Cooperative binding of Agrobacterium tumefaciens VirE2 protein to single-stranded DNA

A tau class GST, OsGSTU5, interacts with VirE2 and modulates the Agrobacterium-mediated transformation in rice

OsGSTU5 interacts and glutathionylates the VirE2 protein of Agrobacterium and its (OsGSTU5) overexpression and downregulation showed a low and high AMT efficiency in rice, respectively. During Agrobacterium-mediated transformation (AMT), T-DNA along with several virulence proteins such as VirD2, VirE2, VirE3, VirD5, and VirF enter the plant cytoplasm. VirE2 serves as a single-stranded DNA binding (SSB) protein that assists the cytoplasmic trafficking of T-DNA inside the host cell. Though the regulatory roles of VirE2 have been established, the cellular reaction of their host, especially in monocots, has not been characterized in detail. This study identified a cellular interactor of VirE2 from the cDNA library of rice. The identified plant protein encoded by the gene cloned from rice was designated OsGSTU5, it interacted specifically with VirE2 in the host cytoplasm. OsGSTU5 was upregulated during Agrobacterium infection and involved in the post-translational glutathionylation of VirE2 (gVirE2). Interestingly, the in silico analysis showed that the 'gVirE2 + ssDNA' complex was structurally less stable than the 'VirE2 + ssDNA' complex. The gel shift assay also confirmed the attenuated SSB property of gVirE2 over VirE2. Moreover, knock-down and overexpression of OsGSTU5 in rice showed increased and decreased T-DNA expression, respectively after Agrobacterium infection. The present finding establishes the role of OsGSTU5 as an important target for modulation of AMT efficiency in rice.

Agrobacterium VirE2 Protein Modulates Plant Gene Expression and Mediates Transformation From Its Location Outside the Nucleus

Agrobacterium effector protein VirE2 is important for plant transformation. VirE2 likely coats transferred DNA (T-DNA) in the plant cell and protects it from degradation. VirE2 localizes to the plant cytoplasm and interacts with several host proteins. Plant-expressed VirE2 can complement a virE2 mutant Agrobacterium strain to support transformation. We investigated whether VirE2 could facilitate transformation from a nuclear location by affixing to it a strong nuclear localization signal (NLS) sequence. Only cytoplasmic-, but not nuclear-localized, VirE2 could stimulate transformation. To investigate the ways VirE2 supports transformation, we generated transgenic Arabidopsis plants containing a virE2 gene under the control of an inducible promoter and performed RNA-seq and proteomic analyses before and after induction. Some differentially expressed plant genes were previously known to facilitate transformation. Knockout mutant lines of some other VirE2 differentially expressed genes showed altered transformation phenotypes. Levels of some proteins known to be important for transformation increased in response to VirE2 induction, but prior to or without induction of their corresponding mRNAs. Overexpression of some other genes whose proteins increased after VirE2 induction resulted in increased transformation susceptibility. We conclude that cytoplasmically localized VirE2 modulates both plant RNA and protein levels to facilitate transformation.

Agrobacterium-delivered VirE2 interacts with host nucleoporin CG1 to facilitate the nuclear import of VirE2-coated T complex

Agrobacterium tumefaciens is the causal agent of crown gall disease. The bacterium is capable of transferring a segment of single-stranded DNA (ssDNA) into recipient cells during the transformation process, and it has been widely used as a genetic modification tool for plants and nonplant organisms. Transferred DNA (T-DNA) has been proposed to be escorted by two virulence proteins, VirD2 and VirE2, as a nucleoprotein complex (T-complex) that targets the host nucleus. However, it is not clear how such a proposed large DNA-protein complex is delivered through the host nuclear pore in a natural setting. Here, we studied the natural nuclear import of the Agrobacterium-delivered ssDNA-binding protein VirE2 inside plant cells by using a split-GFP approach with a newly constructed T-DNA-free strain. Our results demonstrate that VirE2 is targeted into the host nucleus in a VirD2- and T-DNA-dependent manner. In contrast with VirD2 that binds to plant importin α for nuclear import, VirE2 directly interacts with the host nuclear pore complex component nucleoporin CG1 to facilitate its nuclear uptake and the transformation process. Our data suggest a cooperative nuclear import model in which T-DNA is guided to the host nuclear pore by VirD2 and passes through the pore with the assistance of interactions between VirE2 and host nucleoporin CG1. We hypothesize that this large linear nucleoprotein complex (T-complex) is targeted to the nucleus by a "head" guide from the VirD2-importin interaction and into the nucleus by a lateral assistance from the VirE2-nucleoporin interaction.

Agrobacterium VirE3 Uses Its Two Tandem Domains at the C-Terminus to Retain Its Companion VirE2 on the Cytoplasmic Side of the Host Plasma Membrane

Agrobacterium tumefaciens is the causal agent of crown gall disease in nature; in the laboratory the bacterium is widely used for plant genetic modification. The bacterium delivers a single-stranded transferred DNA (T-DNA) and a group of crucial virulence proteins into host cells. A putative T-complex is formed inside host cells that is composed of T-DNA and virulence proteins VirD2 and VirE2, which protect the foreign DNA from degradation and guide its way into the host nucleus. However, little is known about how the T-complex is assembled inside host cells. We combined the split-GFP and split-sfCherry labeling systems to study the interaction of Agrobacterium-delivered VirE2 and VirE3 in host cells. Our results indicated that VirE2 co-localized with VirE3 on the cytoplasmic side of the host cellular membrane upon the delivery. We identified and characterized two tandem domains at the VirE3 C-terminus that interacted with VirE2 in vitro. Deletion of these two domains abolished the VirE2 accumulation on the host plasma membrane and affected the transformation. Furthermore, the two VirE2-interacting domains of VirE3 exhibited different affinities with VirE2. Collectively, this study demonstrates that the anchorage protein VirE3 uses the two tandem VirE2-interacting domains to facilitate VirE2 protection for T-DNA at the cytoplasmic side of the host cell entrance.

Agrobacterium Delivers Anchorage Protein VirE3 for Companion VirE2 to Aggregate at Host Entry Sites for T-DNA Protection

Agrobacterium tumefaciens transfers oncogenic DNA (T-DNA) and effector proteins into various host plants. T-DNA is generated inside the bacteria and subsequently delivered into plant cells along with the companion effectors VirD2, VirE2, and VirE3. However, it is not clear how the T-complex consisting of VirD2 and VirE2 is assembled inside plant cells. Here, we report that the effector protein VirE3 localized to plant plasma membranes as an anchorage through a conserved α-helical-bundle domain. VirE3 interacted with itself and enabled VirE2 accumulation at host entry sites through direct interactions. VirE3 was critical for VirE2 function in T-DNA protection. Our data indicate that VirE3 functions as a previously unrecognized anchorage protein consisting of membrane-binding, self-interacting, and VirE2-interacting domains. Both VirE2 and VirE3 are conserved among Agrobacterium and rhizobia species but not other organisms, suggesting that a group of anchorage proteins have been generated through evolution to facilitate the nucleoprotein assembly at plant membranes.

The Agrobacterium VirE2 effector interacts with multiple members of the Arabidopsis VIP1 protein family

T-DNA transfer from Agrobacterium to its host plant genome relies on multiple interactions between plant proteins and bacterial effectors. One such plant protein is the Arabidopsis VirE2 interacting protein (AtVIP1), a transcription factor that binds Agrobacterium tumefaciens C58 VirE2, potentially acting as an adaptor between VirE2 and several other host factors. It remains unknown, however, whether the same VirE2 protein has evolved to interact with multiple VIP1 homologues in the same host, and whether VirE2 homologues encoded by different bacterial strains/species recognize AtVIP1 or its homologues. Here, we addressed these questions by systematic analysis (using the yeast two-hybrid and co-immunoprecipitation approaches) of interactions between VirE2 proteins encoded by four major representatives of known bacterial species/strains with functional T-DNA transfer machineries and eight VIP1 homologues from Arabidopsis and tobacco. We also analysed the determinants of the VirE2 sequence involved in these interactions. These experiments showed that the VirE2 interaction is degenerate: the same VirE2 protein has evolved to interact with multiple VIP1 homologues in the same host, and different and mutually independent VirE2 domains are involved in interactions with different VIP1 homologues. Furthermore, the VIP1 functionality related to the interaction with VirE2 is independent of its function as a transcriptional regulator. These observations suggest that the ability of VirE2 to interact with VIP1 homologues is deeply ingrained into the process of Agrobacterium infection. Indeed, mutations that abolished VirE2 interaction with AtVIP1 produced no statistically significant effects on interactions with VIP1 homologues or on the efficiency of genetic transformation.

How does Agrobacterium virulence protein VirE2 hijack host actomyosin motility system?

Agrobacterium tumefaciens has the ability to transform a wide range of eukaryotes by transferring single-stranded (ss) DNA (T-DNA) into the host cells. During the process, the bacterium also delivers abundant amount of ssDNA binding protein VirE2 into the host cytoplasm, where multiple VirE2 molecules bind and coat the T-DNA and thus generate large-sized nucleoprotein complex. VirE2 trafficking should be representative of T-DNA movement. Recently, we demonstrated that plant actomyosin motility system facilitated the trafficking of Agrobacterium-delivered VirE2 inside host cytoplasm. We showed that actomyosin-driven VirE2 moved towards the nucleus along the endoplasmic reticulum (ER). Here we hypothesize that the presence of VirE2 on the cytoplasmic side of the ER may provide VirE2 with convenient access to the opening of nuclear pore complexes (NPCs). We propose models to explain how VirE2 is trafficked in the context of actin, myosin and ER.

Agrobacterium delivers VirE2 protein into host cells via clathrin-mediated endocytosis

Agrobacterium tumefaciens can cause crown gall tumors on a wide range of host plants. As a natural genetic engineer, the bacterium can transfer both single-stranded DNA (ssDNA) [transferred DNA (T-DNA)] molecules and bacterial virulence proteins into various recipient cells. Among Agrobacterium-delivered proteins, VirE2 is an ssDNA binding protein that is involved in various steps of the transformation process. However, it is not clear how plant cells receive the T-DNA or protein molecules. Using a split-green fluorescent protein approach, we monitored the VirE2 delivery process inside plant cells in real time. We observed that A. tumefaciens delivered VirE2 from the bacterial lateral sides that were in close contact with plant membranes. VirE2 initially accumulated on plant cytoplasmic membranes at the entry points. VirE2-containing membranes were internalized through clathrin-mediated endocytosis to form endomembrane compartments. VirE2 colocalized with the early endosome marker SYP61 but not with the late endosome marker ARA6, suggesting that VirE2 escaped from early endosomes for subsequent trafficking inside the cells. Dual endocytic motifs at the carboxyl-terminal tail of VirE2 were involved in VirE2 internalization and could interact with the μ subunit of the plant clathrin-associated adaptor AP2 complex (AP2M). Both the VirE2 cargo motifs and AP2M were important for the transformation process. Because AP2-mediated endocytosis is well conserved, our data suggest that the A. tumefaciens pathogen hijacks conserved endocytic pathways to facilitate the delivery of virulence factors. This might be important for Agrobacterium to achieve both a wide host range and a high transformation efficiency.

Agrobacterium-delivered virulence protein VirE2 is trafficked inside host cells via a myosin XI-K-powered ER/actin network

Agrobacterium tumefaciens causes crown gall tumors on various plants by delivering transferred DNA (T-DNA) and virulence proteins into host plant cells. Under laboratory conditions, the bacterium is widely used as a vector to genetically modify a wide range of organisms, including plants, yeasts, fungi, and algae. Various studies suggest that T-DNA is protected inside host cells by VirE2, one of the virulence proteins. However, it is not clear how Agrobacterium-delivered factors are trafficked through the cytoplasm. In this study, we monitored the movement of Agrobacterium-delivered VirE2 inside plant cells by using a split-GFP approach in real time. Agrobacterium-delivered VirE2 trafficked via the endoplasmic reticulum (ER) and F-actin network inside plant cells. During this process, VirE2 was aggregated as filamentous structures and was present on the cytosolic side of the ER. VirE2 movement was powered by myosin XI-K. Thus, exogenously produced and delivered VirE2 protein can use the endogenous host ER/actin network for movement inside host cells. The A. tumefaciens pathogen hijacks the conserved host infrastructure for virulence trafficking. Well-conserved infrastructure may be useful for Agrobacterium to target a wide range of recipient cells and achieve a high efficiency of transformation.

G5, a Phage Single-Stranded DNA-Binding Protein, Fused with a Nuclear Localization Signal, Attenuates Symptoms and Reduces Begomovirus-Betasatellite Accumulation in Transgenic Plants

Cotton leaf curl disease is caused by several monopartite begomoviruses and is the major threat to cotton production in the Indian subcontinent. The disease has been shown to be associated with four distinct species, including Cotton leaf curl Kokhran virus (CLCuKoV), and a specific betasatellite-Cotton leaf curl Multan betasatellite (CLCuMuB). Transgenic Nicotiana benthamiana plants were produced which constitutively express the Escherichia coli phage M13 encoded, sequence nonspecific single-stranded (ss) DNA-binding protein, G5 alone and fused with the maize opaque-2 nuclear localization signal (NLS), to evaluate resistance against CLCuKoV-CLCuMuB. Transgenic plants expressing only G5 performed poorly exhibiting symptoms of infection and high virus DNA levels upon inoculation with CLCuKoV and CLCuKoV with CLCuMuB. In contrast, plants transformed with G5 fused to the NLS developed mild symptoms and showed a reduction in virus and betasatellite DNA levels in comparison to nontransformed plants. The results show that G5 may be useful in developing broad-spectrum resistance against ssDNA viruses.

Yeast Actin-Related Protein ARP6 Negatively Regulates Agrobacterium-Mediated Transformation of Yeast Cell

The yeasts, including Saccharomyces cerevisiae and Pichia pastoris, are single-cell eukaryotic organisms that can serve as models for human genetic diseases and hosts for large scale production of recombinant proteins in current biopharmaceutical industry. Thus, efficient genetic engineering tools for yeasts are of great research and economic values. Agrobacterium tumefaciens-mediated transformation (AMT) can transfer T-DNA into yeast cells as a method for genetic engineering. However, how the T-DNA is transferred into the yeast cells is not well established yet. Here our genetic screening of yeast knockout mutants identified a yeast actin-related protein ARP6 as a negative regulator of AMT. ARP6 is a critical member of the SWR1 chromatin remodeling complex (SWR-C); knocking out some other components of the complex also increased the transformation efficiency, suggesting that ARP6 might regulate AMT via SWR-C. Moreover, knockout of ARP6 led to disruption of microtubule integrity, higher uptake and degradation of virulence proteins, and increased DNA stability inside the cells, all of which resulted in enhanced transformation efficiency. Our findings have identified molecular and cellular mechanisms regulating AMT and a potential target for enhancing the transformation efficiency in yeast cells.

Surface plasmon resonance imaging reveals multiple binding modes of Agrobacterium transformation mediator VirE2 to ssDNA

VirE2 is the major secreted protein of Agrobacterium tumefaciens in its genetic transformation of plant hosts. It is co-expressed with a small acidic chaperone VirE1, which prevents VirE2 oligomerization. After secretion into the host cell, VirE2 serves functions similar to a viral capsid in protecting the single-stranded transferred DNA en route to the nucleus. Binding of VirE2 to ssDNA is strongly cooperative and depends moreover on protein–protein interactions. In order to isolate the protein–DNA interactions, imaging surface plasmon resonance (SPRi) studies were conducted using surface-immobilized DNA substrates of length comparable to the protein-binding footprint. Binding curves revealed an important influence of substrate rigidity with a notable preference for poly-T sequences and absence of binding to both poly-A and double-stranded DNA fragments. Dissociation at high salt concentration confirmed the electrostatic nature of the interaction. VirE1–VirE2 heterodimers also bound to ssDNA, though by a different mechanism that was insensitive to high salt. Neither VirE2 nor VirE1–VirE2 followed the Langmuir isotherm expected for reversible monomeric binding. The differences reflect the cooperative self-interactions of VirE2 that are suppressed by VirE1.

The Agrobacterium tumefaciens Ti Plasmid Virulence Gene virE2 Reduces Sri Lankan Cassava Mosaic Virus Infection in Transgenic Nicotiana benthamiana Plants

Cassava mosaic disease is a major constraint to cassava cultivation worldwide. In India, the disease is caused by Indian cassava mosaic virus (ICMV) and Sri Lankan cassava mosaic virus (SLCMV). The Agrobacterium Ti plasmid virulence gene virE2, encoding a nuclear-localized, single-stranded DNA binding protein, was introduced into Nicotiana benthamiana to develop tolerance against SLCMV. Leaf discs of transgenic N. benthamiana plants, harboring the virE2 gene, complemented a virE2 mutation in A. tumefaciens and produced tumours. Three tested virE2 transgenic plants displayed reduction in disease symptoms upon agroinoculation with SLCMV DNA A and DNA B partial dimers. A pronounced reduction in viral DNA accumulation was observed in all three virE2 transgenic plants. Thus, virE2 is an effective candidate gene to develop tolerance against the cassava mosaic disease and possibly other DNA virus diseases.

Is VIP1 important for Agrobacterium-mediated transformation?

Agrobacterium genetically transforms plants by transferring and integrating T-(transferred) DNA into the host genome. This process requires both Agrobacterium and host proteins. VirE2 interacting protein 1 (VIP1), an Arabidopsis bZIP protein, has been suggested to mediate transformation through interaction with and targeting of VirE2 to nuclei. We examined the susceptibility of Arabidopsis vip1 mutant and VIP1 overexpressing plants to transformation by numerous Agrobacterium strains. In no instance could we detect altered transformation susceptibility. We also used confocal microscopy to examine the subcellular localization of Venus-tagged VirE2 or Venus-tagged VIP1, in the presence or absence of the other untagged protein, in different plant cell systems. We found that VIP1-Venus localized in both the cytoplasm and the nucleus of Arabidopsis roots, agroinfiltrated Nicotiana benthamiana leaves, Arabidopsis mesophyll protoplasts and tobacco BY-2 protoplasts, regardless of whether VirE2 was co-expressed. VirE2 localized exclusively to the cytoplasm of tobacco and Arabidopsis protoplasts, whether in the absence or presence of VIP1 overexpression. In transgenic Arabidopsis plants and agroinfiltrated N. benthamina leaves we could occasionally detect small aggregates of the Venus signal in nuclei, but these were likely to be imagining artifacts. The vast majority of VirE2 remained in the cytoplasm. We conclude that VIP1 is not important for Agrobacterium-mediated transformation or VirE2 subcellular localization.

Visualization of VirE2 protein translocation by the Agrobacterium type IV secretion system into host cells

Type IV secretion systems (T4SS) can mediate the translocation of bacterial virulence proteins into host cells. The plant pathogen Agrobacterium tumefaciens uses a T4SS to deliver a VirD2-single stranded DNA complex as well as the virulence proteins VirD5, VirE2, VirE3, and VirF into host cells so that these become genetically transformed. Besides plant cells, yeast and fungi can efficiently be transformed by Agrobacterium. Translocation of virulence proteins by the T4SS has so far only been shown indirectly by genetic approaches. Here we report the direct visualization of VirE2 protein translocation by using bimolecular fluorescence complementation (BiFC) and Split GFP visualization strategies. To this end, we cocultivated Agrobacterium strains expressing VirE2 tagged with one part of a fluorescent protein with host cells expressing the complementary part, either fused to VirE2 (for BiFC) or not (Split GFP). Fluorescent filaments became visible in recipient cells 20-25 h after the start of the cocultivation indicative of VirE2 protein translocation. Evidence was obtained that filament formation was due to the association of VirE2 with the microtubuli.

Characterization of the single stranded DNA binding protein SsbB encoded in the Gonoccocal Genetic Island

Background

Most strains of Neisseria gonorrhoeae carry a Gonococcal Genetic Island which encodes a type IV secretion system involved in the secretion of ssDNA. We characterize the GGI-encoded ssDNA binding protein, SsbB. Close homologs of SsbB are located within a conserved genetic cluster found in genetic islands of different proteobacteria. This cluster encodes DNA-processing enzymes such as the ParA and ParB partitioning proteins, the TopB topoisomerase, and four conserved hypothetical proteins. The SsbB homologs found in these clusters form a family separated from other ssDNA binding proteins.

Methodology/Principal Findings

In contrast to most other SSBs, SsbB did not complement the Escherichia coli ssb deletion mutant. Purified SsbB forms a stable tetramer. Electrophoretic mobility shift assays and fluorescence titration assays, as well as atomic force microscopy demonstrate that SsbB binds ssDNA specifically with high affinity. SsbB binds single-stranded DNA with minimal binding frames for one or two SsbB tetramers of 15 and 70 nucleotides. The binding mode was independent of increasing Mg²⁺ or NaCl concentrations. No role of SsbB in ssDNA secretion or DNA uptake could be identified, but SsbB strongly stimulated Topoisomerase I activity.

Conclusions/Significance

We propose that these novel SsbBs play an unknown role in the maintenance of genetic islands.

Traversing the Cell: Agrobacterium T-DNA's Journey to the Host Genome

The genus Agrobacterium is unique in its ability to conduct interkingdom genetic exchange. Virulent Agrobacterium strains transfer single-strand forms of T-DNA (T-strands) and several Virulence effector proteins through a bacterial type IV secretion system into plant host cells. T-strands must traverse the plant wall and plasma membrane, traffic through the cytoplasm, enter the nucleus, and ultimately target host chromatin for stable integration. Because any DNA sequence placed between T-DNA "borders" can be transferred to plants and integrated into the plant genome, the transfer and intracellular trafficking processes must be mediated by bacterial and host proteins that form complexes with T-strands. This review summarizes current knowledge of proteins that interact with T-strands in the plant cell, and discusses several models of T-complex (T-strand and associated proteins) trafficking. A detailed understanding of how these macromolecular complexes enter the host cell and traverse the plant cytoplasm will require development of novel technologies to follow molecules from their bacterial site of synthesis into the plant cell, and how these transferred molecules interact with host proteins and sub-cellular structures within the host cytoplasm and nucleus.

Finding a way to the nucleus

Agrobacterium species transfer single-strand DNA and virulence effector proteins to plants. To understand how Agrobacterium achieves interkingdom horizontal gene transfer, scientists have investigated how the interaction of bacterial effector proteins with host proteins directs T-DNA to the plant nucleus. VirE2, a single-strand DNA binding protein, likely plays a key role in T-DNA nuclear targeting. However, subcellular trafficking of VirE2 remains controversial, with reports of both cytoplasmic and nuclear localization. The recent discovery that phosphorylation of the VirE2 interacting protein VIP1 modulates both nuclear targeting and transformation may provide a solution to this conundrum. Novel experimental systems that allow tracking of VirE2 as it exits Agrobacterium and enters the plant cell will also aid in understanding virulence protein/T-DNA cytoplasmic trafficking.

Agrobacterium tumefaciens and A. rhizogenes use different proteins to transport bacterial DNA into the plant cell nucleus

Agrobacterium tumefaciens and A. rhizogenes transport single‐stranded DNA (ssDNA; T‐strands) and virulence proteins into plant cells through a type IV secretion system. DNA transfer initiates when VirD2 nicks border sequences in the tumour‐inducing plasmid, attaches to the 5′ end, and pilots T‐strands into plant cells. Agrobacterium tumefaciens translocates ssDNA‐binding protein VirE2 into plant cells where it targets T‐strands into the nucleus. Some A. rhizogenes strains lack VirE2 but transfer T‐strands efficiently due to the GALLS gene, which complements an A. tumefaciens virE2 mutant. VirE2 and full‐length GALLS (GALLS‐FL) contain nuclear localization sequences that target these proteins to the plant cell nucleus. VirE2 binds cooperatively to T‐strands allowing it to move ssDNA without ATP hydrolysis. Unlike VirE2, GALLS‐FL contains ATP‐binding and helicase motifs similar to those in TraA, a strand transferase involved in conjugation. VirE2 may accumulate in the nucleus and pull T‐strands into the nucleus using the force generated by cooperative DNA binding. GALLS‐FL accumulates inside the nucleus where its predicted ATP‐dependent strand transferase may pull T‐strands into the nucleus. These different mechanisms for nuclear import of T‐strands may affect the efficiency and quality of transgenic events in plant biotechnology applications.

IMPa-4, an Arabidopsis importin alpha isoform, is preferentially involved in agrobacterium-mediated plant transformation

Successful transformation of plants by Agrobacterium tumefaciens requires that the bacterial T-complex actively escorts T-DNA into the host's nucleus. VirD2 and VirE2 are virulence proteins on the T-complex that have plant-functional nuclear localization signal sequences that may recruit importin alpha proteins of the plant for nuclear import. In this study, we evaluated the involvement of seven of the nine members of the Arabidopsis thaliana importin alpha family in Agrobacterium transformation. Yeast two-hybrid, plant bimolecular fluorescence complementation, and in vitro protein-protein interaction assays demonstrated that all tested Arabidopsis importin alpha members can interact with VirD2 and VirE2. However, only disruption of the importin IMPa-4 inhibited transformation and produced the rat (resistant to Agrobacterium transformation) phenotype. Overexpression of six importin alpha members, including IMPa-4, rescued the rat phenotype in the impa-4 mutant background. Roots of wild-type and impa-4 Arabidopsis plants expressing yellow fluorescent protein-VirD2 displayed nuclear localization of the fusion protein, indicating that nuclear import of VirD2 is not affected in the impa-4 mutant. Somewhat surprisingly, VirE2-yellow fluorescent protein mainly localized to the cytoplasm of both wild-type and impa-4 Arabidopsis cells and to the cytoplasm of wild-type tobacco (Nicotiana tabacum) cells. However, bimolecular fluorescence complementation assays indicated that VirE2 could localize to the nucleus when IMPa-4, but not when IMPa-1, was overexpressed.

Crystal structure of the Agrobacterium virulence complex VirE1-VirE2 reveals a flexible protein that can accommodate different partners

Agrobacterium tumefaciens infects its plant hosts by a mechanism of horizontal gene transfer. This capability has led to its widespread use in artificial genetic transformation. In addition to DNA, the bacterium delivers an abundant ssDNA binding protein, VirE2, whose roles in the host include protection from cytoplasmic nucleases and adaptation for nuclear import. In Agrobacterium, VirE2 is bound to its acidic chaperone VirE1. When expressed in vitro in the absence of VirE1, VirE2 is prone to oligomerization and forms disordered filamentous aggregates. These filaments adopt an ordered solenoidal form in the presence of ssDNA, which was characterized previously by electron microscopy and three-dimensional image processing. VirE2 coexpressed in vitro with VirE1 forms a soluble heterodimer. VirE1 thus prevents VirE2 oligomerization and competes with its binding to ssDNA. We present here a crystal structure of VirE2 in complex with VirE1, showing that VirE2 is composed of two independent domains presenting a novel fold, joined by a flexible linker. Electrostatic interactions with VirE1 cement the two domains of VirE2 into a locked form. Comparison with the electron microscopy structure indicates that the VirE2 domains adopt different relative orientations. We suggest that the flexible linker between the domains enables VirE2 to accommodate its different binding partners.

Plant transformation by Agrobacterium tumefaciens: modulation of single-stranded DNA-VirE2 complex assembly by VirE1

Agrobacterium tumefaciens infects plant cells by the transfer of DNA. A key factor in this process is the bacterial virulence protein VirE2, which associates stoichiometrically with the transported single-stranded (ss) DNA molecule (T-strand). As observed in vitro by transmission electron microscopy, VirE2-ssDNA readily forms an extended helical complex with a structure well suited to the tasks of DNA protection and nuclear import. Here we have elucidated the role of the specific molecular chaperone VirE1 in regulating VireE2-VirE2 and VirE2-ssDNA interactions. VirE2 alone formed functional filamentous aggregates capable of ssDNA binding. In contrast, co-expression with VirE1 yielded monodisperse VirE1-VirE2 complexes. Cooperative binding of VirE2 to ssDNA released VirE1, resulting in a controlled formation mechanism for the helical complex that is further promoted by macromolecular crowding. Based on this in vitro evidence, we suggest that the constrained volume of the VirB channel provides a natural site for the exchange of VirE2 binding from VirE1 to the T-strand.

Agrobacterium rhizogenes GALLS protein substitutes for Agrobacterium tumefaciens single-stranded DNA-binding protein VirE2

Agrobacterium tumefaciens and Agrobacterium rhizogenes transfer plasmid-encoded genes and virulence (Vir) proteins into plant cells. The transferred DNA (T-DNA) is stably inherited and expressed in plant cells, causing crown gall or hairy root disease. DNA transfer from A. tumefaciens into plant cells resembles plasmid conjugation; single-stranded DNA (ssDNA) is exported from the bacteria via a type IV secretion system comprised of VirB1 through VirB11 and VirD4. Bacteria also secrete certain Vir proteins into plant cells via this pore. One of these, VirE2, is an ssDNA-binding protein crucial for efficient T-DNA transfer and integration. VirE2 binds incoming ssT-DNA and helps target it into the nucleus. Some strains of A. rhizogenes lack VirE2, but they still transfer T-DNA efficiently. We isolated a novel gene from A. rhizogenes that restored pathogenicity to virE2 mutant A. tumefaciens. The GALLS gene was essential for pathogenicity of A. rhizogenes. Unlike VirE2, GALLS contains a nucleoside triphosphate binding motif similar to one in TraA, a strand transferase conjugation protein. Despite their lack of similarity, GALLS substituted for VirE2.

Three-dimensional reconstruction of Agrobacterium VirE2 protein with single-stranded DNA

Agrobacterium tumefaciens infects plant cells by a unique mechanism involving an interkingdom genetic transfer. A single-stranded DNA substrate is transported across the two cell walls along with the bacterial virulence proteins VirD2 and VirE2. A single VirD2 molecule covalently binds to the 5'-end of the single-stranded DNA, while the VirE2 protein binds stoichiometrically along the length of the DNA, without sequence specificity. An earlier transmission/scanning transmission electron microscopy study indicated a solenoidal ("telephone coil") organization of the VirE2-DNA complex. Here we report a three-dimensional reconstruction of this complex using electron microscopy and single-particle image-processing methods. We find a hollow helical structure of 15.7-nm outer diameter, with a helical rise of 51.5 nm and 4.25 VirE2 proteins/turn. The inner face of the protein units contains a continuous wall and an inward protruding shelf. These structures appear to accommodate the DNA binding. Such a quaternary arrangement naturally sequesters the DNA from cytoplasmic nucleases and suggests a mechanism for its nuclear import by decoration with host cell factors. Coexisting with the helices, we also found VirE2 tetrameric ring structures. A two-dimensional average of the latter confirms the major features of the three-dimensional reconstruction.

Site-specific integration of Agrobacterium tumefaciens T-DNA via double-stranded intermediates

Agrobacterium tumefaciens-mediated genetic transformation involves transfer of a single-stranded T-DNA molecule (T strand) into the host cell, followed by its integration into the plant genome. The molecular mechanism of T-DNA integration, the culmination point of the entire transformation process, remains largely obscure. Here, we studied the roles of double-stranded breaks (DSBs) and double-stranded T-DNA intermediates in the integration process. We produced transgenic tobacco (Nicotiana tabacum) plants carrying an I-SceI endonuclease recognition site that, upon cleavage with I-SceI, generates DSB. Then, we retransformed these plants with two A. tumefaciens strains: one that allows transient expression of I-SceI to induce DSB and the other that carries a T-DNA with the I-SceI site and an integration selection marker. Integration of this latter T-DNA as full-length and I-SceI-digested molecules into the DSB site was analyzed in the resulting plants. Of 620 transgenic plants, 16 plants integrated T-DNA into DSB at their I-SceI sites; because DSB induces DNA repair, these results suggest that the invading T-DNA molecules target to the DNA repair sites for integration. Furthermore, of these 16 plants, seven plants incorporated T-DNA digested with I-SceI, which cleaves only double-stranded DNA. Thus, T-strand molecules can be converted into double-stranded intermediates before their integration into the DSB sites within the host cell genome.

VirE2, a type IV secretion substrate, interacts with the VirD4 transfer protein at cell poles of Agrobacterium tumefaciens

Agrobacterium tumefaciens transfers oncogenic DNA and effector proteins to plant cells during the course of infection. Substrate translocation across the bacterial cell envelope is mediated by a type IV secretion (TFS) system composed of the VirB proteins, as well as VirD4, a member of a large family of inner membrane proteins implicated in the coupling of DNA transfer intermediates to the secretion machine. In this study, we demonstrate with novel cytological screens - a two-hybrid (C2H) assay and bimolecular fluorescence complementation (BiFC) - and by immunoprecipitation of chemically cross-linked protein complexes that the VirE2 effector protein interacts directly with the VirD4 coupling protein at cell poles of A. tumefaciens. Analyses of truncation derivatives showed that VirE2 interacts via its C terminus with VirD4, and, further, an NH2-terminal membrane-spanning domain of VirD4 is dispensable for complex formation. VirE2 interacts with VirD4 independently of the virB-encoded transfer machine and T pilus, the putative periplasmic chaperones AcvB and VirJ, and the T-DNA transfer intermediate. Finally, VirE2 is recruited to polar-localized VirD4 as a complex with its stabilizing secretion chaperone VirE1, yet the effector-coupling protein interaction is not dependent on chaperone binding. Together, our findings establish for the first time that a protein substrate of a type IV secretion system is recruited to a member of the coupling protein superfamily.

Agrobacterium-mediated plant transformation: the biology behind the "gene-jockeying" tool

Agrobacterium tumefaciens and related Agrobacterium species have been known as plant pathogens since the beginning of the 20th century. However, only in the past two decades has the ability of Agrobacterium to transfer DNA to plant cells been harnessed for the purposes of plant genetic engineering. Since the initial reports in the early 1980s using Agrobacterium to generate transgenic plants, scientists have attempted to improve this "natural genetic engineer" for biotechnology purposes. Some of these modifications have resulted in extending the host range of the bacterium to economically important crop species. However, in most instances, major improvements involved alterations in plant tissue culture transformation and regeneration conditions rather than manipulation of bacterial or host genes. Agrobacterium-mediated plant transformation is a highly complex and evolved process involving genetic determinants of both the bacterium and the host plant cell. In this article, I review some of the basic biology concerned with Agrobacterium-mediated genetic transformation. Knowledge of fundamental biological principles embracing both the host and the pathogen have been and will continue to be key to extending the utility of Agrobacterium for genetic engineering purposes.

VIP1, an Arabidopsis protein that interacts with Agrobacterium VirE2, is involved in VirE2 nuclear import and Agrobacterium infectivity

T-DNA nuclear import is a central event in genetic transformation of plant cells by Agrobacterium. This event is thought to be mediated by two bacterial proteins, VirD2 and VirE2, which are associated with the transported T-DNA molecule. While VirD2 is imported into the nuclei of plant, animal and yeast cells, nuclear uptake of VirE2 occurs most efficiently in plant cells. To understand better the mechanism of VirE2 action, a cellular interactor of VirE2 was identified and its encoding gene cloned from Arabidopsis. The identified plant protein, designated VIP1, specifically bound VirE2 and allowed its nuclear import in non-plant systems. In plants, VIP1 was required for VirE2 nuclear import and Agrobacterium tumorigenicity, participating in early stages of T-DNA expression.

The Legionella pneumophila IcmR protein exhibits chaperone activity for IcmQ by preventing its participation in high-molecular-weight complexes

A key event in legionellosis is the ability of Legionella pneumophila to survive and proliferate inside alveolar macrophages. The dot/icm genes, which are necessary for intracellular growth, show sequence similarity to genes encoding conjugative transfer systems, and it is believed that they are responsible for the formation of a secretion apparatus. Evidence is provided here that the IcmR and IcmQ proteins participate in a chaperone-substrate relationship similar to that observed for translocated proteins in type III and type IV secretion apparatuses. Immobilized IcmQ was found to bind IcmR from crude bacterial extracts efficiently. Furthermore, purified IcmR and IcmQ bind with high affinity. This interaction was also observed in vivo by co-immunoprecipitation. The presence of IcmR was found to affect the physical state of IcmQ directly. In the absence of IcmR, IcmQ formed high-molecular-weight complexes both in vivo and in vitro, whereas IcmR prevented and reversed the formation of these complexes.

Import of Agrobacterium T-DNA into plant nuclei: two distinct functions of VirD2 and VirE2 proteins

To study the mechanism of nuclear import of T-DNA, complexes consisting of the virulence proteins VirD2 and VirE2 as well as single-stranded DNA (ssDNA) were tested for import into plant nuclei in vitro. Import of these complexes was fast and efficient and could be inhibited by a competitor, a nuclear localization signal (NLS) coupled to BSA. For import of short ssDNA, VirD2 was sufficient, whereas import of long ssDNA additionally required VirE2. A VirD2 mutant lacking its C-terminal NLS was unable to mediate import of the T-DNA complexes into nuclei. Although free VirE2 molecules were imported into nuclei, once bound to ssDNA they were not imported, implying that when complexed to DNA, the NLSs of VirE2 are not exposed and thus do not function. RecA, another ssDNA binding protein, could substitute for VirE2 in the nuclear import of T-DNA but not in earlier events of T-DNA transfer to plant cells. We propose that VirD2 directs the T-DNA complex to the nuclear pore, whereas both proteins mediate its passage through the pore. Therefore, by binding to ssDNA, VirE2 may shape the T-DNA complex such that it is accepted for translocation into the nucleus.

An Agrobacterium VirE2 channel for transferred-DNA transport into plant cells

Transferred DNA (T-DNA) transfer from Agrobacterium tumefaciens into eukaryotic cells is the only known example of interkingdom DNA transfer. T-DNA is a single-stranded segment of Agrobacterium's tumor-inducing plasmid that enters the plant cell as a complex with the bacterial virulence proteins VirD2 and VirE2. The VirE2 protein is highly induced on contact of A. tumefaciens with a plant host and has been reported to act in late steps of transfer. One of its previously demonstrated functions is binding to the single-stranded (ss) T-DNA and protecting it from degradation. Recent experiments suggest other functions of the protein. A combination of planar lipid bilayer experiments, vesicle swelling assays, and DNA transport experiments demonstrated that VirE2 can insert itself into artificial membranes and form channels. These channels are voltage gated, anion selective, and single-stranded DNA-specific and can facilitate the efficient transport of single-stranded DNA through membranes. These experiments demonstrate a VirE2 function as a transmembrane DNA transporter, which could have applications in gene delivery systems.

An Agrobacterium VirE2 channel for transferred-DNA transport into plant cells

Transferred DNA (T-DNA) transfer from Agrobacterium tumefaciens into eukaryotic cells is the only known example of interkingdom DNA transfer. T-DNA is a single-stranded segment of Agrobacterium's tumor-inducing plasmid that enters the plant cell as a complex with the bacterial virulence proteins VirD2 and VirE2. The VirE2 protein is highly induced on contact of A. tumefaciens with a plant host and has been reported to act in late steps of transfer. One of its previously demonstrated functions is binding to the single-stranded (ss) T-DNA and protecting it from degradation. Recent experiments suggest other functions of the protein. A combination of planar lipid bilayer experiments, vesicle swelling assays, and DNA transport experiments demonstrated that VirE2 can insert itself into artificial membranes and form channels. These channels are voltage gated, anion selective, and single-stranded DNA-specific and can facilitate the efficient transport of single-stranded DNA through membranes. These experiments demonstrate a VirE2 function as a transmembrane DNA transporter, which could have applications in gene delivery systems.

AGROBACTERIUM AND PLANT GENES INVOLVED IN T-DNA TRANSFER AND INTEGRATION

The phytopathogenic bacterium Agrobacterium tumefaciens genetically transforms plants by transferring a portion of the resident Ti-plasmid, the T-DNA, to the plant. Accompanying the T-DNA into the plant cell is a number of virulence (Vir) proteins. These proteins may aid in T-DNA transfer, nuclear targeting, and integration into the plant genome. Other virulence proteins on the bacterial surface form a pilus through which the T-DNA and the transferred proteins may translocate. Although the roles of these virulence proteins within the bacterium are relatively well understood, less is known about their roles in the plant cell. In addition, the role of plant-encoded proteins in the transformation process is virtually unknown. In this article, I review what is currently known about the functions of virulence and plant proteins in several aspects of the Agrobacterium transformation process.

The N- and C-terminal portions of the Agrobacterium VirB1 protein independently enhance tumorigenesis

Genetic transformation of plants by Agrobacterium tumefaciens is mediated by a virulence (vir)-specific type IV secretion apparatus assembled from 11 VirB proteins and VirD4. VirB1, targeted to the periplasm by an N-terminal signal peptide, is processed to yield VirB1*, comprising the C-terminal 73 amino acids. The N-terminal segment, which shares homology with chicken egg white lysozyme as well as lytic transglycosylases, may provide local lysis of the peptidoglycan cell wall to create channels for transporter assembly. Synthesis of VirB1* followed by its secretion to the exterior of the cell suggests that VirB1* may also have a role in virulence. In the present study, we provide evidence for the dual roles of VirB1 in tumorigenesis as well as the requirements for processing and secretion of VirB1*. Complementation of a virB1 deletion strain with constructs expressing either the N-terminal lysozyme-homologous region or VirB1* results in tumors intermediate in size between those induced by a wild-type strain and a virB1 deletion strain, suggesting that each domain has a unique role in tumorigenesis. The secretion of VirB1* translationally fused to the signal peptide indicates that processing and secretion are not coupled. When expressed independently of all other vir genes, VirB1 was processed and VirB1* was secreted. When restricted to the cytoplasm by deletion of the signal peptide, VirB1 was neither processed nor secreted and did not restore virulence to the virB1 deletion strain. Thus, factors that mediate processing of VirB1 and secretion of VirB1* are localized in the periplasm or outer membrane and are not subject to vir regulation.

The Agrobacterium tumefaciens chaperone-like protein, VirE1, interacts with VirE2 at domains required for single-stranded DNA binding and cooperative interaction

Agrobacterium tumefaciens transfers single-stranded DNA (ssDNA) into plants. Efficient tumorigenesis requires VirE1-dependent export of ssDNA-binding (SSB) protein VirE2. VirE1 binds VirE2 domains involved in SSB and self-association, and VirE1 may facilitate VirE2 export by preventing VirE2 aggregation and the premature binding of VirE2 to ssDNA.

Mutagenesis of the Agrobacterium VirE2 single-stranded DNA-binding protein identifies regions required for self-association and interaction with VirE1 and a permissive site for hybrid protein construction

The VirE2 single-stranded DNA-binding protein (SSB) of Agrobacterium tumefaciens is required for delivery of T-DNA to the nuclei of susceptible plant cells. By yeast two-hybrid and immunoprecipitation analyses, VirE2 was shown to self-associate and to interact with VirE1. VirE2 mutants with small deletions or insertions of a 31-residue oligopeptide (i31) at the N or C terminus or with an i31 peptide insertion at Leu236 retained the capacity to form homomultimers. By contrast, VirE2 mutants with modifications outside a central region located between residues 320 and 390 retained the capacity to interact with VirE1. These findings suggest the tertiary structure of VirE2 is important for homomultimer formation whereas a central domain mediates formation of a complex with VirE1. The capacity of VirE2 mutants to interact with full-length VirE2 in the yeast Saccharomyces cerevisiae correlated with the abundance of the mutant proteins in A. tumefaciens, suggesting that VirE2 is stabilized by homomultimerization in the bacterium. We further characterized the promoter and N- and C-terminal sequence requirements for synthesis of functional VirE2. A PvirB::virE2 construct yielded functional VirE2 protein as defined by complementation of a virE2 null mutation. By contrast, PvirE or Plac promoter constructs yielded functional VirE2 only if virE1 was coexpressed with virE2. Deletion of 10 or 9 residues from the N or C terminus of VirE2, respectively, or addition of heterologous peptides or proteins to either terminus resulted in a loss of protein function. However, an i31 peptide insertion at Tyr39 had no effect on protein function as defined by the capacity of the mutant protein to (i) interact with native VirE2, (ii) interact with VirE1, (iii) accumulate at abundant levels in A. tumefaciens, and (iv) restore wild-type virulence to a virE2 null mutant. We propose that Tyr39 of VirE2 corresponds to a permissive site for insertion of heterologous peptides or proteins of interest for delivery across kingdom boundaries.

Import of DNA into mammalian nuclei by proteins originating from a plant pathogenic bacterium

Import of DNA into mammalian nuclei is generally inefficient. Therefore, one of the current challenges in human gene therapy is the development of efficient DNA delivery systems. Here we tested whether bacterial proteins could be used to target DNA to mammalian cells. Agrobacterium tumefaciens, a plant pathogen, efficiently transfers DNA as a nucleoprotein complex to plant cells. Agrobacterium-mediated T-DNA transfer to plant cells is the only known example for interkingdom DNA transfer and is widely used for plant transformation. Agrobacterium virulence proteins VirD2 and VirE2 perform important functions in this process. We reconstituted complexes consisting of the bacterial virulence proteins VirD2, VirE2, and single-stranded DNA (ssDNA) in vitro. These complexes were tested for import into HeLa cell nuclei. Import of ssDNA required both VirD2 and VirE2 proteins. A VirD2 mutant lacking its C-terminal nuclear localization signal was deficient in import of the ssDNA-protein complexes into nuclei. Import of VirD2-ssDNA-VirE2 complexes was fast and efficient, and was shown to depended on importin alpha, Ran, and an energy source. We report here that the bacterium-derived and plant-adapted protein-DNA complex, made in vitro, can be efficiently imported into mammalian nuclei following the classical importin-dependent nuclear import pathway. This demonstrates the potential of our approach to enhance gene transfer to animal cells.

VirE1 is a specific molecular chaperone for the exported single-stranded-DNA-binding protein VirE2 in Agrobacterium

Agrobacterium tumefaciens induces tumours on plants by transferring a nucleoprotein complex, the T-complex, from the bacterium to the plant cell. The T-complex consists of a single-stranded DNA (ssDNA) segment, the T-DNA, and VirD2, an endonuclease covalently attached to the 5' end of the T-DNA. A type IV secretion system encoded by the virB operon and virD4 is required for the entry of the T-complex and VirE2, a ssDNA-binding protein, into plant cells. The VirE1 protein is specifically required for the export of the VirE2 protein, as demonstrated by extracellular complementation and tumour formation. In this report, using a yeast two-hybrid system, we demonstrated that the VirE1 and VirE2 proteins interact and confirmed this interaction by in vitro binding assays. Although VirE2 is a ssDNA-binding protein, addition of ssDNA into the binding buffer did not interfere with the interaction of VirE1 and VirE2. VirE2 also interacts with itself, but the interaction between VirE1 and VirE2 is stronger than the VirE2 self-interaction, as measured in a lacZ reporter gene assay. In addition, the interaction of VirE2 with itself is inhibited by VirE1, indicating that VirE2 binds VirE1 preferentially. Analysis of various virE2 deletions indicated that the VirE1 interaction domain of VirE2 overlaps the VirE2 self-interaction domain. Incubation of extracts from Escherichia coli overexpressing His-VirE1 with the extracts of E. coli overexpressing His-VirE2 increased the yield of His-VirE2 in the soluble fraction. In a similar purified protein solubility assay, His-VirE1 increased the amount of His-VirE2 partitioning into the soluble fraction. In Agrobacterium, VirE2 was undetectable in the soluble protein fraction unless VirE1 was co-expressed. When urea was added to solubilize any large protein aggregates, a low level of VirE2 was detected. These results indicate that VirE1 prevents VirE2 from aggregating, enhances the stability of VirE2 and, perhaps, maintains VirE2 in an export-competent state. Analysis of the deduced amino acid sequence of the VirE1 protein revealed that the VirE1 protein shares a number of properties with molecular chaperones that are involved in the transport of specific proteins into animal and plant cells using type III secretion systems. We suggest that VirE1 functions as a specific molecular chaperone for VirE2, the first such chaperone linked to the presumed type IV secretion system.

The molecular structure of agrobacterium VirE2-single stranded DNA complexes involved in nuclear import

Nuclear import of DNA is a central event in genetic transformation of plant cells by Agrobacterium tumefaciens. Agrobacterium elicits tumors on plant hosts by transporting a single-stranded (ss) copy of the bacterial transferred DNA (T-DNA) from its Ti (tumor-inducing) plasmid into the plant cell nucleus. Presumably, the process of T-DNA nuclear import is mediated by two agrobacterium proteins, VirD2 and VirE2, which are thought to directly associate with the transported T-DNA. Both proteins have been shown to contain functional nuclear localizations signals (NLS). Recently, VirE2 alone has been shown to actively transport ssDNA into the plant cell nucleus. To understand the process of DNA nuclear import, it is important to know the structure of the transport intermediate. To this end, complexes of VirE2 and ssDNA were analyzed by scanning transmission electron microscopy (STEM). This analysis suggests that VirE2 packages ssDNA into semi-rigid, hollow cylindrical filaments with a telephone cord-like coiled structure. The outer diameter of these complexes is too large to enter the nucleus by diffusion but is within the size exclusion limits of the active nuclear import. Detailed mass analysis of VirE2-ssDNA filaments is presented and a structural model is proposed.

Functional domains of Agrobacterium tumefaciens single-stranded DNA-binding protein VirE2

The transferred DNA (T-DNA) portion of the Agrobacterium tumefaciens tumor-inducing (Ti) plasmid enters infected plant cells and integrates into plant nuclear DNA. Direct repeats define the T-DNA ends; transfer begins when the VirD2 endonuclease produces a site-specific nick in the right-hand border repeat and attaches to the 5' end of the nicked strand. Subsequent events liberate the lower strand of the T-DNA from the Ti plasmid, producing single-stranded DNA molecules (T strands) that are covalently linked to VirD2 at their 5' ends. A. tumefaciens appears to transfer T-DNA into plant cells as a T-strand-VirD2 complex. The bacterium also transports VirE2, a cooperative single-stranded DNA-binding protein, into plant cells during infection. Both VirD2 and VirE2 contain nuclear localization signals that may direct these proteins, and bound T strands, into plant nuclei. Here we report the locations of functional regions of VirE2 identified by eight insertions of XhoI linker oligonucleotides, and one deletion mutation, throughout virE2. We examined the effects of these mutations on virulence, single-stranded DNA (ssDNA) binding, and accumulation of VirE2 in A. tumefaciens. Two of the mutations in the C-terminal half of VirE2 eliminated ssDNA binding, whereas two insertions in the N-terminal half altered cooperativity. Four of the mutations, distributed throughout virE2, decreased the stability of VirE2 in A. tumefaciens. In addition, we isolated a mutation in the central region of VirE2 that decreased tumorigenicity but did not affect ssDNA binding or VirE2 accumulation. This mutation may affect export of VirE2 into plant cells or nuclear localization of VirE2, or it may affect an uncharacterized activity of VirE2.

Early transcription of Agrobacterium T-DNA genes in tobacco and maize

We developed a sensitive procedure to investigate the kinetics of transcription of an Agrobacterium tumefaciens transferred (T)-DNA-encoded beta-glucuronidase gusA (uidA) gene soon after infection of plant suspension culture cells. The procedure uses a reverse transcriptase-polymerase chain reaction and enables detection of gusA transcripts within 18 to 24 hr after cocultivation of the bacteria with either tobacco or maize cells. Detection of gusA transcripts depended absolutely on the intact virulence (vir) genes virB, virD1/virD2, and virD4 within the bacterium. Mutations in virC and virE resulted in delayed and highly attenuated expression of the gusA gene. A nonpolar transposon insertion into the C-terminal coding region of virD2 resulted in only slightly decreased production of gusA mRNA, although this insertion resulted in the loss of the nuclear localization sequence and the important omega region from VirD2 protein and rendered the bacterium avirulent. However, expression of gusA transcripts in tobacco infected by this virD2 mutant was more transient than in cells infected by a wild-type strain. Infection of tobacco cells with an Agrobacterium strain harboring a mutant virD2 allele from which the omega region had been deleted resulted in similar transient expression of gusA mRNA. These data indicate that the C-terminal nuclear localization signal of the VirD2 protein is not essential for nuclear uptake of T-DNA and further suggest that the omega domain of VirD2 may be required for efficient integration of T-DNA into the plant genome. The finding that the initial kinetics of gusA gene expression in maize cells are similar to those shown in infected tobacco cells but that the presence of gusA mRNA in maize is highly transient suggests that the block to maize transformation involves T-DNA integration and not T-DNA entry into the cell or nuclear targeting.

Early transcription of Agrobacterium T-DNA genes in tobacco and maize

We developed a sensitive procedure to investigate the kinetics of transcription of an Agrobacterium tumefaciens transferred (T)-DNA-encoded beta-glucuronidase gusA (uidA) gene soon after infection of plant suspension culture cells. The procedure uses a reverse transcriptase-polymerase chain reaction and enables detection of gusA transcripts within 18 to 24 hr after cocultivation of the bacteria with either tobacco or maize cells. Detection of gusA transcripts depended absolutely on the intact virulence (vir) genes virB, virD1/virD2, and virD4 within the bacterium. Mutations in virC and virE resulted in delayed and highly attenuated expression of the gusA gene. A nonpolar transposon insertion into the C-terminal coding region of virD2 resulted in only slightly decreased production of gusA mRNA, although this insertion resulted in the loss of the nuclear localization sequence and the important omega region from VirD2 protein and rendered the bacterium avirulent. However, expression of gusA transcripts in tobacco infected by this virD2 mutant was more transient than in cells infected by a wild-type strain. Infection of tobacco cells with an Agrobacterium strain harboring a mutant virD2 allele from which the omega region had been deleted resulted in similar transient expression of gusA mRNA. These data indicate that the C-terminal nuclear localization signal of the VirD2 protein is not essential for nuclear uptake of T-DNA and further suggest that the omega domain of VirD2 may be required for efficient integration of T-DNA into the plant genome. The finding that the initial kinetics of gusA gene expression in maize cells are similar to those shown in infected tobacco cells but that the presence of gusA mRNA in maize is highly transient suggests that the block to maize transformation involves T-DNA integration and not T-DNA entry into the cell or nuclear targeting.

VirE1 protein mediates export of the single-stranded DNA-binding protein VirE2 from Agrobacterium tumefaciens into plant cells

Agrobacterium tumefaciens transfers single-stranded DNAs (T strands) into plant cells. VirE1 and VirE2, which is a single-stranded DNA binding protein, are important for tumorigenesis. We show that T strands and VirE2 can enter plant cells independently and that export of VirE2, but not of T strands, depends on VirE1.

Integration of complete transferred DNA units is dependent on the activity of virulence E2 protein of Agrobacterium tumefaciens

Agrobacterium tumefaciens transfers transferred DNA (T-DNA), a single-stranded segment of its tumor-inducing (Ti) plasmid, to the plant cell nucleus. The Ti-plasmid-encoded virulence E2 (VirE2) protein expressed in the bacterium has single-stranded DNA (ssDNA)-binding properties and has been reported to act in the plant cell. This protein is thought to exert its influence on transfer efficiency by coating and accompanying the single-stranded T-DNA (ss-T-DNA) to the plant cell genome. Here, we analyze different putative roles of the VirE2 protein in the plant cell. In the absence of VirE2 protein, mainly truncated versions of the T-DNA are integrated. We infer that VirE2 protects the ss-T-DNA against nucleolytic attack during the transfer process and that it is interacting with the ss-T-DNA on its way to the plant cell nucleus. Furthermore, the VirE2 protein was found not to be involved in directing the ss-T-DNA to the plant cell nucleus in a manner dependent on a nuclear localization signal, a function which is carried by the NLS of VirD2. In addition, the efficiency of T-DNA integration into the plant genome was found to be VirE2 independent. We conclude that the VirE2 protein of A. tumefaciens is required to preserve the integrity of the T-DNA but does not contribute to the efficiency of the integration step per se.