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

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

Periplasmic electron carriers and photo-induced electron transfer in the photosynthetic bacterium Ectothiorhodospira sp.

International audienceA detailed analysis of the periplasmic electron carriers of the photosynthetic bacterium Ectothiorhodospira sp. has been performed. Two low mid-point redox potential electron carriers, cytochrome c' and cytochrome c, are detected. A high potential iron-sulfur protein is the only high mid-point redox potential electron transfer component present in the periplasm. Analysis of light-induced absorption changes shows that this high potential iron-sulfur protein acts in vivo as efficient electron donor to the photo-oxidized high potential heme of the Ectothiorhodospira sp. reaction center

Identification of novel genes putatively involved in the photosystem synthesis of Bradyrhizobium sp ORS 278

In aerobic anoxygenic phototrophs, oxygen is required for both the formation of the photosynthetic apparatus and an efficient cyclic electron transfer. Mutants of Bradyrhizobium sp. ORS278 affected in photosystem synthesis were selected by a bacteriochlorophyll fluorescence-based screening. Out of the 9,600 mutants of a random Tn5 insertion library, 50 clones, corresponding to insertions in 28 different genes, present a difference in fluorescence intensity compared to the WT. Besides enzymes and regulators known to be involved in photosystem synthesis, 14 novel components of the photosynthesis control are identified. Among them, two genes, hsIU and hsIV, encode components of a protein degradation complex, probably linked to the renewal of photosystem, an important issue in Bradyrhizobia which have to deal with harmful reactive oxygen species. The presence of homologs in non-photosynthetic bacteria for most of the regulatory genes identified during study suggests that they could be global regulators, as the RegA-RegB system

Effect of Bradyrhizobium photosynthesis on stem nodulation of Aeschynomene sensitiva

Some leguminous species of the genus #Aeschynomene are specifically stem-nodulated by photosynthetic bradyrhizobia. To study the effect of bacterial photosynthesis during symbiosis, we generated a photosynthesis-negative mutant of the #Bradyrhizobium sp. strain ORS278 symbiont of #Aeschynomene sensitiva$. The presence of a functional photosynthetic unit in bacterioids and the high expression of the photosynthetic genes observed in stem nodules demonstrate that the bacteria are photosynthetically active during stem symbiosis. Stem inoculation by the photosynthetic mutant gave a 50% decrease in stem-nodule number, which reduced nitrogen fixation activity and plant growth in the same proportion. These results indicate an important role of bacterial photosynthesis in the efficiency of stem nodulation. (Résumé d'auteur

Supramolecular organization of the photosynthetic apparatus of Rhodobacter sphaeroides.

Native tubular membranes were purified from the purple non-sulfur bacterium Rhodobacter sphaeroides. These tubular structures contain all the membrane components of the photosynthetic apparatus, in the relative ratio of one cytochrome bc1 complex to two reaction centers, and approximately 24 bacteriochlorophyll molecules per reaction center. Electron micrographs of negative-stained membranes diffract up to 25 A and allow the calculation of a projection map at 20 A. The unit cell (a = 198 A, b = 120 A and gamma = 103 degrees) contains an elongated S-shaped supercomplex presenting a pseudo-2-fold symmetry. Comparison with density maps of isolated reaction center and light-harvesting complexes allowed interpretation of the projection map. Each supercomplex is composed of light-harvesting 1 complexes that take the form of two C-shaped structures of approximately 112 A in external diameter, facing each other on the open side and enclosing the two reaction centers. The remaining positive density is tentatively attributed to one cytochrome bc1 complex. These features shed new light on the association of the reaction center and the light-harvesting complexes. In particular, the organization of the light-harvesting complexes in C-shaped structures ensures an efficient exchange of ubihydroquinone/ubiquinone between the reaction center and the cytochrome bc1 complex

Dual role for a bacteriophytochrome in the bioenergetic control of Rhodopsdeudomonas palustris : enhancement of photosystem synthesis and limitation of respiration

In the purple photosynthetic bacterium Rhodopseudomonas palustris, far-red illumination induces photosystem synthesis via the action of the bacteriophytochrome RpBphP1. This bacteriophytochrome antagonizes the repressive effect of the transcriptional regulator PpsR2 under aerobic condition. We show here that, in addition to photosystem synthesis, far-red light induces a significant growth rate limitation, compared to cells grown in the dark, linked to a decrease in the respiratory activity. The phenotypes of mutants inactivated in RpBphP1 and PpsR2 show their involvement in this regulation. Based on enzymatic and transcriptional studies, a 30% decrease in the expression of the alpha-ketoglutarate dehydrogenase complex, a central enzyme of the Krebs cycle, is observed under far-red light. We propose that this decrease is responsible for the down-regulation of respiration in this condition. This regulation mechanism at the Krebs cycle level still allows the formation of the photosynthetic apparatus via the synthesis of key biosynthesis precursors but lowers the production of NADH, i.e. the respiratory activity. Overall, the dual action of RpBphP1 on the regulation of both the photosynthesis genes and the Krebs cycle allows a fine adaptation of bacteria to environmental conditions by enhancement of the most favorable bioenergetic process in the light, photosynthesis versus respiration