Quorum-Dependent Mannopine-Inducible Conjugative Transfer of an Agrobacterium Opine-Catabolic Plasmid

Abstract

The Ti plasmid in Agrobacterium tumefaciens strain 15955 carries two alleles of traR that regulate conjugative transfer. The first is a functional allele, called traR, that is transcriptionally induced by the opine octopine. The second, trlR, is a nonfunctional, dominant-negative mutant located in an operon that is inducible by the opine mannopine (MOP). Based on these findings, we predicted that there exist wild-type agrobacterial strains harboring plasmids in which MOP induces a functional traR and, hence, conjugation. We analyzed 11 MOP-utilizing field isolates and found five where MOP induced transfer of the MOP-catabolic element and increased production of the acyl-homoserine lactone (acyl-HSL) quormone. The transmissible elements in these five strains represent a set of highly related plasmids. Sequence analysis of one such plasmid, pAoF64/95, revealed that the 176-kb element is not a Ti plasmid but carries genes for catabolism of MOP, mannopinic acid (MOA), agropinic acid (AGA), and the agrocinopines. The plasmid additionally carries all of the genes required for conjugative transfer, including the regulatory genes traR, traI, and traM. The traR gene, however, is not located in the MOP catabolism region. The gene, instead, is monocistronic and located within the tra-trb-rep gene cluster. A traR mutant failed to transfer the plasmid and produced little to no quormone even when grown with MOP, indicating that TraR pAoF64/95 is the activator of the tra regulon. A traM mutant was constitutive for transfer and acyl-HSL production, indicating that the anti-activator function of TraM is conserved.

Discovery of a Conjugative Megaplasmid in Bifidobacterium breve

“…Bacterial conjugation is one of the possible means of horizontal transfer of genetic material and it has been intensively studied in Gram-negative bacteria, in particular in the plant pathogen Agrobacterium tumefaciens due to its pathogenic and biotechnological relevance (e.g., tumor-inducing plasmid Ti) (14)(15)(16). Conjugation has been described for Gram-positive bacteria, with the difference that a somewhat simpler conjugative machinery appears to be required due to the fact that DNA traverses just a single membrane layer (17).…”

Evolutionary aspects of toxin-producing bacteria

“…It appears that the high rates of HGT observed in bacterial biofilms may be attributable to quorum-sensing control of the conjugation and production of exoenzymes, toxins, and other virulence factors, in addition to biofilm formation [247,251,263,307,[315][316][317]. Roseobacter species and other environmental bacteria besides Vibrio associate with marine snow and also produce acyl homoserine lactones.…”

“…Since HGT occurs at maximal frequencies when bacterial densities of potential recipients are high [246][247][248][249][250][251], one might ask how genetic exchange can occur in aquatic environments, where some bacteria, such as Vibrio, spend most of their time and where their concentrations are expected to be too low to be conducive to HGT. Studies have shown, however, that bacteria found in marine environments are often adherent to marine animals or particles, such as zooplankton or marine snow [252][253][254].…”

Quorum‐dependent transfer of the opine‐catabolic plasmid pAoF64/95 is regulated by a novel mechanism involving inhibition of the TraR antiactivator TraM

“…In these four cases, conjugation is strongly controlled, with expression of the transfer genes requiring two exogenous signals (Fuqua & Winans, ; Oger & Farrand, ; Piper, Beck von Bodman, Hwang, & Farrand, ; Wetzel et al., ). The first is a particular opine, a type of novel carbon compound produced by the crown gall tumors induced by pathogenic members of the two genera.…”

“…It also carries a full set of genes for uptake and catabolism of the mannityl opines. However, traR is not associated with the mot operon and, while located in the canonical site between traM and the traAFBH operon, traR is, in fact, monocistronic (Wetzel et al., ). Mutational analysis showed that traR of this plasmid is essential for MOP‐dependent induction of the synthesis of the acyl‐HSL QS signal and the resultant conjugative transfer of the element (Wetzel et al., ).…”