N -Acyl-Homoserine Lactone Inhibition of Rhizobial Growth Is Mediated by Two Quorum-Sensing Genes That Regulate Plasmid Transfer

Abstract

The growth of some strains of Rhizobium leguminosarum bv. viciae is inhibited by N-(3-hydroxy-7-cis tetradecenoyl)-L-homoserine lactone (3OH-C 14:1 -HSL), which was previously known as the small bacteriocin before its characterization as an N-acyl homoserine lactone (AHL). Tn5-induced mutants of R. leguminosarum bv. viciae resistant to 3OH-C 14:1 -HSL were isolated, and mutations in two genes were identified. These genes, bisR and triR, which both encode LuxR-type regulators required for plasmid transfer, were found downstream of an operon containing trb genes involved in the transfer of the symbiotic plasmid pRL1JI. The first gene in this operon is traI, which encodes an AHL synthase, and the trbBCDEJKLFGHI genes were found between traI and bisR. Mutations in bisR, triR, traI, or trbL blocked plasmid transfer. Using gene fusions, it was demonstrated that bisR regulates triR in response to the presence of 3OH-C 14:1 -HSL. In turn, triR is then required for the induction of the traI-trb operon required for plasmid transfer. bisR also represses expression of cinI, which is chromosomally located and determines the level of production of 3OH-C 14:1 -HSL. The cloned bisR and triR genes conferred 3OH-C 14:1 -HSL sensitivity to strains of R. leguminosarum bv. viciae normally resistant to this AHL. Furthermore, bisR and triR made Agrobacterium tumefaciens sensitive to R. leguminosarum bv. viciae strains producing 3OH-C 14:1 -HSL. Analysis of patterns of growth inhibition using mutant strains and synthetic AHLs revealed that maximal growth inhibition required, in addition to 3OH-C 14:1 -HSL, the presence of other AHLs such as N-octanoyl-L-homoserine lactone and/or N-(3-oxo-octanoyl)-L-homoserine lactone. In an attempt to identify the causes of growth inhibition, a strain of R. leguminosarum bv. viciae carrying cloned bisR and triR was treated with an AHL extract containing 3OH-C 14:1 -HSL. N-terminal sequencing of induced proteins revealed one with significant similarity to the protein translation factor Ef-Ts.

Quorum Sensing in Nitrogen-Fixing Rhizobia

“…Recent work has also identified a cluster of genes on pRL1JI (51,57,62,130,180). Interestingly, in addition to a traR regulator, R. leguminosarum carries a second regulatory gene, bisR, downstream of the trb operon.…”

“…Mutations in the rai, cin, and tra systems do not have any apparent defects in nodulation (95,180,184). The rhi system seems to play a role in nodulation efficiency, but no dramatic defect has been observed for rhi mutants that might suggest a possible mechanism (29,139).…”

Potential Emergence of Multi-quorum Sensing Inhibitor Resistant (MQSIR) Bacteria

“…The raiI and raiR genes are involved in nodulation with the help of OHC8HSL, where as TraR leads to transfer of the symbiotic plasmid. In continuation of the QS process BisR and TraR regulate conjugation ( Table 2) [79]. For nodulation efficiency, RhiI synthesized C6HSL, C7HSL and C8-HSL activate the rhiABC genes.…”

Future research trends in the major chemical language of bacteria

“…Functions required for symbiosis (Lithgow et al, 2000;Rodelas et al, 1999;Wilkinson et al, 2002;Wisniewski-Dye et al, 2002) Serratia liquefaciens SwrI/R C4-HSL Swarming motility …”

“…viciae strain A34, is a regulator of growth inhibition of this strain in the presence of 3OH-C14:1-HSL, produced by the CinI/R QS system, and controls the frequency of pRL1J1 plasmid transfer (Wilkinson et al, 2002). BisR, located on the pRL1J1 plasmid, represses the expression of cinI and thereby maintains minimal amounts of 3OH-C14:1-HSL in donor cells.…”