experiments using the hydrophobic fluorescence probe ANS revealed that eukaryotic PsbO proteins exhibit acid–base hysteresis. This hysteresis is a dynamic effect accompanied by changes in the accessibility of the protein's hydrophobic core and is not due to reversible oligomerization or unfolding of the PsbO protein. These results confirm the hypothesis that pH‐dependent dynamic behavior at physiological pH ranges is a common feature of PsbO proteins and causes reversible opening and closing of their β‐barrel domain in response to the fluctuating acidity of the thylakoid lumen.
BackgroundThe facultative anoxygenic photosynthetic bacterium Rhodospirillum rubrum exhibits versatile metabolic activity allowing the adaptation to rapidly changing growth conditions in its natural habitat, the microaerobic and anoxic zones of stagnant waters. The microaerobic growth mode is of special interest as it allows the high-level expression of photosynthetic membranes when grown on succinate and fructose in the dark, which could significantly simplify the industrial production of compounds associated with PM formation. However, recently we showed that PM synthesis is no longer inducible when R. rubrum cultures are grown to high cell densities under aerobic conditions. In addition a reduction of the growth rate and the continued accumulation of precursor molecules for bacteriochlorophyll synthesis were observed under high cell densities conditions.ResultsIn the present work, we demonstrate that the cell density-dependent effects are reversible if the culture supernatant is replaced by fresh medium. We identified six N-acylhomoserine lactones and show that four of them are produced in varying amounts according to the growth phase and the applied growth conditions. Further, we demonstrate that N-acylhomoserine lactones and tetrapyrrole compounds released into the growth medium affect the growth rate and PM expression in high cell density cultures.ConclusionsIn summary, we provide evidence that R. rubrum possesses a Lux-type quorum sensing system which influences the biosynthesis of PM and the growth rate and is thus likely to be involved in the phenotypes of high cell density cultures and the rapid adaptation to changing environmental conditions.
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