Cell-cell communication in the plant pathogen Agrobacterium tumefaciens

Abstract

The plant pathogen Agrobacterium tumefaciens induces the formation of crown gall tumours at wound sites on host plants by directly transforming plant cells. This disease strategy benefits the bacteria as the infected plant tissue produces novel nutrients, called opines, that the colonizing bacteria can use as nutrients. Almost all of the genes that are required for virulence, and all of the opine uptake and utilization genes, are carried on large tumour-inducing (Ti) plasmids. The observation more than 25 years ago that specific opines are required for Ti plasmid conjugal transfer led to the discovery of a cell-cell signalling system on these plasmids that is similar to the LuxR-LuxI system first described in Vibrio fischeri. All Ti plasmids that have been described to date carry a functional LuxI-type N-acylhomoserine lactone synthase (TraI), and a LuxR-type signal receptor and transcriptional regulator called TraR. The traR genes are expressed only in the presence of specific opines called conjugal opines. The TraR-TraI system provides an important model for LuxR-LuxI-type systems, especially those found in the agriculturally important Rhizobiaceae family. In this review, we discuss current advances in the biochemistry and structural biology of the TraR-TraI system.

Figure 1

A model of the quorum-sensing system in octopine-type Ti plasmids. The traR gene is transcribed in response to octopine and the apo-protein binds to 3-oxo-C8 HSL, the quorum-sensing signal produced by TraI. TraR–3-oxo-C8 HSL dimers activate transcription of traM and the tra, trb and rep operons of the Ti plasmid. TraR–3-oxo-C8 HSL complexes can be inactivated through direct interactions with TraM or TrlR. Transcription of trlR is activated by MocR in response to mannopine. The approximate locations of the T-DNA and vir genes are shown for reference.

Figure 2

A comparison of traR regulation via opines on different types of Ti plasmids. On octopine-type Ti plasmids, traR is activated by OccR in response to octopine. On nopaline-type Ti plasmids, traR is expressed when AccR repression is relieved by agrocinopines A and B. Regulation of traR on the chrysopine-type plasmid is similar, except that the inducing opines are agrocinopines C and D. There are two copies of traR on pAtK84b, one thought to be activated by NocR in response to nopaline and the transcription of the other is activated in response to agrocinopines A and B via the derepression of AccR.

Figure 3

Ribbon model of a TraR dimer, complexed with 3-oxo-C8 HSL, bound to tra box DNA. One monomer of the dimer is in dark grey, the other in light grey and the DNA is black. One molecule of 3-oxo-C8 HSL, shown using space-filling atoms, is bound in the N-terminal domain (NTD) of each monomer, while the C-terminal domain (CTD) of each monomer is bound to DNA. (a) The helices of the NTD (helix 9) that form the dimerization interface are highlighted, and helix 13 of each CTD that aid in dimerization. The recognition helix of the HTH, helix 12, of each monomer is also labelled. Note that each binds in the major groove of each half site of the tra box. The N-terminus (N) of the left monomer and the C-termini (C) of both are labelled. The unstructured linker between the NTD and CTD of the right monomer was disordered in the crystal, but the linker of the left monomer is visible. (b) A side view of the model shown in (a), to illustrate the overall asymmetry of the dimer. Coordinates from Vannini et al. (2002) and Zhang et al. (2002b).

Figure 4

Interactions between TraR and DNA. (a) A model of contact surfaces between TraR and the tra box. Specific contacts between residues of TraR (a valine and two arginines in each monomer) and four bases in the major groove are in dark grey. Interactions between TraR and the DNA backbone (non-specific) are white. (b) TraR interactions with the tra box. (White & Winans in press).

Figure 5

Interactions in the ligand-binding pocket of the TraR N-terminal domain. An α-helix and β-strand of TraR are shown, with critical residues and the 3-oxo-C8 HSL shown at the centre. Hydrogen bonds and their lengths (in angstroms) are shown. One water molecule is bound in the ligand-binding pocket, and coordinates interactions between a threonine residue and the 3-oxo group of 3-oxo-C8 HSL. See the text for details. Structural coordinates from Vannini et al. (2002) and Zhang et al. (2002b).

Figure 6

GBL degradation pathway via AttKLM (Carlier et al. 2004; Chai et al. in press). The pathway for 3-oxo-C8 HSL degradation is thought to be similar. At least the first step can result in hydrolysis of the lactone ring through activity of AttM.

Figure 7

Transcription activation at TraR-dependent promoters on the octopine-type Ti plasmid. The tra boxes are marked as open squares and correspond to tra boxes I, II, III and IV. Thick bars represent both the –10 and the –35 elements of the promoter, and the transcription start sites are marked with arrows. The four promoters of the rep operon are labelled P₁–P₄, as described in the text. Activation at P₄ by phosphor-VirG and repression of the same promoter by RepA–RepB complexes are also shown. Only the first gene (or part of the first gene) is shown for each operon.