Recipient-induced transfer of the symbiotic plasmid pRL1JI in Rhizobium leguminosarum bv. viciae is regulated by a quorum-sensing relay

Abstract

Analysis of the regulation of plasmid transfer genes on the symbiotic plasmid pRL1JI in Rhizobium leguminosarum bv. viciae has revealed a novel regulatory relay that is specifically poised to detect an N-acylhomoserine lactone (AHL) made by different cells (potential recipients of pRL1JI). Adjacent to the tral-trbBCDEJKLFGHI plasmid transfer operon on pRL1JI are two regulatory genes, bisR and traR, which encode LuxR-type quorum-sensing regulators required for conjugation. Potential recipients of pRL1JI induce the tral-trb operon and plasmid transfer via a quorum-sensing relay involving BisR, TraR and the tral-trb operon in donor cells. BisR induces expression of traR in response to N-(3-hydroxy-7-cis-tetradecenoyl)-L-homoserine lactone (3-OH-C 14:1-HSL), which is produced by Cinl in potential recipient strains. In donor strains (carrying pRL1JI), BisR represses the expression of the chromosomal gene cinl; this repression results in a very low level of formation of 3-OH-C14:1-HSL and hence relatively low levels of expression of traR and the tral-trb operon in strains carrying pRL1JI. However, if 3-OH-C 14:1-HSL from potential recipients is present, then traR and plasmid transfer are induced. The induction of traR occurs at very low concentrations of 3-OH-C14:1-HSL (around 1 nM). TraR then induces the tral-trb operon in a quorum-sensing dependent manner in response to the TraI-made AHLs, N-(3-oxo-octanoyl)-L-homoserine lactone and N-(octanoyl)-L-homoserine lactone. The resulting autoinduction results in high levels of expression of the tral-trb operon. Premature expression of the tral-trb operon is reduced by TraM, which probably titres out TraR preventing expression of tral when there are low levels of traR expression. Expression of traR in stationary phase cells is limited by feedback inhibition mediated by TraI-made AHLs.