# Riboregulation in Nitrogen-Fixing Endosymbiotic Bacteria
## 4 Assigning Functions to Rhizbial _Trans_-sRNAs
### Quorum Sensing  
Bacteria are able to communicate with their pairs and adjust their expression profiles to cell density thanks to the quorum sensing (QS) response. This collective behavior system has been extensively studied in Gram-negative bacteria and it is mainly based on the synthesis of autoinducers, such as acyl-homoserine lactones (AHLs) [77]. These molecules are usually recognized by their cognate LuxR receptors, triggering gene expression changes involved in important biological processes like biofilm formation and virulence. Autoinducers diffuse easily through bacterial membranes and can also stimulate Luxl-type AHLs synthases in neighboring microorganisms, orchestrating transcriptional control. In _Vibrio_, the model microorganism to study AHLs-based QS regulation, several sRNAs modulating this process have been identified. In the absence of AHLs, _V. harveyi_ expresses five Hfq-dependent homologous QS Regulatory RNAs (Qrr) that regulate multiple mRNAs, including four low cell density master regulators that repress LuxR by four distinct mechanisms [78].  
In _S. melilioti_, AHLs are synthesized by the Luxl-type synthase SinI, which is regulated by other components of the Sin QS system, like SinR and ExpR, under cell density-dependent and stress conditions [79]. Accumulation of \(sinI\) mRNA in an _hfq_ knock-out mutant together with co-immunoprecipitation with Hfq anticipated QS riboregulation in _S. melilioti_[32; 80]. On the other hand, the conserved endoribonuclease RNAse E is involved in \(sinI\) 5'-UTR degradation, thereby influencing mRNA turnover and AHLs levels [81]. A _trans_-sRNA that regulates _S. melilioti sinI_ post-transcriptionally has been also unveiled. This sRNA, initially referred to as SmelC587 (Table 1), was identified by inverse bioinformatics target predictions (i.e., using the 5'-UTR of \(sinI\) mRNA sequence as query) as a putative \(sinI\) base-pair riboregulator [82]. Abundance of SmelC587 transcript does not significantly vary with cell densities but with temperature and salt stress [26] and, accordingly, it was renamed Rhizbial Cold and Salinity stress Riboregulator 1 (RcsR1). Accumulation of this sRNA was very low in the presence of 500 mM NaCl in several Rhizaceae, matching the growth defects observed upon RcsR1 overproduction at high salinity conditions [82]. RcsR1 is predicted to fold into three distinct domains: two highly-conserved SLs and a less conserved region that targets the \(sinI\) 5'-UTR. This interaction was genetically confirmed in vivo and by in vitro by RNA degradation assays using native RNase E. Interestingly, the results show that RcsR1 competes with the endonuclease for \(sinI\) 5'-UTR cleavage site, impairing SinI translation [82]. This work represents an example of negative sRNA-mediated modulation of key bacterial processes under stress signals. Remarkably, RcsR1 is transcribed from an identical position to the \(trpL\)-derived attenuator RNA mTrpL. Ectopic RcsR1 overproduction results in \(trpDC\) mRNA level decrease as mTrpL down-regulates \(sinI\), suggesting a co-regulation of both QS and tryptophan biosynthesis [57].