Table 1.

Examples of MGEs from each type of conjugation system. P, plasmid; I, ICEsym.

MGE	replicon type	details	references
type 1: quorum sensing (QS) mediated conjugation
pNGR234a in Rhizobium sp. strain NGR234	P	Tra AHL mobilized plasmid. Conjugation rate estimated at 10−9.	[39]
pRL1JI in Rhizobium leguminosarum sv. viciae 2483841	P	Well-studied pSym that is transferred at very high frequencies. QS is dependent on plasmid-free recipients.	[31]
		This plasmid seems to be made up of 3 modules: (1) a basic replicon with repABC genes and bacteriocin production and other genes that is similar to two other (unsequenced) plasmids pRL3JI and pRL4JI as well as transfer genes (Type I, QS regulated system); (2) a symbiosis region virtually identical to that in pRL10JI (from strain 3841); and (3) an extended region that looks like a catabolic region from pRL8JI.	[40,41]
ICEMlSymR7A in Mesorhizobium loti strain R7A	I	ICE excision is highly controlled by TraR. Experimental derepression has shown that conjugation is functional but it has yet to be observed in wild-type strains. In addition, it has a second regulatory system, which also acts to further limit excision and transfer.	[42,43]
pSfr64b in Ensifer/Sinorhizobium fredii GR64	P	pSfr64b carries its own conjugative machinery but transfer is mutually dependent on a second plasmid, pSfr64a, for conjugation. Both plasmids carry regulatory genes that initiate conjugation of the other in response to QS molecules.	[44]
type II: RctA repression system
pRetCFN42d in Rhizobium etli CFN42	P	pRetCFN42d carries its own conjugation machinery but this is heavily repressed and the environmental trigger is unknown. Transfer has been observed within nodules.	[45]
		This plasmid can also exploit other transfer machineries—mobilization has been shown to occur via integration and mobilization of the class I QS-induced plasmid p42a.	[46,47]
pSymA in Ensifer/Sinorhizobium melliloti strain 1021	P	Large (1354 kb) conjugative plasmid. Transfer has yet to be observed in the laboratory although there is evidence for transfer within nodules.	[33,34,48]
		63 kb region that contains the key symbiosis genes (nod, nif and regulatory genes).
type III: mobilizable plasmids
pRleVF39d in Rhizobium leguminosarum VF39SM	P	sym Plasmid carrying a chemotaxis gene.	[30]
pRL10JI in Rhizobium leguminosarum 3841	P	Plasmid carries a compact approximately 60 kb symbiosis gene cassette that is flanked by inverted repeat regions, suggesting the sym genes may be readily mobilizable.	[26]
type IV
(type IVa) pRleVF39b in Rhizobium leguminosarum VF39SM	P	Plasmid carries the distinct type IVa conjugation system containing a small relaxase gene (traA) producing a shorter TraA protein, amongst other differences to the above systems. Mutagenesis studies highlighted the importance of trcA-F in conjugative transfer and alleviation of the repressor TrbR.	[37,38]
(type IVb) pSmed03 in Ensifer medicae WSM419	P	Plasmid carries the distinct type IV relaxase group (MOBP0) but clusters on a separate branch from type IVa systems.	[38,49]
alternative conjugation mechanisms
ICEAc in Azorhizobium caulinodans	I	An 87.6 kb sym ICE found to excise and transfer in response to the host plant flavonoid naringenin. Increased transfers were also found after exposure to non-host plants, highlighting the rhizosphere as a promotive environment for HGT events.	[18]