## 2 Quorum Sensing Genes in _Sinorhizobium melilioti_
Transcriptional activation of _sinR_ expression mediated by PhoB and ReIA in response to nutrient limitation is also represented. Cyclic diguanylate (cdG) negatively affects _sinl_ expression and AHL synthesis by an as yet unknown mechanism which is _expR_ independent. Post-transcriptional regulation of the _sinl_ transcript mediated by the RNA chaperone Hfq, RNAse E and the small RNA RcsR1 is shown in orange. See text for more details. The protein FadL that facilitates sensing of the long-chain AHLs produced by _S. melidi_ is represented. Genes are represented by wide arrows, proteins by rectangles, and second messenger by ovals. Green arrows represent activation; red flat-ended lines indicate repression; orange arrows and flat-ended lines indicate positive and negative post-transcriptional regulations, respectively. Question mark indicates that the mechanism of regulation is unknown. AHL: _N_-acyl homoserine lactone; IM: inner membrane; OM: outer membrane. cultivation of _S. meliloti_ strains. This inactivation confers a selective advantage due to ExpR-dependent restraint of bacterial growth whose mechanism is unknown [40]. Interestingly, ExpR and QS are stable in natural environments, suggesting that maintaining an intact QS system confers bacterial benefits that remain to be discovered. Despite the loss of ExpR, these mutant strains still produce significant levels of AHLs [24; 25], but the presence of these AHLs affects the expression of only few genes [29; 38] via an unknown mechanism. In addition to ExpR and SinR, the _S. meliloti_ Rm1021 genome potentially codes for at least eight additional LuxR-type proteins. These are SMc00658, SMc00877, SMc00878, SMc03015 (VisN), SMc03016 (VisR), SMc04032 (NesR) and SMc03150. In addition, many other open reading frames (ORFs) could code for proteins harboring LuxR-domains, but differ from the standard LuxR-like proteins, e.g., SMc01630 which has a larger size. Thus, the number of genes coding for LuxR-type proteins could be higher. For example, several species of the Gammaproteobacterium _Photorhabus_ are reported to contain up to 40 _luxR_-like orphans [32]. All of the _luxR_-like genes in _S. meliloti_, with the exception of _sinR_, are not associated with a known AHL synthase-encoding locus, and therefore, as with ExpR, can be considered orphan LuxR homologs [31; 41]. LuxR solos can respond to endogenous or exogenous AHLs, to endogenous signals which are not AHLs, or to low molecular weight plant compounds [41]. In _S. meliloti_, only ExpR is known to respond to AHLs as the effector molecule. For the other orphan LuxR-type proteins and SinR, the mechanisms and potential ligands involved in activating these transcriptional regulators remain to be discovered. Likewise, little is known about the biological role of most of these LuxR homologs. VisN and VisR are the master regulators of the _S. meliloti_ flagellar regulon and are essential for the expression of flagellar and motility genes [42]. These are encoded within an operon whose promoter binds to and is repressed by the ExpR/AHL complex [43]. VisN and VisR are proposed to form a heterodimer and variability in their ligand-binding domains suggest that these proteins bind different effector molecules whose nature remains unknown [42]. SMc00878 and SMc00877 are highly homologous to AviR of _Agrobacteriumvinitis_[44] and the former seems to affect the denitrification pathway of _S. meliloti_[31]. In the case of _nesR_, its promoter also binds to the ExpR/AHL complex [28] and its expression and phenotypic analyses suggest that this regulator contributes to the bacterial adaptation to environmental stresses such as high osmotic conditions or nutrient deprivation, as well as to the bacterial ability to compete for nodule formation [45]. NesR belongs to a LuxR solo sub-family which is only found in plant associated bacteria, whose members do not bind AHLs but plant-produced compounds [46]. NesR is highly homologous to XccR and OryR of _Xanthomonas campestris_ pv. campestris and _Xanthomonas oryzae_ pv. oryzae, respectively.