The general stress response in Sinorhizobium meliloti
The general stress response has been well studied in Escherichia coli: in response to various stress conditions, the alternative sigma factor sigmaS accumulates and permits the RNA polymerase to transcribe hundreds of new genes, some of which are involved in stress adaptation. This allows bacteria to survive not only the stress they currently experience, but also many other stresses that they could potentially face in the future, in particular in periods of prolonged nutrient starvation. This multiple and preventive protection is believed to be of primary importance for the survival of bacteria in nature. However, no sigmaS orthologue is encoded in the genome of S. meliloti and other a-proteobacteria.
For the first time, we identified the σ factor RpoE2 as a major regulator of the general stress response in S. meliloti (Sauviac et al. 2007). Indeed, in response to various stress conditions, RpoE2 controls the expression of >45 genes including stress resistance genes. Interestingly, RpoE2 orthologues are widely distributed among α-proteobacteria where they play various roles in stress adaptation and/or host colonization, which suggests they are the long-searched functional analogs of E. coli sigmaS. We more recently characterized the mechanism of activation of RpoE2 in response to stress. A complex model was proposed, involving 2 anti-sigma and 2 anti-anti-sigma factors working together in a partner-switching cascade (Bastiat et al. 2010).
Our present objectives are i) to better characterize the RpoE2-dependent response, in particular the mechanisms of stress perception and signal transduction, ii) to better understand the physiological functions of the RpoE2-dependent response, iii) to identify other regulators of the general stress response in S. meliloti.
Study of the NO response in Sinorhizobium meliloti
NO (nitric oxide) plays a key role in intracellular signaling in eukaryotic cells. In animal as well as in plant cells NO at toxic concentration is also part of the anti-bacterial arsenal against pathogens. Surprisingly, several indications of the occurrence of NO during legume–rhizobia interactions have also been reported, and recently NO was detected in M. truncatula nodules infected by S. meliloti. On the other hand, NO is also produced in the soil by denitrifying bacteria. In both cases, this toxic molecule may represent a stress for S. meliloti and it is interesting to understand how rhizobia cope with the presence of NO and what role is played by the bacterial NO response in the interaction with the host plant. Using a transcriptomic approach on bacteria in free living conditions, we identified about 100 genes induced by NO, and two major transcriptional regulators (FixLJ and NnrR) involved in this response (Meilhoc et al. 2010). We showed that one of the genes belonging to the NO stimulon, hmp, encodes a flavohemoglobin involved in NO detoxification in S. meliloti. By using hmp as a tool to modulate the NO level in planta we established a dual role for NO: positive on the early steps of symbiosis and negative on nitrogen fixation efficiency (del Giudice et al. 2011).
Our present objectives are i) to decipher the role of the bacterial response to NO in symbiosis ii) to characterize NO role(s) at the different steps of the symbiotic process.
- ANR STAYPINK 2016-2019 (Coord. C. Bruand)
- TULIP New Frontiers 2016-2018 (B. Gourion)
- INRA SPE 2016-2019 (B. Gourion)
- ANR Trolesinfidels 2018-2022 (B. Gourion)