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A double-negative feedback loop between NtrBC and a small RNA rewires nitrogen metabolism in legume symbiontsA double-negative feedback loop between NtrBC and a small RNA rewires nitrogen metabolism in legume symbiontsGarcía-Tomsig NI, García-Rodriguez FM, Guedes-García SK, Millán V, Becker A, Robledo M, Jiménez-Zurdo JI2023-12-18T23:00:00Z<h3>​​ABSTRACT</h3><div><br></div><div>The nitrogen (N) status transduced via the NtrBC two-component system is a major signaling cue in the root nodule endosymbiosis of diazotrophic rhizobia with legumes. NtrBC is upregulated in the N-limiting rhizosphere environment at the onset of nodulation but silenced in nodules to favor the assimilation of the fixed N into plant biomass. We reported that the trans-acting sRNA NfeR1 (Nodule Formation Efficiency RNA) broadly influences the symbiotic performance of the α-rhizobium Sinorhizobium meliloti. Here, we show that NfeR1 is indeed an N-responsive sRNA that fine-tunes NtrBC output during the symbiotic transition. Biochemical and genetic approaches unveiled that NtrC and the LysR-type symbiotic regulator LsrB bind at distinct nearby sites in the NfeR1 promoter, acting antagonistically as repressor and activator of transcription, respectively. This complex transcriptional control specifies peak NfeR1 steady-state levels in N-starved and endosymbiotic bacteria. Furthermore, NfeR1 base pairs the translation initiation region of the histidine kinase coding mRNA ntrB, causing a decrease in both NtrB and NtrC abundance as assessed by double-plasmid genetic assays. In the context of endogenous regulation, NfeR1-mediated ntrBC silencing most likely amends the effective strength of the known operon autorepression exerted by NtrC. Accordingly, a lack of NfeR1 shifts the wild-type NtrBC output, restraining the fitness of free-living rhizobia under N stress and plant growth upon nodulation. The mixed NtrBC-NfeR1 double-negative feedback loop is thus an unprecedented adaptive network motif that helps α-rhizobia adjust N metabolism to the demands of an efficient symbiosis with legume plants.​<br></div><p><br></p><p><a href="https://journals.asm.org/doi/10.1128/mbio.02003-23">mBio. Dec 19; 14(6):e0200323.</a><br></p>471

 
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