Octopine and nopaline strains of Agrobacterium tumefaciens differ in virulence; molecular characterization of the virF locus

The Ti Plasmid, Driver of Agrobacterium Pathogenesis

The phytopathogenic bacterium Agrobacterium tumefaciens causes crown gall disease in plants, characterized by the formation of tumor-like galls where wounds were present. Nowadays, however, the bacterium and its Ti (tumor-inducing) plasmid is better known as an effective vector for the genetic manipulation of plants and fungi. In this review, I will briefly summarize some of the major discoveries that have led to this bacterium now playing such a prominent role worldwide in plant and fungal research at universities and research institutes and in agricultural biotechnology for the production of genetically modified crops. I will then delve a little deeper into some aspects of Agrobacterium biology and discuss the diversity among agrobacteria and the taxonomic position of these bacteria, the diversity in Ti plasmids, the molecular mechanism used by the bacteria to transform plants, and the discovery of protein translocation from the bacteria to host cells as an essential feature of Agrobacterium-mediated transformation.

JAZ8 Interacts With VirE3 Attenuating Agrobacterium Mediated Root Tumorigenesis

Agrobacterium tumefaciens can cause crown gall tumors by transferring both an oncogenic piece of DNA (T-DNA) and several effector proteins into a wide range of host plants. For the translocated effector VirE3 multiple functions have been reported. It acts as a transcription factor in the nucleus binding to the Arabidopsis thaliana pBrp TFIIB-like protein to activate the expression of VBF, an F-box protein involved in degradation of the VirE2 and VIP1 proteins, facilitating Agrobacterium-mediated transformation. Also VirE3 has been found at the plasma membrane, where it could interact with VirE2. Here, we identified AtJAZ8 in a yeast two-hybrid screening with VirE3 as a bait and confirmed the interaction by pull-down and bimolecular fluorescence complementation assays. We also found that the deletion of virE3 reduced Agrobacterium virulence in a root tumor assay. Overexpression of virE3 in Arabidopsis enhanced tumorigenesis, whereas overexpression of AtJAZ8 in Arabidopsis significantly decreased the numbers of tumors formed. Further experiments demonstrated that AtJAZ8 inhibited the activity of VirE3 as a plant transcriptional regulator, and overexpression of AtJAZ8 in Arabidopsis activated AtPR1 gene expression while it repressed the expression of AtPDF1.2. Conversely, overexpression of virE3 in Arabidopsis suppressed the expression of AtPR1 whereas activated the expression of AtPDF1.2. Our results proposed a novel mechanism of counter defense signaling pathways used by Agrobacterium, suggesting that VirE3 and JAZ8 may antagonistically modulate the salicylic acid/jasmonic acid (SA/JA)-mediated plant defense signaling response during Agrobacterium infection.

F-Box Gene D5RF Is Regulated by Agrobacterium Virulence Protein VirD5 and Essential for Agrobacterium-Mediated Plant Transformation

We previously reported that the Agrobacterium virulence protein VirD5 possesses transcriptional activation activity, binds to a specific DNA element D5RE, and is required for Agrobacterium-mediated stable transformation, but not for transient transformation. However, direct evidence for a role of VirD5 in plant transcriptional regulation has been lacking. In this study, we found that the Arabidopsis gene D5RF (coding for VirD5 response F-box protein, At3G49480) is regulated by VirD5. D5RF has two alternative transcripts of 930 bp and 1594 bp that encode F-box proteins of 309 and 449 amino acids, designated as D5RF.1 and D5RF.2, respectively. D5RF.2 has a N-terminal extension of 140 amino acids compared to D5RF.1, and both of them are located in the plant cell nucleus. The promoter of the D5RF.1 contains two D5RE elements and can be activated by VirD5. The expression of D5RF is downregulated when the host plant is infected with virD5 deleted Agrobacterium. Similar to VirD5, D5RF also affects the stable but not transient transformation efficiency of Agrobacterium. Some pathogen-responsive genes are downregulated in the d5rf mutant. In conclusion, this study further confirmed Agrobacterium VirD5 as the plant transcription activator and identified Arabidopsis thalianaD5RF.1 as the first target gene of VirD5 in regulation.

Pathways of DNA Transfer to Plants from Agrobacterium tumefaciens and Related Bacterial Species

Genetic transformation of host plants by Agrobacterium tumefaciens and related species represents a unique model for natural horizontal gene transfer. Almost five decades of studying the molecular interactions between Agrobacterium and its host cells have yielded countless fundamental insights into bacterial and plant biology, even though several steps of the DNA transfer process remain poorly understood. Agrobacterium spp. may utilize different pathways for transferring DNA, which likely reflects the very wide host range of Agrobacterium. Furthermore, closely related bacterial species, such as rhizobia, are able to transfer DNA to host plant cells when they are provided with Agrobacterium DNA transfer machinery and T-DNA. Homologs of Agrobacterium virulence genes are found in many bacterial genomes, but only one non-Agrobacterium bacterial strain, Rhizobium etli CFN42, harbors a complete set of virulence genes and can mediate plant genetic transformation when carrying a T-DNA-containing plasmid.

Presence of an Agrobacterium-Type Tumor-Inducing Plasmid in Neorhizobium sp. NCHU2750 and the Link to Phytopathogenicity

The genus Agrobacterium contains a group of plant-pathogenic bacteria that have been developed into an important tool for genetic transformation of eukaryotes. To further improve this biotechnology application, a better understanding of the natural genetic variation is critical. During the process of isolation and characterization of wild-type strains, we found a novel strain (i.e., NCHU2750) that resembles Agrobacterium phenotypically but exhibits high sequence divergence in several marker genes. For more comprehensive characterization of this strain, we determined its complete genome sequence for comparative analysis and performed pathogenicity assays on plants. The results demonstrated that this strain is closely related to Neorhizobium in chromosomal organization, gene content, and molecular phylogeny. However, unlike the characterized species within Neorhizobium, which all form root nodules with legume hosts and are potentially nitrogen-fixing mutualists, NCHU2750 is a gall-forming pathogen capable of infecting plant hosts across multiple families. Intriguingly, this pathogenicity phenotype could be attributed to the presence of an Agrobacterium-type tumor-inducing plasmid in the genome of NCHU2750. These findings suggest that these different lineages within the family Rhizobiaceae are capable of transitioning between ecological niches by having novel combinations of replicons. In summary, this work expanded the genomic resources available within Rhizobiaceae and provided a strong foundation for future studies of this novel lineage. With an infectivity profile that is different from several representative Agrobacterium strains, this strain may be useful for comparative analysis to better investigate the genetic determinants of host range among these bacteria.

Application of phiLOV2.1 as a fluorescent marker for visualization of Agrobacterium effector protein translocation

Agrobacterium tumefaciens can genetically transform plants by translocating a piece of oncogenic DNA, called T-DNA, into host cells. Transfer is mediated by a type IV secretion system (T4SS). Besides the T-DNA, which is transferred in a single-stranded form and at its 5' end covalently bound to VirD2, several other effector proteins (VirE2, VirE3, VirD5, and VirF) are translocated into the host cells. The fate and function of the translocated proteins inside the host cell are only partly known. Therefore, several studies were conducted to visualize the translocation of the VirE2 protein. As GFP-tagged effector proteins are unable to pass the T4SS, other approaches like the split GFP system were used, but these require specific transgenic recipient cells expressing the complementary part of GFP. Here, we investigated whether use can be made of the photostable variant of LOV, phiLOV2.1, to visualize effector protein translocation from Agrobacterium to non-transgenic yeast and plant cells. We were able to visualize the translocation of all five effector proteins, both to yeast cells, and to cells in Nicotiana tabacum leaves and Arabidopsis thaliana roots. Clear signals were obtained that are easily distinguishable from the background, even in cases in which by comparison the split GFP system did not generate a signal.

Complete sequence of the tumor-inducing plasmid pTiChry5 from the hypervirulent Agrobacterium tumefaciens strain Chry5

Agrobacterium tumefaciens strain Chry5 is hypervirulent on many plants including soybean that are poorly transformed by other A. tumefaciens strains. Therefore, it is considered as a preferred vector for genetic transformation of plants. Here we report the complete nucleotide sequence of its chrysopine-type Ti-plasmid pTiChry5. It is comprised of 197,268 bp with an overall GC content of 54.5%. Two T-DNA regions are present and 219 putative protein-coding sequences could be identified in pTiChry5. Roughly one half of the plasmid is highly similar to the agropine-type Ti plasmid pTiBo542, including the virulence genes with an identical virG gene, which is responsible for the supervirulence caused by pTiBo542. The remaining part of pTiChry5 is less related to that of pTiBo542 and embraces the trb operon of conjugation genes, genes involved in the catabolism of Amadori opines and the gene for chrysopine synthase, which replaces the gene for agropine synthase in pTiBo542. With the exception of an insertion of IS869, these Ti plasmids differ completely in the set of transposable elements present, reflecting a different evolutionary history from a common ancestor.

Agrobacterium-mediated plant transformation: biology and applications

Plant genetic transformation heavily relies on the bacterial pathogen Agrobacterium tumefaciens as a powerful tool to deliver genes of interest into a host plant. Inside the plant nucleus, the transferred DNA is capable of integrating into the plant genome for inheritance to the next generation (i.e. stable transformation). Alternatively, the foreign DNA can transiently remain in the nucleus without integrating into the genome but still be transcribed to produce desirable gene products (i.e. transient transformation). From the discovery of A. tumefaciens to its wide application in plant biotechnology, numerous aspects of the interaction between A. tumefaciens and plants have been elucidated. This article aims to provide a comprehensive review of the biology and the applications of Agrobacterium-mediated plant transformation, which may be useful for both microbiologists and plant biologists who desire a better understanding of plant transformation, protein expression in plants, and plant-microbe interaction.

Virulence protein VirD5 of Agrobacterium tumefaciens binds to kinetochores in host cells via an interaction with Spt4

The bacterium Agrobacterium tumefaciens causes crown gall tumor formation in plants. During infection the bacteria translocate an oncogenic piece of DNA (transferred DNA, T-DNA) into plant cells at the infection site. A number of virulence proteins are cotransported into host cells concomitantly with the T-DNA to effectuate transformation. Using yeast as a model host, we find that one of these proteins, VirD5, localizes to the centromeres/kinetochores in the nucleus of the host cells by its interaction with the conserved protein Spt4. VirD5 promotes chromosomal instability as seen by the high-frequency loss of a minichromosome in yeast. By using both yeast and plant cells with a chromosome that was specifically marked by a lacO repeat, chromosome segregation errors and the appearance of aneuploid cells due to the presence of VirD5 could be visualized in vivo. Thus, VirD5 is a prokaryotic virulence protein that interferes with mitosis.

Nopaline-type Ti plasmid of Agrobacterium encodes a VirF-like functional F-box protein

During Agrobacterium-mediated genetic transformation of plants, several bacterial virulence (Vir) proteins are translocated into the host cell to facilitate infection. One of the most important of such translocated factors is VirF, an F-box protein produced by octopine strains of Agrobacterium, which presumably facilitates proteasomal uncoating of the invading T-DNA from its associated proteins. The presence of VirF also is thought to be involved in differences in host specificity between octopine and nopaline strains of Agrobacterium, with the current dogma being that no functional VirF is encoded by nopaline strains. Here, we show that a protein with homology to octopine VirF is encoded by the Ti plasmid of the nopaline C58 strain of Agrobacterium. This protein, C58VirF, possesses the hallmarks of functional F-box proteins: it contains an active F-box domain and specifically interacts, via its F-box domain, with SKP1-like (ASK) protein components of the plant ubiquitin/proteasome system. Thus, our data suggest that nopaline strains of Agrobacterium have evolved to encode a functional F-box protein VirF.

Agroinfiltration by cytokinin-producing Agrobacterium sp. strain GV3101 primes defense responses in Nicotiana tabacum

Transient infiltrations in tobacco are commonly used in plant studies, but the host response to different disarmed Agrobacterium strains is not fully understood. The present study shows that pretreatment with disarmed Agrobacterium tumefaciens GV3101 primes the defense response to subsequent infection by Pseudomonas syringae in Nicotiana tabacum. The presence of a trans-zeatin synthase (tzs) gene in strain GV3101 may be partly responsible for the priming response, as the tzs-deficient Agrobacterium sp. strain LBA4404 only weakly imparts such responses. Besides inducing the expression of defense-related genes like PR-1 and NHL10, GV3101 pretreatment increased the expression of tobacco mitogen-activated protein kinase (MAPK) pathway genes like MEK2, WIPK (wound-induced protein kinase), and SIPK (salicylic acid-induced protein kinase). Furthermore, the GV3101 strain showed a stronger effect than the LBA4404 strain in activating phosphorylation of the tobacco MAPK, WIPK and SIPK, which presumably prime the plant immune machinery. Lower doses of exogenously applied cytokinins increased the activation of MAPK, while higher doses decreased the activation, suggesting a balanced level of cytokinins is required to generate defense response in planta. The current study serves as a cautionary warning for plant researchers over the choice of Agrobacterium strains and their possible consequences on subsequent pathogen-related studies.

The putative Agrobacterium transcriptional activator-like virulence protein VirD5 may target T-complex to prevent the degradation of coat proteins in the plant cell nucleus

Agrobacterium exports at least five virulence proteins (VirE2, VirE3, VirF, VirD2, VirD5) into host cells and hijacks some host plant factors to facilitate its transformation process. Random DNA binding selection assays (RDSAs), electrophoretic mobility shift assays (EMSAs) and yeast one-hybrid systems were used to identify protein-bound DNA elements. Bimolecular fluorescence complementation, glutathione S-transferase pull-down and yeast two-hybrid assays were used to detect protein interactions. Protoplast transformation, coprecipitation, competitive binding and cell-free degradation assays were used to analyze the relationships among proteins. We found that Agrobacterium VirD5 exhibits transcriptional activation activity in yeast, is located in the plant cell nucleus, and forms homodimers. A specific VirD5-bound DNA element designated D5RE (VirD5 response element) was identified. VirD5 interacted directly with Arabidopsis VirE2 Interacting Protein 1 (AtVIP1). However, the ternary complex of VirD5-AtVIP1-VirE2 could be detected, whereas that of VirD5-AtVIP1-VBF (AtVIP1 Binding F-box protein) could not. We demonstrated that VirD5 competes with VBF for binding to AtVIP1 and stabilizes AtVIP1 and VirE2 in the cell-free degradation system. Our results indicated that VirD5 may act as both a transcriptional activator-like effector to regulate host gene expression and a protector preventing the coat proteins of the T-complex from being quickly degraded by the host's ubiquitin proteasome system (UPS).

Agrobacterium: nature's genetic engineer

Agrobacterium was identified as the agent causing the plant tumor, crown gall over 100 years ago. Since then, studies have resulted in many surprising observations. Armin Braun demonstrated that Agrobacterium infected cells had unusual nutritional properties, and that the bacterium was necessary to start the infection but not for continued tumor development. He developed the concept of a tumor inducing principle (TIP), the factor that actually caused the disease. Thirty years later the TIP was shown to be a piece of a tumor inducing (Ti) plasmid excised by an endonuclease. In the next 20 years, most of the key features of the disease were described. The single-strand DNA (T-DNA) with the endonuclease attached is transferred through a type IV secretion system into the host cell where it is likely coated and protected from nucleases by a bacterial secreted protein to form the T-complex. A nuclear localization signal in the endonuclease guides the transferred strand (T-strand), into the nucleus where it is integrated randomly into the host chromosome. Other secreted proteins likely aid in uncoating the T-complex. The T-DNA encodes enzymes of auxin, cytokinin, and opine synthesis, the latter a food source for Agrobacterium. The genes associated with T-strand formation and transfer (vir) map to the Ti plasmid and are only expressed when the bacteria are in close association with a plant. Plant signals are recognized by a two-component regulatory system which activates vir genes. Chromosomal genes with pleiotropic functions also play important roles in plant transformation. The data now explain Braun's old observations and also explain why Agrobacterium is nature's genetic engineer. Any DNA inserted between the border sequences which define the T-DNA will be transferred and integrated into host cells. Thus, Agrobacterium has become the major vector in plant genetic engineering.

The efficiency of Arabidopsis thaliana floral dip transformation is determined not only by the Agrobacterium strain used but also by the physiology and the ecotype of the dipped plant

To evaluate the chromosomal background of different Agrobacterium strains on floral dip transformation frequency, eight wild-type Agrobacterium strains, provided by Laboratorium voor Microbiologie Gent (LMG) and classified in different genomic groups, were compared with the commonly used Agrobacterium strains C58C1 Rif(r) (pMP90) and LBA4404 in Arabidopsis thaliana Columbia (Col-0) and C24 ecotypes. The C58C1 Rif(r) chromosomal background in combination with the pMP90 virulence plasmid showed high Col-0 floral dip transformation frequencies (0.76 to 1.57%). LMG201, which is genetically close to the Agrobacterium C58 strain, with the same virulence plasmid showed comparable or even higher transformation frequencies (1.22 to 2.28%), whereas the LBA4404 strain displayed reproducibly lower transformation frequencies (<0.2%). All other tested LMG Agrobacterium chromosomal backgrounds had transformation frequencies between those of the C58C1 Rif(r) (pMP90) and LBA4404 reference strains. None of the strains could transform the C24 ecotype with a frequency higher than 0.1%. Strikingly, all Arabidopsis Col-0 floral dip transformation experiments showed a high transformation variability from plant to plant (even more than 50-fold) within and across the performed biological repeats for all analyzed Agrobacterium strains. Therefore, the physiology of the plant and, probably, the availability of competent flowers to be transformed determine, to a large extent, floral dip transformation frequencies.

Several components of SKP1/Cullin/F-box E3 ubiquitin ligase complex and associated factors play a role in Agrobacterium-mediated plant transformation

• Successful genetic transformation of plants by Agrobacterium tumefaciens requires the import of bacterial T-DNA and virulence proteins into the plant cell that eventually form a complex (T-complex). The essential components of the T-complex include the single stranded T-DNA, bacterial virulence proteins (VirD2, VirE2, VirE3 and VirF) and associated host proteins that facilitate the transfer and integration of T-DNA. The removal of the proteins from the T-complex is likely achieved by targeted proteolysis mediated by VirF and the plant ubiquitin proteasome complex. • We evaluated the involvement of the host SKP1/culin/F-box (SCF)-E3 ligase complex and its role in plant transformation. Gene silencing, mutant screening and gene expression studies suggested that the Arabidopsis homologs of yeast SKP1 (suppressor of kinetochore protein 1) protein, ASK1 and ASK2, are required for Agrobacterium-mediated plant transformation. • We identified the role for SGT1b (suppressor of the G2 allele of SKP1), an accessory protein that associates with SCF-complex, in plant transformation. We also report the differential expression of many genes that encode F-box motif containing SKP1-interacting proteins (SKIP) upon Agrobacterium infection. • We speculate that these SKIP genes could encode the plant specific F-box proteins that target the T-complex associated proteins for polyubiquitination and subsequent degradation by the 26S proteasome.

Hijacking of the Host SCF Ubiquitin Ligase Machinery by Plant Pathogens

The SCF (SKP1-CUL1-F-box protein) ubiquitin ligase complex mediates polyubiquitination of proteins targeted for degradation, thereby controlling a plethora of biological processes in eukaryotic cells. Although this ubiquitination machinery is found and functional only in eukaryotes, many non-eukaryotic pathogens also encode F-box proteins, the critical subunits of the SCF complex. Increasing evidence indicates that such non-eukaryotic F-box proteins play an essential role in subverting or exploiting the host ubiquitin/proteasome system for efficient pathogen infection. A recent bioinformatic analysis has identified more than 70 F-box proteins in 22 different bacterial species, suggesting that use of pathogen-encoded F-box effectors in the host cell may be a widespread infection strategy. In this review, we focus on plant pathogen-encoded F-box effectors, such as VirF of Agrobacterium tumefaciens, GALAs of Ralstonia solanacearum, and P0 of Poleroviruses, and discuss the molecular mechanism by which plant pathogens use these factors to manipulate the host cell for their own benefit.

Agrobacterium counteracts host-induced degradation of its effector F-box protein

The SCF (Skp1-Cul1-F-box protein) ubiquitin ligase complex plays a pivotal role in various biological processes, including host-pathogen interactions. Many pathogens exploit the host SCF machinery to promote efficient infection by translocating pathogen-encoded F-box proteins into the host cell. How pathogens ensure sufficient amounts of the F-box effectors in the host cell despite the intrinsically unstable nature of F-box proteins, however, remains unclear. We found that the Agrobacterium F-box protein VirF, an important virulence factor, undergoes rapid degradation through the host proteasome pathway. This destabilization of VirF was counteracted by VirD5, another bacterial effector that physically associated with VirF. These observations reveal a previously unknown counterdefense strategy used by pathogens against potential host antimicrobial responses.

Agrobacterium-produced and exogenous cytokinin-modulated Agrobacterium-mediated plant transformation

Agrobacterium tumefaciens is a plant pathogenic bacterium that causes neoplastic growths, called 'crown gall', via the transfer and integration of transferred DNA (T-DNA) from the bacterium into the plant genome. We characterized an acetosyringone (AS)-induced tumour-inducing (Ti) plasmid gene, tzs (trans-zeatin synthesizing), that is responsible for the synthesis of the plant hormone cytokinin in nopaline-type A. tumefaciens strains. The loss of Tzs protein expression and trans-zeatin secretions by the tzs frameshift (tzs-fs) mutant is associated with reduced tumorigenesis efficiency on white radish stems and reduced transformation efficiencies on Arabidopsis roots. Complementation of the tzs-fs mutant with a wild-type tzs gene restored wild-type levels of trans-zeatin secretions and transformation efficiencies. Exogenous application of cytokinin during infection increased the transient transformation efficiency of Arabidopsis roots infected by strains lacking Tzs, which suggests that the lower transformation efficiency resulted from the lack of Agrobacterium-produced cytokinin. Interestingly, although the tzs-fs mutant displayed reduced tumorigenesis efficiency on several tested plants, the loss of Tzs enhanced tumorigenesis efficiencies on green pepper and cowpea. These data strongly suggest that Tzs, by synthesizing trans-zeatin at early stage(s) of the infection process, modulates plant transformation efficiency by A. tumefaciens.

Plant proteins involved in Agrobacterium-mediated genetic transformation

Agrobacterium species genetically transform plants by transferring a region of plasmid DNA, T-DNA, into host plant cells. The bacteria also transfer several virulence effector proteins. T-DNA and virulence proteins presumably form T-complexes within the plant cell. Super-T-complexes likely also form by interaction of plant-encoded proteins with T-complexes. These protein-nucleic acid complexes traffic through the plant cytoplasm, enter the nucleus, and eventually deliver T-DNA to plant chromatin. Integration of T-DNA into the plant genome establishes a permanent transformation event, permitting stable expression of T-DNA-encoded transgenes. The transformation process is complex and requires participation of numerous plant proteins. This review discusses our current knowledge of plant proteins that contribute to Agrobacterium-mediated transformation, the roles these proteins play in the transformation process, and the modern technologies that have been employed to elucidate the cell biology of transformation.

Proteasomal degradation in plant-pathogen interactions

The ubiquitin/26S proteasome pathway is a basic biological mechanism involved in the regulation of a multitude of cellular processes. Increasing evidence indicates that plants utilize the ubiquitin/26S proteasome pathway in their immune response to pathogen invasion, emphasizing the role of this pathway during plant-pathogen interactions. The specific functions of proteasomal degradation in plant-pathogen interactions are diverse, and do not always benefit the host plant. Although in some cases, proteasomal degradation serves as an effective barrier to help plants ward off pathogens, in others, it is used by the pathogen to enhance the infection process. This review discusses the different roles of the ubiquitin/26S proteasome pathway during interactions of plants with pathogenic viruses, bacteria, and fungi.

Exploitation of eukaryotic ubiquitin signaling pathways by effectors translocated by bacterial type III and type IV secretion systems

The specific and covalent addition of ubiquitin to proteins, known as ubiquitination, is a eukaryotic-specific modification central to many cellular processes, such as cell cycle progression, transcriptional regulation, and hormone signaling. Polyubiquitination is a signal for the 26S proteasome to destroy earmarked proteins, but depending on the polyubiquitin chain topology, it can also result in new protein properties. Both ubiquitin-orchestrated protein degradation and modification have also been shown to be essential for the host's immune response to pathogens. Many animal and plant pathogenic bacteria utilize type III and/or type IV secretion systems to inject effector proteins into host cells, where they subvert host signaling cascades as part of their infection strategy. Recent progress in the determination of effector function has taught us that playing with the host's ubiquitination system seems a general tactic among bacteria. Here, we discuss how bacteria exploit this system to control the timing of their effectors' action by programming them for degradation, to block specific intermediates in mammalian or plant innate immunity, or to target host proteins for degradation by mimicking specific ubiquitin/proteasome system components. In addition to analyzing the effectors that have been described in the literature, we screened publicly available bacterial genomes for mimicry of ubiquitin proteasome system subunits and detected several new putative effectors. Our understanding of the intimate interplay between pathogens and their host's ubiquitin proteasome system is just beginning. This exciting research field will aid in better understanding this interplay, and may also provide new insights into eukaryotic ubiquitination processes.

The Agrobacterium VirE3 effector protein: a potential plant transcriptional activator

During the infection of plants, Agrobacterium tumefaciens introduces several Virulence proteins including VirE2, VirF, VirD5 and VirE3 into plant cells in addition to the T-DNA. Here, we report that double mutation of virF and virE3 leads to strongly diminished tumor formation on tobacco, tomato and sunflower. The VirE3 protein is translated from a polycistronic mRNA containing the virE1, virE2 and virE3 genes, in Agrobacterium. The VirE3 protein has nuclear localization sequences, which suggests that it is transported into the plant cell nucleus upon translocation. Indeed we show here that VirE3 interacts in vitro with importin-α and that a VirE3–GFP fusion protein is localized in the nucleus. VirE3 also interacts with two other proteins, viz. pCsn5, a component of the COP9 signalosome and pBrp, a plant specific general transcription factor belonging to the TFIIB family. We found that VirE3 is able to induce transcription in yeast when bound to DNA through the GAL4-BD. Our data indicate that the translocated effector protein VirE3 is transported into the nucleus and there it may interact with the transcription factor pBrp to induce the expression of genes needed for tumor development.

Will you let me use your nucleus? How Agrobacterium gets its T-DNA expressed in the host plant cell

Agrobacterium is the only known bacterium capable of natural DNA transfer into a eukaryotic host. The genes transferred to host plants are contained on a T-DNA (transferred DNA) molecule, the transfer of which begins with its translocation, along with several effector proteins, from the bacterial cell to the host-cell cytoplasm. In the host cytoplasm, the T-complex is formed from a single-stranded copy of the T-DNA (T-strand) associated with several bacterial and host proteins and it is imported into the host nucleus via interactions with the host nuclear import machinery. Once inside the nucleus, the T-complex is most likely directed to the host genome by associating with histones. Finally, the chromatin-associated T-complex is uncoated from its escorting proteins prior to the conversion of the T-strand to a double-stranded form and its integration into the host genome.

Agrobacterium rhizogenes GALLS protein contains domains for ATP binding, nuclear localization, and type IV secretion

Agrobacterium tumefaciens and Agrobacterium rhizogenes are closely related plant pathogens that cause different diseases, crown gall and hairy root. Both diseases result from transfer, integration, and expression of plasmid-encoded bacterial genes located on the transferred DNA (T-DNA) in the plant genome. Bacterial virulence (Vir) proteins necessary for infection are also translocated into plant cells. Transfer of single-stranded DNA (ssDNA) and Vir proteins requires a type IV secretion system, a protein complex spanning the bacterial envelope. A. tumefaciens translocates the ssDNA-binding protein VirE2 into plant cells, where it binds single-stranded T-DNA and helps target it to the nucleus. Although some strains of A. rhizogenes lack VirE2, they are pathogenic and transfer T-DNA efficiently. Instead, these bacteria express the GALLS protein, which is essential for their virulence. The GALLS protein can complement an A. tumefaciens virE2 mutant for tumor formation, indicating that GALLS can substitute for VirE2. Unlike VirE2, GALLS contains ATP-binding and helicase motifs similar to those in TraA, a strand transferase involved in conjugation. Both GALLS and VirE2 contain nuclear localization sequences and a C-terminal type IV secretion signal. Here we show that mutations in any of these domains abolished the ability of GALLS to substitute for VirE2.

Proteomic analysis ofAgrobacterium tumefaciens response to thevir gene inducer acetosyringone

Agrobacterium tumefaciens causes crown gall disease in a wide range of plants by transforming plants through the transfer and integration of its transferred DNA (T-DNA) into the host genome. In the present study, we used two-dimensional gel electrophoresis to examine the protein expression profiles of A. tumefaciens in response to the phenolic compound acetosyringone (AS), a known plant-released virulence (vir) gene inducer. Using mass spectrometry, we identified 11 proteins consisting of 9 known AS-induced Vir proteins and 2 newly discovered AS-induced proteins, an unknown protein Y4mC (Atu6162) and a small heat shock protein HspL (Atu3887). Further expression analysis revealed that the AS-induced expression of Y4mC and HspL is regulated by the VirA/VirG two-component system. This report presents the first proteomics study successfully identifying both known and new AS-induced proteins that are implicated in Agrobacterium virulence.

Symbiotic phenotypes and translocated effector proteins of the Mesorhizobium loti strain R7A VirB/D4 type IV secretion system

The symbiosis island of Mesorhizobium loti strain R7A contains genes with strong similarity to the structural vir genes (virB1-11; virD4) of Agrobacterium tumefaciens that encode the type IV secretion system (T4SS) required for T-DNA transfer to plants. In contrast, M. loti strain MAFF303099 lacks these genes but contains genes not present in strain R7A that encode a type III secretion system (T3SS). Here we show by hybridization analysis that most M. loti strains contain the VirB/D4 T4SS and not the T3SS. Strikingly, strain R7A vir gene mutants formed large nodules containing bacteroids on Leucaena leucocephala in contrast to the wild-type strain that formed only uninfected tumour-like structures. A rhcJ T3SS mutant of strain MAFF303099 also nodulated L. leucocephala, unlike the wild type. On Lotus corniculatus, the vir mutants were delayed in nodulation and were less competitive compared with the wild type. Two strain R7A genes, msi059 and msi061, were identified through their mutant phenotypes as possibly encoding translocated effector proteins. Both Msi059 and Msi061 were translocated through the A. tumefaciens VirB/D4 system into Saccharomyces cerevisiae and Arabidopsis thaliana, as shown using the Cre recombinase Reporter Assay for Translocation (CRAfT). Taken together, these results suggest that the VirB/D4 T4SS of M. loti R7A plays an analogous symbiotic role to that of T3SS found in other rhizobia. The heterologous translocation of rhizobial proteins by the Agrobacterium VirB/D4 T4SS is the first demonstration that rhizobial effector proteins are translocated into plant cells and confirms functional conservation between the M. loti and A. tumefaciens T4SS.

Involvement of targeted proteolysis in plant genetic transformation by Agrobacterium

Genetic transformation of plant cells by Agrobacterium represents a unique case of trans-kingdom DNA transfer. During this process, Agrobacterium exports its transferred (T) DNA and several virulence (Vir) proteins into the host cell, within which T-DNA nuclear import is mediated by VirD2 (ref. 3) and VirE2 (ref. 4) and their host cell interactors AtKAP-alpha and VIP1 (ref. 6), whereas its integration is mediated mainly by host cell proteins. The factors involved in the uncoating of T-DNA from its cognate proteins, which occurs before integration into the host genome, are still unknown. Here, we report that VirF-one of the few known exported Vir proteins whose function in the host cell remains unknown-is involved in targeted proteolysis of VIP1 and VirE2. We show that VirF localizes to the plant cell nucleus and interacts with VIP1, a nuclear protein. VirF, which contains an F-box motif, significantly destabilizes both VIP1 and VirE2 in yeast cells. Destabilization of VIP1 in the presence of VirF was then confirmed in planta. These results suggest that VIP1 and its cognate VirE2 are specifically targeted by the VirF-containing Skp1-Cdc53-cullin-F-box complex for proteolysis. The critical role of proteasomal degradation in Agrobacterium-mediated genetic transformation was also evident from inhibition of T-DNA expression by a proteasomal inhibitor.

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.

Role of bacterial virulence proteins in Agrobacterium-mediated transformation of Aspergillus awamori

The Agrobacterium-mediated transformation of Aspergillus awamori was optimized using defined co-cultivation conditions, which resulted in a reproducible and efficient transformation system. Optimal co-cultivation conditions were used to study the role of Agrobacterium tumefaciens virulence proteins in T-DNA transfer. This study revealed that inactivation of either of the regulatory proteins (VirA, VirG), any of the transport pore proteins (VirB), proteins involved in generation of the T-strand (VirD, VirC) or T-strand protection and targeting (VirE2) abolishes or severely reduces the formation of transformants. The results indicate that the Agrobacterium-mediated transformation of A. awamori requires an intact T-DNA machinery for efficient transformation; however, the plant host range factors, like VirE3, VirH, and VirF, are not important.

Recognition of the Agrobacterium tumefaciens VirE2 translocation signal by the VirB/D4 transport system does not require VirE1

Agrobacterium tumefaciens uses a type IV secretion system to deliver a nucleoprotein complex and effector proteins directly into plant cells. The single-stranded DNA-binding protein VirE2, the F-box protein VirF and VirE3 are delivered into host cells via this VirB/D4 encoded translocation system. VirE1 functions as a chaperone of VirE2 by regulating its efficient translation and preventing VirE2-VirE2 aggregation in the bacterial cell. We analyzed whether the VirE1 chaperone is also essential for transport recognition of VirE2 by the VirB/D4 encoded type IV secretion system. In addition, we assayed whether translocation of VirF and VirE3, which also forms part of the virE operon, is affected by the absence of VirE1. We employed the earlier developed CRAFT (Cre recombinase Reporter Assay For Translocation) assay to detect transfer of Cre::Vir fusion proteins from A. tumefaciens into plants, monitored by stable reconstitution of a kanamycin resistance marker, and into yeast, screened by loss of the URA3 gene. We show that the C-terminal 50 amino acids of VirE2 and VirE3 are sufficient to mediate Cre translocation into host cells, confirming earlier indications of a C-terminal transport signal. This transfer was independent of the presence or absence of VirE1. Besides, the translocation efficiency of VirF is not altered in a virE1 mutant. The results unambiguously show that the VirE1 chaperone is not essential for the recognition of the VirE2 transport signal by the transport system and the subsequent translocation across the bacterial envelope into host cells.

Functional subsets of the virB type IV transport complex proteins involved in the capacity of Agrobacterium tumefaciens to serve as a recipient in virB-mediated conjugal transfer of plasmid RSF1010

The virB-encoded type IV transport complex of Agrobacterium tumefaciens mediates the transfer of DNA and proteins into plant cells, as well as the conjugal transfer of IncQ plasmids, such as RSF1010, between Agrobacterium strains. While several studies have indicated that there are physical interactions among the 11 VirB proteins, the functional significance of the interactions has been difficult to establish since all of the proteins are required for substrate transfer. Our previous studies, however, indicated that although all of the VirB proteins are required for the capacity of a strain to serve as an RSF1010 donor, only a subset of these proteins in the recipient is necessary to increase the conjugal frequency by 3 to 4 logs. The roles of particular groups of VirB proteins in this increased recipient activity were examined in the study reported here. Examination of the expression of subgroups of virB genes revealed that translation of virB6 is necessary for expression of downstream open reading frames. Expression of limited subsets of the VirB proteins in a recipient strain lacking the Ti plasmid revealed that the VirB7 to VirB10 proteins yield a subcomplex that is functional in the recipient assay but that the VirB1 to VirB4 proteins, as a group, dramatically increase this activity in strains expressing VirB7 to VirB10. Finally, the membrane distribution and cross-linking patterns of VirB10, but not of VirB8 or VirB9, in a strain expressing only VirB7 to VirB10 are significantly altered compared to the patterns of the wild type. These characteristics are, however, restored to the wild-type status by coexpression of VirB1 to VirB3. Taken together, these results define subsets of type IV transport complex proteins that are critical in allowing a strain to participate as a recipient in virB-mediated conjugal RSF1010 transfer.

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.

Analysis of Vir protein translocation from Agrobacterium tumefaciens using Saccharomyces cerevisiae as a model: evidence for transport of a novel effector protein VirE3

Agrobacterium tumefaciens causes crown gall disease on a variety of plants. During the infection process Agrobacterium transfers a nucleoprotein complex, the VirD2 T-complex, and at least two Vir proteins, VirE2 and VirF, into the plant cell via the VirB/VirD4 type IV secretion system. Recently, we found that T-DNA could also be transferred from Agrobacterium to Saccharomyces cerevisiae. Here, we describe a novel method to also detect trans-kingdom Vir protein transfer from Agrobacterium to yeast, using the Cre/lox system. Protein fusions between Cre and VirE2 or VirF were expressed in AGROBACTERIUM: Transfer of the Cre-Vir fusion proteins from Agrobacterium to yeast was monitored by a selectable excision event resulting from site-specific recombination mediated by Cre on a lox-flanked transgene in yeast. The VirE2 and VirF proteins were transported to yeast via the virB-encoded transfer system in the presence of coupling factor VirD4, analogous to translocation into plant cells. The yeast system therefore provides a suitable and fast model system to study basic aspects of trans-kingdom protein transport from Agrobacterium into host cells. Using this method we showed that VirE2 and VirF protein transfer was inhibited by the presence of the Osa protein. Besides, we found evidence for a novel third effector protein, VirE3, which has a similar C-terminal signature to VirE2 and VirF.

Agrobacterium type IV secretion is a two-step process in which export substrates associate with the virulence protein VirJ in the periplasm

Type IV secretion systems are virulence determinants in many bacteria and share extensive homology with many conjugal transfer systems. Although type IV systems and their homologues have been studied widely, the mechanism by which substrates are secreted remains unclear. In Agrobacterium, we show that type IV secretion substrates that lack signal peptides form a soluble complex in the periplasm with the virulence protein VirJ. Additionally, these proteins co-precipitate with constituents of the type IV transporter: the VirB pilus and the VirD4 protein. Our findings suggest that the substrate proteins localized to the periplasm may associate with the pilus in a manner that is mediated by VirJ, and suggest a two-step process for type IV secretion in Agrobacterium. Our analyses of protein-protein interactions in a variety of mutant backgrounds indicate that substrates are probably secreted independently of one another.

The carboxy-terminus of VirE2 from Agrobacterium tumefaciens is required for its transport to host cells by the virB-encoded type IV transport system

Agrobacterium tumefaciens transfers DNA from the resident 'tumour-inducing' (Ti) plasmid into plant cells, where it can be stably integrated into the plant genome, ultimately resulting in crown gall tumour formation. The mobilized DNA molecule is a single-stranded intermediate with VirD2 covalently bound to its 5' end. Successful transport of the transferred DNA (T-DNA) and integration of the DNA into the genome requires that additional proteins be transported to the plant as well, including the single-stranded (ss)DNA-binding protein, VirE2. The transport of these two different substrates occurs as a result of the activities of a type IV secretion system encoded by the virB operon. Although the substrates have been identified, the mechanism of their transport remains unknown. In the experiments described here, a region in one of these substrates, VirE2, necessary for transport is identified. The addition of a C-terminal FLAG epitope tag to VirE2, or the deletion of its C-terminal 18 amino acids, renders it non-functional in A. tumefaciens. However, transgenic plants expressing either of these virE2 genes respond to virE2 mutants of A. tumefaciens by forming wild-type tumours. These results indicate that this region of VirE2 is necessary for the protein to be transported into the plant cells, but is not necessary for its function within the plant. Additionally, these studies demonstrate that mutant forms of VirE2 lacking this region do not disrupt the activities of the VirB transporter and support the hypothesis that VirE2 and the VirD2 T-strand are transported independently, even when they co-exist in the same cell.

Interaction of the virulence protein VirF of Agrobacterium tumefaciens with plant homologs of the yeast Skp1 protein

The infection of plants by Agrobacterium tumefaciens leads to the formation of crown gall tumors due to the transfer of a nucleoprotein complex into plant cells that is mediated by the virulence (vir) region-encoded transport system (reviewed in [1-5]). In addition, A. tumefaciens secretes the Vir proteins, VirE2 and VirF, directly into plant cells via the same VirB/VirD4 transport system [6], and both assist there in the transformation of normal cells into tumor cells. The function of the 22 kDa VirF protein is not clear. Deletion of the virF gene in A. tumefaciens leads to diminished virulence [7, 8] and can be complemented by the expression of the virF gene in the host plant. This finding indicates that VirF functions within the plant cell [8]. Here, we report that the VirF protein is the first prokaryotic protein with an F box by which it can interact with plant homologs of the yeast Skp1 protein. The presence of the F box turned out to be essential for the biological function of VirF. F box proteins and Skp1p are both subunits of a class of E3 ubiquitin ligases referred to as SCF complexes. Thus, VirF may be involved in the targeted proteolysis of specific host proteins in early stages of the transformation process.

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 conjugal intermediate of plasmid RSF1010 inhibits Agrobacterium tumefaciens virulence and VirB-dependent export of VirE2

Agrobacterium tumefaciens causes crown gall disease by transferring oncogenic, single-stranded DNA (T strand), covalently attached to the VirD2 protein, across the bacterial envelope into plant cells where its expression results in tumor formation. The single-stranded DNA binding protein VirE2 is also transferred into the plant cell, though the location at which VirE2 interacts with the T strand is still under investigation. The movement of the transferred DNA and VirE2 from A. tumefaciens to the plant cell depends on the membrane-localized VirB and VirD4 proteins. Further, the movement of the IncQ broad-host-range plasmid RSF1010 between Agrobacterium strains or from Agrobacterium to plants also requires the virB-encoded transfer system. Our earlier studies showed that the presence of the RSF1010 plasmid in wild-type strains of Agrobacterium inhibits both their virulence and their capacity to transport VirE2, as assayed by coinfection with virE mutants. Here we demonstrate that the capacity to form a conjugal intermediate of RSF1010 is necessary for this inhibition, suggesting that the transferred form of the plasmid competes with the VirD2-T strand and/or VirE2 for a common export site.

The Ti plasmid increases the efficiency of Agrobacterium tumefaciens as a recipient in virB-mediated conjugal transfer of an IncQ plasmid

The T-DNA transfer apparatus of Agrobacterium tumefaciens mediates the delivery of the T-DNA into plant cells, the transfer of the IncQ plasmid RSF1010 into plant cells, and the conjugal transfer of RSF1010 between Agrobacteria. We show in this report that the Agrobacterium-to-Agrobacterium conjugal transfer efficiencies of RSF1010 increase dramatically if the recipient strain, as well as the donor strain, carries a wild-type Ti plasmid and is capable of vir gene expression. Investigation of possible mechanisms that could account for this increased efficiency revealed that the VirB proteins encoded by the Ti plasmid were required. Although, with the exception of VirB1, all of the proteins that form the putative T-DNA transfer apparatus (VirB1-11, VirD4) are required for an Agrobacterium strain to serve as an RSF1010 donor, expression of only a subset of these proteins is required for the increase in conjugal transfer mediated by the recipient. Specifically, VirB5, 6, 11, and VirD4 are essential donor components but are dispensable for the increased recipient capacity. Defined point mutations in virB9 affected donor and recipient capacities to the same relative extent, suggesting that similar functions of VirB9 are important in both of these contexts.

The presence and characterization of a virF gene on Agrobacterium vitis Ti plasmids

Octopine and nopaline strains of Agrobacterium tumefaciens differ in their ability to induce tumors on Nicotiana glauca. The presence of a virF locus on the octopine Ti plasmid makes N. glauca a host plant for these strains, indicating that the VirF protein is a host-range determinant. Here we show the presence of a virF locus not only on the Agrobacterium vitis octopine/cucumopine plasmids pTiAg57 and pTiTm4, but also on the nopaline Ti plasmids pTiAT1, pTiAT66a, and pTiAT66b. On the octopine Ti plasmids from A. tumefaciens the virF gene is located between the virE locus and the left border of the T-region. In contrast, the virF gene on Ti plasmids of A. vitis is located at the very left end of the vir-region near the virA locus. The virF gene of pTiAg57 has been sequenced and codes for a protein of 202 amino acids with a molecular mass of 22,280 Da. Comparison showed that the virF gene from A. vitis strain Ag57 is almost identical to that from A. tumefaciens octopine strains. The transcription of the pTiAg57 virF is inducible by the plant phenolic compound acetosyringone through the presence of a vir-box consensus sequence in its promoter region. The VirF protein from pTiAg57 can complement octopine A. tumefaciens strains deleted for virF as shown by tumor formation on N. glauca.

A specific protease encoded by the conjugative DNA transfer systems of IncP and Ti plasmids is essential for pilus synthesis

TraF, an essential component of the conjugative transfer apparatus of the broad-host-range plasmid RP4 (IncP), which is located at the periplasmic side of the cytoplasmic membrane, encodes a specific protease. The traF gene products of IncP and Ti plasmids show extensive similarities to prokaryotic and eukaryotic signal peptidases. Mutational analysis of RP4 TraF revealed that the mechanism of the proteolytic cleavage reaction resembles that of signal and LexA-like peptidases. Among the RP4 transfer functions, the product of the Tra2 gene, trbC, was identified as a target for the TraF protease activity. TrbC is homologous to VirB2 of Ti plasmids and thought to encode the RP4 prepilin. The maturation of TrbC involves three processing reactions: (i) the removal of the N-terminal signal peptide by Escherichia coli signal peptidase I (Lep), (ii) a proteolytic cleavage at the C terminus by an as yet unidentified host cell enzyme, and (iii) C-terminal processing by TraF. The third reaction of the maturation process is critical for conjugative transfer, pilus synthesis, and the propagation of the donor-specific bacteriophage PRD1. Thus, cleavage of TrbC by TraF appears to be one of the initial steps in a cascade of processes involved in export of the RP4 pilus subunit and pilus assembly mediated by the RP4 mating pair formation function.

Ti plasmid conjugation is independent of vir: reconstitution of the tra functions from pTiC58 as a binary system

Two regions of the nopaline-type Ti plasmid pTiC58 are important for conjugal transfer of this element to recipient bacteria. These two regions were cloned into two independent replicons to produce a binary transfer system. For one region, oriT/tra, we constructed two derivatives, pFRtra and pDCtra-5. Each contains the oriT site and the two flanking, divergently transcribed tra operons that encode the DNA processing functions associated with the relaxosome. These two plasmids also carry traR, which encodes the transcriptional activator necessary for expression of transfer genes. The two plasmids differ by the amounts of traB sequence or sequence downstream of traG present in the construct. The second replicon, pPLE2, carries the traI/trb region. The traI gene confers production of the Agrobacterium tumefaciens N-acyl homoserine lactone autoinducer, while the remaining genes in the trb operon encode components of the mating bridge. Donors harboring the two plasmids mobilized the transfer of the plasmid carrying the oriT/tra region to an A. tumefaciens recipient at frequencies similar to that at which the intact Ti plasmid transferred. Plasmid pFRtra, which encodes most of traB, was mobilized at a frequency almost 10-fold higher than was pDCtra-5, which lacks most of the gene. A. tumefaciens donors also mobilized pFRtra to Escherichia coli and Pseudomonas fluorescens recipients at frequencies similar to those observed with A. tumefaciens recipients. Rhizobium meliloti harboring the binary system also transferred the oriT/tra component to these recipients. However, E. coli or P. fluorescens donors harboring the binary system did not transfer pFRtra to any of the recipients. Furthermore, while the A. tumefaciens and R. meliloti donors produced high levels of the autoinducer, the P. fluorescens and E. coli donors produced only trace amounts of this signal molecule. These results indicate that the tra system of pTiC58 is fully contained within the characterized tra and trb regions of the Ti plasmid, that conjugation does not require functions encoded by the vir system for maximal activity, and that while the Ti plasmid tra system recognizes diverse gram-negative bacteria as recipients, of the hosts tested, it functions only in members of the family Rhizobiaceae.

The tra region of the nopaline-type Ti plasmid is a chimera with elements related to the transfer systems of RSF1010, RP4, and F

The Ti plasmids of Agrobacterium tumefaciens encode two transfer systems. One mediates the translocation of the T-DNA from the bacterium to a plant cell, while the other is responsible for the conjugal transfer of the entire Ti plasmid from one bacterium to another. The determinants responsible for conjugal transfer map to two regions, tra and trb, of the nopaline-type Ti plasmid pTiC58. By using transposon mutagenesis with Tn3HoHo1, we localized the tra determinants to an 8.5-kb region that also contains the oriT region. Fusions to lacZ formed by transposon insertions indicated that this region is expressed as two divergently transcribed units. We determined the complete nucleotide sequence of an 8,755-bp region of the Ti plasmid encompassing the transposon insertions defining tra. The region contains six identifiable genes organized as two units divergently transcribable from a 258-bp inter-genic region that contains the oriT site. One unit encodes traA, traF, and traB, while the second encodes traC, traD, and traG. Reporter insertions located downstream of both sets of genes did not affect conjugation but were expressed, suggesting that the two units encode additional genes that are not involved in transfer under the conditions tested. Proteins of the predicted sizes were expressible from traA, traC, traD, and traG. The products of several Ti plasmid tra genes are related to those of other conjugation systems. The 127-kDa protein expressed from traA contains domains related to MobA of RSF1O1O and to the helicase domain of TraI of plasmid F. The translation product of traF is related to TraF of RP4, and that of traG is related to TraG of RP4 and to VirD4 of the Ti plasmid T-DNA transfer system. Genetic analysis indicated that at least traG and traF are essential for conjugal transfer, while sequence analysis predicts that traA also encodes an essential function. traB, while not essential, is required for maximum frequency of transfer. Patterns of sequence relatedness indicate that the oriT and the predicted cognate site-specific endonuclease encoded by traA share lineage with those of the transfer systems of RSF1010 and plasmid F, while genes of the Ti plasmid encoding other essential tra functions share common ancestry with genes of the RP4 conjugation system.