Chapter 22 : The Ti Plasmids

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This chapter summarizes the latest information about Ti-encoded functions, with emphasis on the mechanistic studies of proteins associated with pathogenesis and intercellular signaling. The ability to transfer T-DNA across kingdom boundaries is a hallmark feature of the infection process. Moreover, the presence of overdrive is correlated with increased production of single-stranded T-DNA, the T-strand, which corresponds to the translocation-competent form of the T-DNA. strains in the vicinity of the transformed plant cells can then import the opines by mechanisms described for use as sources of carbon and energy and, in some cases, nitrogen. The converse model, that VirA interacts directly with phenolic signals, is supported by two genetic lines of study. First, genes from different strains of , when introduced into an isogenic background, encode VirA proteins that sense different types of inducers. Second, a recent study reported the reconstitution of the VirA/VirG two-component system in . VirE1 interacts with an N-terminal domain and at least one internal domain of VirE2. Finally, VirD4 recently was shown to assemble at the poles of . One of the most striking features of the Ti plasmids is the evolution of an extensive regulatory network that serves to link the activities of the five conserved gene sets. Studies on the regulatory circuitries governing Ti plasmid gene expression will lead to a refined, mechanistic understanding of the paradigmatic regulatory factors VirA and VirG, OccR and AccR, and TraR.

Citation: Christie P. 2004. The Ti Plasmids, p 455-472. In Funnell B, Phillips G (ed), Plasmid Biology. ASM Press, Washington, DC. doi: 10.1128/9781555817732.ch22
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Figure 1

Physical maps of the ocropine-type pTiA6NC and the nopaline-type pTiC58 tumor-inducing plasmids. The maps are oriented with the left border of the T-DNA at 0°. The five conserved gene modules present on these and other Ti plasmids are denoted i through v with corresponding shading patterns (see text). The black triangles in region i correspond to T-DNA border repeats, and the black bars of various sizes denote insertion sequence elements or fragments. Note that the conserved regions do not necessarily encode the same functions, e.g., region iv of pTiA6NC encodes for uptake and catabolism of octopine, mannopine, agropine, etc., whereas the corresponding region of pTiC58 encodes for utilization of nopaline (at ∼2 o'clock) or agrocinopine (at ∼8 o'clock). The regions denoted Misc./Hypothetical are composed of nonconserved genes whose functions are either postulated on the basis of homologies with genes in the database or are unknown.

Citation: Christie P. 2004. The Ti Plasmids, p 455-472. In Funnell B, Phillips G (ed), Plasmid Biology. ASM Press, Washington, DC. doi: 10.1128/9781555817732.ch22
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Figure 2

Processing reactions for DNA and protein effectors translocated to plant cells during infection. Left reaction: VirD2/T-strand is derived from a strand-specific displacement/replication replacement reaction at the T-DNA, as described in the text. Triangles represent T-DNA borders; OD, . The step(s) at which VirC1 and VirD1 dissociate from the T-DNA transfer intermediate are not defined. Right reaction: Cotranslation of and facilitates complex formation between the VirE1 chaperone and the VirE2 effector protein. VirE1 (65 amino acids) binds minimally at two sites (E1 boxes) of VirE2 (533 amino acids). The C-terminal half of VirE2 binds ssDNA (SSB box). Two NLSs (shaded boxes) are located to the left of the SSB box. The C terminus of VirE2 is thought to encode a secretion signal (SecSig box) for recognition/docking with translocase. The step at which VirE1 chaperone dissociates from VirE2 is not defined. In the plant, VirE2 associates with the VirD2/T-strand to form the T-complex; this complex then translocates to the plant nuclear pore via interactions with plant proteins, e.g., VIP1 and 2, karopherins, and cyclophilins.

Citation: Christie P. 2004. The Ti Plasmids, p 455-472. In Funnell B, Phillips G (ed), Plasmid Biology. ASM Press, Washington, DC. doi: 10.1128/9781555817732.ch22
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Figure 3

Gene arrangements of type IV secretion loci. Genes encoding VirB homologues are similarly shaded, and those encoding proteins unrelated to VirB are not shaded. (Top) is the first species shown to carry three distinct type IV secretion systems whose substrates are defined, e.g., VirB system transfers T-DNA and protein effectors to plants, whereas Avh transfers pATC58 and Trb transfers the Ti plasmid, respectively, to other bacteria. (Middle) Representative type IV secretion systems of other species that direct conjugal DNA transfer. (Bottom) Representative type IV secretion systems that direct protein transfer during the course of infection.

Citation: Christie P. 2004. The Ti Plasmids, p 455-472. In Funnell B, Phillips G (ed), Plasmid Biology. ASM Press, Washington, DC. doi: 10.1128/9781555817732.ch22
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Figure 4

Possible architectures of type IV secretion machines. The precise physical relationships between the VirD4/TraG coupling proteins, the Mpf structure spanning the cell envelope, and the conjugative T pilus are not yet defined. In each model, substrates are delivered across the cytoplasmic membrane via the VirD4/TraG coupling protein. Models 1 and 3: The coupling protein, located adjacent to the Mpf structure, delivers substrates to the Mpf for secretion across the outer membrane (two-step translocation). Model 2: The coupling protein, embedded in the Mpf, directs substrates through the Mpf (one-step translocation). Models 1 and 2: The T pilus is part of the Mpf and participates directly in substrate transfer. Model 3: The T pilus is not physically associated with an Mpf secretion channel and participates only indirectly in substrate transfer.

Citation: Christie P. 2004. The Ti Plasmids, p 455-472. In Funnell B, Phillips G (ed), Plasmid Biology. ASM Press, Washington, DC. doi: 10.1128/9781555817732.ch22
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Figure 5

Chemical signaling between and the transformed plant cell. The VirA/VirG kinase/response regulator activates expression of the genes in response to recognition of plant signals. T-DNA in transformed plant cells directs synthesis of plant hormones, leading to tumorigenesis, as well as opines that serve as nutrients for the infecting bacterium and modulators of Ti gene expression. TraR and Tra1 direct synthesis of the OOHL autoinducer, and TraR and OOHL direct expression of Ti, Trb and Rep genes, stimulating conjugal transfer of the Ti plasmid.

Citation: Christie P. 2004. The Ti Plasmids, p 455-472. In Funnell B, Phillips G (ed), Plasmid Biology. ASM Press, Washington, DC. doi: 10.1128/9781555817732.ch22
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