## 4 Quorum Sensing Regulation in Other Rhizobia
CinI produces 3-OH-C14:1-HL and CinR regulates the expression of _cinI_ in response to the CinI-made AHL. Rhil produces C6-HL and RhiR strongly induces the expression of _rhil_ and the _rhiABC_ operon in response to Rhil-made AHL. RaiI produces 3-OH-C8-HL and RaiR regulates the expression of _rail_ in response to the RaiI-made AHL [84]. Apart from regulating the production of 3-OH-C8-HL, no other phenotypes have been associated to the Rai system of Rlv. A distinguishing feature of QS regulation in Rlv is that gene regulation relies on the population density-dependent accumulation of an antirepressor: CinS [81]. CinS is a small protein encoded by a gene which is co-transcribed with _cinI_, and is required to activate the Rhi and Rai systems [80] (Figure 2). CinS has the ability to bind PraR, a transcriptional regulator that represses _rhiiR_ and _raiR_ expression. The anti-repressor activity of CinS does not require AHL. Binding of CinS to PraR displaces the repressor from the _rhiR_ and _raiR_ promoters, thereby inducing their expression. Full activation of Rhi and Rai systems also requires ExpR, which represses transcription of the _praR_ gene (Figure 2). In _S. meliloti_ an ortholog of _praR_ is present (named _pltrR_), but not a _cinS_ orthologo [81]. As in Rlv, ExpR represses the promoter of the _pltrR_ gene of _S. meliloti_ upon binding of ExpR/AHL to this promoter [28]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Rhizobial Strain** & **QS Signal** & **Genes** & **Phenotypes Regulated** & **References** \\ \hline \hline _Rhizobium Iguminosarum_ & & & \\ \hline bv. viciae & 3-OH-C14:1-HL & _cinI/cinI/cinI_S & Growth inhibition, regulation of EPS cleavage, biofilm & [79; 80; 81] \\  & C6-HL, C7-HL, C8-HL & _rlili/rliR_ & Modulation * & [4; 82] \\  & 3-oxo-C8-HL, C8-HL & _trai/tnR_, _bisR_ & Plasmid transfer & [4; 83] \\  & C6-HL, C7-HL, C8-HL, & _rail/ruiR_ & Unknown & [84] \\  & 3-OH-C8-HL & & & \\  & Unknown effector & _expR_ & Regulation of EPS cleavage, & [80; 81] \\ \hline \hline _Sinorhineshuman fmdii_ & & & \\ \hline SMH12 & C8-HL, C14-HL & Unknown & Biofilm formation & [85; 86] \\  & 3-oxo-C8-HL & _trai/trai_ & Plasmid transfer & \\  & 3-oxo-C8-HL & _trai/trai_ & Plasmid transfer, growth rate, sedimentation, motility, & \\  & Non-characterized AHL & _ngr1/ngrR_ & biofilm formation, regulation of EPS genes, regulation of & [87; 88; 89] \\  & \(\epsilon\) & the copy number of the symbiotic plasmid & \\ \hline \hline _Sinorhineshuman milli_ & & & \\ (_Esnfer melidi_) & & & \\ \hline Rm1021 & C12-HL, C14-HL, & _sint/sint/sintR, expR_ & EPS production, surface translocation, regulation of & [16; 24; 25; 26; 28; 34; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 72; 74; 76; 79; 81; 82; 83; 84; 85; 86; 87; 88; 89] \\  & 3-oxo-C16-HL, C16-HL, & & & \\  & 3-oxo-C16-HL, C18-HL & & & \\ Rm41 1 & 3-oxo-C8-HL & _trai/trai_ & Plasmid transfer & [27] \\ \hline \hline \end{tabular}  
* Effects observed by adding synthetic AHLs.
* Effects observed by adding purified extracts of AHLs.
* Directed by using biosensors but not characterized by mass spectrometry analysis.
* Starated long chain 3-hydroxy-acyl-HL.
* Only in a mutant strain already compromised for modulation.
* Rm41 also harbors the ExpR/Sin system of Rm1021  
\end{table} 
Table 1: _Cont._In _R. etli_ two different isolates have been investigated, which share orthologous genes with strains of _R. leguminosarum_ (Table 1). _R. etli_ strain CNPAF512 has two QS regulatory systems, CinIR and RaiIR, both of which control symbiosis [73; 75]. In contrast, strain CFN42 has a complex but less well characterized QS system composed of three different LuxI-type AHL synthases (CinI, RaiI, and TraI) and four cognate LuxR-type regulators (CinR, RaiR, TraR1, and TraR2).