# Quorum Sensing and Quorum Quenching in _Agrobacteriumum_: A \"Go/No Go System\"?
## 2 Ti Plasmid Transfer is Regulated by Quorum Sensing and Opines  
Ti plasmid conjugal transfer has been best described in _Agrobacterium barium_ strain C58. In this strain, Ti plasmid transfer is also regulated by quorum sensing (QS), a cell-cell communication system responding to bacterial cell concentration via the production of the acylhomoserine lactone (AHL) signal, 3-oxo-octanoylhomoserine lactone (OOHL), that accumulates in increasing concentrations in the bacterial environment as the bacterial population grows [16; 17]. In that strain, the conjugative opines are agrocinopines A and B [18]. Once bound to the catabolite of agrocinopines, arabinose-2-phosphate [19], the master repressor AccR [20], promotes the expression of two Ti plasmid-encoded operons (Figure 1). The first one, _accABCDEFG_ operon, is responsible for the importation and degradation of agrocinopines [21]. A second, the _arc_ operon (divergently transcribed from the former one), noticeably includes a _traR_ gene that encodes TraR, a LuxR-like protein [16]. TraR exhibits two domains, one that binds DNA, the other that binds OOHL [22]. The TraR-OOHL complex activates the transcription of the _traAFB_, _traCDG_, and _trb_ operons [23]. Remarkably, the first gene of the _trb_ operon is _tral_, a _luxl_-like gene that encodes the OOHL signal synthase TraI [24; 25]. This positive regulatory loop amplifies OOHL synthesis in the presence of agrocinopines. The _traAFB_ and _traCDG_ operons encode the DNA transfer and replication (Dtr) system, a protein complex also known as the relaxosome [26]. The relaxosome recognizes and cleaves the _nic_ site at the origin of transfer (_oriT_) of Ti plasmids [27]. The _trb_ operon determines the components of the T4 SS that physically permits the conjugative transfer of Ti plasmids from one strain to another [25]. In this scheme, TraG may be the so-called coupling protein that bridges the relaxosome and its cognate T4 SS [27]. The above data are related to _A. fabrum_ strain C58. However, Ti plasmid conjugation and its regulation involve similar--if not identical--mechanisms and elements in other agrobacterial strains [28; 29]. Figure 1: Global scheme of tumour inducing (Ti) plasmid organization and conjugation functions. The global organization of the C58 Ti plasmid is shown with magnified regions involved in Ti plasmid  
conjugation. In the absence of the inducing agrocinopines, the master regulator AccR prevents the transcription of both the _acc_ and _arc_ operons. In the presence of arabinose-2-phosphate, a catabolite of agrocinopines that binds the AccR master regulator, the repression is released, and the transcription of both the _acc_ and _arc_ operons occurs. Acc proteins are directing the uptake and degradation of agrocinopines. One of the genes of the _arc_ operon is _traR_, a _luxR_-like gene involved in the sensing of the acylhomoserine lactone (AHL) quorum sensing (QS) signal OOHL). OOHL is synthesized by TraI, encoded by the first gene of the _trb_ operon that determines the T4 SS that permits the physical transfer of the Ti plasmid from one strain to another. The sensing of OOHL by TraR is antagonized by TraM that interacts with TraR to favor its proteolytic degradation. In the presence of OOHL, TraM interaction with TraR is reduced, TraR dimerizes, and becomes activated. The TraR/OOHL complex activates the transcription of the _trb_ operon and that of the _traAFB_ and _traCDG_ operons coding the DNA transfer and replication (Dtr) system, a protein complex also known as the relaxosome. The relaxosome recognizes and cleaves the _nic_ site at the origin of transfer (_oriT_) of Ti plasmids. TraG may be the coupling protein that bridges the relaxosome and its cognate T4 SS. The activation of all these systems permits the conjugation of the Ti plasmid.