Supramolecular organisation of the photosynthetic chain in anoxygenic bacteria

AbstractThis minireview summarizes our present view of the supramolecular organization of the photosynthetic apparatus of Rhodobacter sphaeroides and Rhodobacter capsulatus. These two species present a close association between two reaction centers (RCs), one cytochrome (cyt) bc1 and one cyt c. In R. sphaeroides, the RCs are only partially surrounded by LH1 complexes. This open ring of LH1 complexes is required for an efficient photoinduced cyclic electron transfer only under conditions where the quinone pool totally reduced. When the quinone pool is partially oxidized, a closed ring of LH1 complexes around the RCs does not impair the exchange of quinone molecules between the RC and the cyt bc1 complex. To explain the efficient photochemistry of the various species which possess a RC surrounded by a closed ring of LH, it is proposed that their quinone pool is partially oxidized even under anaerobic condition

Fast oxidation of the primary electron acceptor under anaerobic conditions requires the organization of the photosynthetic chain of Rhodobacter sphaeroides in supercomplexes

AbstractThe kinetics of reoxidation of the primary acceptor Qa has been followed by measuring the changes in the fluorescence yield induced by a series of saturating flashes in intact cells of Rhodobacter sphaeroides in anaerobic conditions. At 0 °C, about half of Qa− is reoxidized in about 200 ms while reoxidation of the remaining fraction is completed in several seconds to minutes. The fast phase is associated with the transfer of ubiquinone formed at site Qo of the cytochrome bc1 complex while the slowest phase is associated with the diffusion of ubiquinone present in the membrane prior to the flash excitation. The biphasic kinetics of Qa− oxidation is interpreted assuming that the electron chain is organized in supercomplexes that associate two RCs and one cyt bc1 complex, which allows a fast transfer of quinone formed at the level of cyt bc1 complex to the RCs. In agreement with this model, the fast phase of Qa− reoxidation is inhibited by myxothiazol, a specific inhibitor of cyt bc1. The PufX-deleted mutant displays only the slowest phase of Qa− oxidation; it is interpreted by the lack of supramolecular organization of the photosynthetic chain that leads to a larger average distance between cyt bc1 and RCs

Comparative genomics of Aeschynomene symbionts : insights into the ecological lifestyle of nod-independent photosynthetic Bradyrhizobia

Tropical aquatic species of the legume genus Aeschynomene are stem- and root-nodulated by bradyrhizobia strains that exhibit atypical features such as photosynthetic capacities or the use of a nod gene-dependent (ND) or a nod gene-independent (NI) pathway to enter into symbiosis with legumes. In this study we used a comparative genomics approach on nine Aeschynomene symbionts representative of their phylogenetic diversity. We produced draft genomes of bradyrhizobial strains representing different phenotypes: five NI photosynthetic strains (STM3809, ORS375, STM3847, STM4509 and STM4523) in addition to the previously sequenced ORS278 and BTAi1 genomes, one photosynthetic strain ORS285 hosting both ND and NI symbiotic systems, and one NI non-photosynthetic strain (STM3843). Comparative genomics allowed us to infer the core, pan and dispensable genomes of Aeschynomene bradyrhizobia, and to detect specific genes and their location in Genomic Islands (GI). Specific gene sets linked to photosynthetic and NI/ND abilities were identified, and are currently being studied in functional analyses

Electron Accepting Units of the Diheme Cytochrome c TsdA, a Bifunctional Thiosulfate Dehydrogenase/Tetrathionate Reductase

The enzymes of the thiosulfate dehydrogenase (TsdA) family are wide-spread diheme c-type cytochromes. Here, redox carriers were studied mediating the flow of electrons arising from thiosulfate oxidation into respiratory or photosynthetic electron chains. In a number of organisms, including Thiomonas intermedia and Sideroxydans lithotrophicus the tsdA gene is immediately preceded by tsdB encoding for another diheme cytochrome. Spectrophotometric experiments in combination with enzymatic assays in solution showed that TsdB acts as an effective electron acceptor of TsdA in vitro when TsdA and TsdB originate from the same source organism. While TsdA covers a range from -300 mV to +150 mV, TsdB is redox active between -100 to +300 mV, thus enabling electron transfer between these hemoproteins. The three-dimensional structure of the TsdB-TsdA fusion protein from the purple sulfur bacterium Marichromatium purpuratum was solved by X-ray crystallography to 2.75 Å resolution providing insights into internal electron transfer. In the oxidized state, this tetraheme cytochrome c contains three hemes with axial His/Met ligation, while heme 3 exhibits the His/Cys coordination typical for TsdA active sites. Interestingly, thiosulfate is covalently bound to Cys330 on heme 3. In several bacteria including Allochromatium vinosum, TsdB is not present, precluding a general and essential role for electron flow. Both, AvTsdA and the MpTsdBA fusion react efficiently in vitro with high potential iron sulfur protein from A. vinosum (Em +350 mV). HiPIP not only acts as direct electron donor to the reaction center in anoxygenic phototrophs but can also be involved in aerobic respiratory chains

The Cyst-Dividing Bacterium Ramlibacter tataouinensis TTB310 Genome Reveals a Well-Stocked Toolbox for Adaptation to a Desert Environment

Ramlibacter tataouinensis TTB310T (strain TTB310), a betaproteobacterium isolated from a semi-arid region of South Tunisia (Tataouine), is characterized by the presence of both spherical and rod-shaped cells in pure culture. Cell division of strain TTB310 occurs by the binary fission of spherical “cyst-like” cells (“cyst-cyst” division). The rod-shaped cells formed at the periphery of a colony (consisting mainly of cysts) are highly motile and colonize a new environment, where they form a new colony by reversion to cyst-like cells. This unique cell cycle of strain TTB310, with desiccation tolerant cyst-like cells capable of division and desiccation sensitive motile rods capable of dissemination, appears to be a novel adaptation for life in a hot and dry desert environment. In order to gain insights into strain TTB310's underlying genetic repertoire and possible mechanisms responsible for its unusual lifestyle, the genome of strain TTB310 was completely sequenced and subsequently annotated. The complete genome consists of a single circular chromosome of 4,070,194 bp with an average G+C content of 70.0%, the highest among the Betaproteobacteria sequenced to date, with total of 3,899 predicted coding sequences covering 92% of the genome. We found that strain TTB310 has developed a highly complex network of two-component systems, which may utilize responses to light and perhaps a rudimentary circadian hourglass to anticipate water availability at the dew time in the middle/end of the desert winter nights and thus direct the growth window to cyclic water availability times. Other interesting features of the strain TTB310 genome that appear to be important for desiccation tolerance, including intermediary metabolism compounds such as trehalose or polyhydroxyalkanoate, and signal transduction pathways, are presented and discussed

Connectivity of the intracytoplasmic membrane of Rhodobacter sphaeroides: a functional approach

International audienc

Tribute in memory of Jacques Breton (1942–2018)

International audienceJacques Breton spent his 39 years of professional life at Saclay, a center of the French Atomic Energy Commission. He studied photosynthesis with various advanced biophysical tools, often developed by himself and his numerous coworkers, obtaining a large number of new information on the structure and the functioning of antenna and of reaction centers of plants and bacteria: excitation migration in the antenna, orientation of molecules, rate of primary reactions, binding of pigments and electron transfer cofactors. Although it is much too short to illustrate his impressive work, we hope that this contribution will help maintaining the souvenir of Jacques Breton as an active and enthusiastic person, full of qualities, devoted to research and to his family as well. We include personal comments from N. E. Geacintov, A. Dobek, W. Leibl, M. Vos and W. W. Parson