## 3 Regulatory Mechanisms Controlling AHL Synthesis in _Sinorhizobium meliloti_
### Post-Transcriptional Regulation  
Regulation of QS in _S. meliloti_ has been studied mainly at the level of transcription and little is known about factors acting post-transcriptionally. In other bacteria, small regulatory RNAs (sRNAs) have been found to regulate QS by controlling translation and mRNA levels of the LuxR-like transcriptional regulators or other QS-dependent genes [57; 58]. Typically, sRNAs interact with mRNAs with the help of the RNA chaperone Hfq and influence the translation rate and/or half-life of the mRNA targets. Usually, this interaction acts as a repressor mechanism by inducing degradation of the sRNA-mRNA complex in an endoribonuclease E (RNase E)-dependent manner. RNase E plays a major role in mRNA decay in bacteria [59]. Interestingly, RNase E of _S. meliloti_ affects the production of AHLs by interacting with a specific target sequence located in the 5'-untranslated region (UTR) of the autoinducer synthase encoding _sinl_ mRNA (Figure 1). Independently of ExpR and Hfq, and without affecting _sinR_ mRNA stability, overexpression of the RNase E coding sequence, _rne_, results in enhanced degradation of _sinl_ transcript. Therefore, RNase E-dependent degradation of _sinl_ mRNA from the 5'-end could be one of the steps mediating a high turn-over of _sinl_ mRNA, which allows the Sin QS system to respond rapidly to changes in transcriptional control of AHL production [60]. Although not linked with RNase E activity, the RNA-binding protein Hfq contributes to the regulation of AHL production in _S. meliloti_ (Figure 1). This regulation is exerted via both ExpR-dependent and -independent mechanisms [61]. In the _expR+_ strain, the lack of Hfq leads to hyper-accumulation of QS signals at low population densities, and a sharp decrease in AHL accumulation in stationary phase. These results indicated that Hfq, directly or indirectly, exerts negative control over production or stability of AHLs at low population densities, and a positive control over AHL synthesis and/or accumulation, at high population densities. No regulatory effect of Hfq on _sinR_ mRNA was observed. In contrast, it was demonstrated that Hfq influences _expR_ and _sinl_ expression. Whereas Hfq directly controls _expR_ mRNA at higher population densities, the effects of Hfq on _sinl_ at lower population densities seem to be mediated by other mechanisms [61]. More recently, a structurally conserved, trans-acting sRNA has been shown to directly interact with and to destabilize the _sinl_ mRNA in _S. meliloti_. This sRNA, called RcsR1, is not regulated in a cell density-dependent manner, but shows similar expression profiles under salt and cold stress in several Rhizaceae members, which suggests a conserved role in response to environmental stress. Thus, RcsR1 links stress responses to QS in _S. meliloti_[62; 63]. In _S. meliloti_ the _sinl_ mRNA steady-state levels and the AHL amounts diminish upon overexpression of _rcsR1_, a process that requires an intact _rne_ gene, indicating that RcsR1 influences QS in an RNase E-dependent manner. Consistent with the RNase E effect on _sinl_ mRNA degradation, RcsR1 operates by an Hfq-independent mechanism. Interestingly, although RcsR1 binds closely to the Shine-Dalgarno sequence and therefore prevents accessibility of RNase E to the 5'-UTR region of the _sinl_ transcript, the sRNA increases the negative effect of RNase E on the _sinl_ mRNA half-life. This can be explained by a block of ribosomal machinery by RcsR1 during translation leading to a better accessibility of RNase E cleavage sites in the _sinl_ coding region [63]. Apart from the direct interaction between RcsR1 and _sinl_ mRNA, three other targets have been verified: _phoR_ encoding a sensor kinase for the response to phosphate limitation, _motE_ encoding a chaperone for a periplasmic motility protein and _sm2011_c01420_ encoding the anti-oE1 factor. As mentioned above, PhoB (the response regulator in the two-component system PhoR/PhoB) induces _sinR_. This implies that RcsR1 does not only influence QS in _S.