Processing of the T-DNA of Agrobacterium tumefaciens generates border nicks and linear, single-stranded T-DNA

Association of the virD2 protein with the 5' end of T strands in Agrobacterium tumefaciens

The soil bacterium Agrobacterium tumefaciens can incite tumors in many dicotyledonous plants by transferring a portion (T-DNA) of its Ti plasmid into susceptible plant cells. The T-DNA is flanked by border sequences that serve as recognition sites for specific cleavage by an endonuclease that comprises two virD-encoded proteins (VirD1 and VirD2). After cleavage, both double-stranded, nicked T-DNA molecules and single-stranded T-DNA molecules (T strands) were present. We have determined that a protein is tightly associated with, and probably covalently attached to, the 5' end of the T strands. Analysis of deletion derivatives in Escherichia coli, immunoprecipitation, and a procedure combining immunoblot and nucleic acid hybridization data identified this protein as the gene product of virD2.

Right-border sequences enable the left border of an Agrobacterium tumefaciens nopaline Ti-plasmid to produce single-stranded DNA

The T-region of nopaline-type Ti-plasmids (the portion of the plasmid that is transferred to plant cells) of Agrobacterium tumefaciens is delimited by 23-25 bp direct repeats. They are nicked by the products of the virD locus and the presence of these nicked sites is correlated with the synthesis of single-stranded T-region copies. Despite previous indications to the contrary, we show that the pTiT37 T-region left border is capable of producing single-stranded DNA with high efficiency and that its ability to do so is totally dependent on right border-proximal cis-acting sequences, most probably overdrive, located several kilobases from the border. The absence of overdrive does not affect the single-strand nicking activity of the virD product but only the production of single-stranded copies from the nicked substrate.

The Agrobacterium tumefaciens virE2 gene product is a single-stranded-DNA-binding protein that associates with T-DNA

Agrobacterium tumefaciens transfers T-DNA into the plant genome by a process mediated by Ti plasmid-encoded vir genes. Cleavage at T-DNA border sequences by the VirD endonuclease generates linear, single-stranded T-DNA molecules. In the work described in this report, we used electrophoretic mobility shift assays to show that the purified virE2 gene product binds to single-stranded DNA. VirE2 protein associates with T-DNA as shown by immunoprecipitation studies with VirE2-specific antiserum. The VirE2 protein was detected primarily in the cytoplasm, but also in the inner and outer membrane and periplasmic fractions. Virulence of a virE2 mutant was restored by mixed infection with strains carrying an intact vir region, but not with virA, virB, virD, virE, or virG mutants or chvA, chvB, or exoC mutants. We propose that the VirE2 protein is involved in the processing of T-DNA and in T-strand protection during transfer to the plant cell.

Agrobacterium tumefaciens virE operon encodes a single-stranded DNA-binding protein

The virulence (vir) genes of Agrobacterium tumefaciens Ti plasmid are essential for transformation of plant cells. Overproduction of a virE-encoded gene product in Escherichia coli was achieved by construction of an operon fusion with the E. coli tryptophan (trp) operon. The virE2 gene product in E. coli partitioned into the insoluble membrane fraction. The protein was solubilized by treatment with 4 M urea at 0 degree C. DNA-protein binding experiments showed that a strong single-stranded (ss) DNA-binding activity was present in protein fractions containing the virE2 gene product. The binding was highly specific with little or no binding observed with either double-stranded DNA or ssRNA. No significant binding to Ti plasmid DNA sequences was observed. Protein blotting studies indicated that the ssDNA-binding activity was associated with the 68-kDa virE2 polypeptide.

Virulence genes, borders, and overdrive generate single-stranded T-DNA molecules from the A6 Ti plasmid of Agrobacterium tumefaciens

Agrobacterium tumefaciens transfers the T-DNA portion of its Ti plasmid to the nuclear genome of plant cells. Upon cocultivation of A. tumefaciens A348 with regenerating tobacco leaf protoplasts, six distinct single-stranded T-DNA molecules (T strands) were generated in addition to double-stranded T-DNA border cleavages which we have previously reported (K. Veluthambi, R.K. Jayaswal, and S.B. Gelvin, Proc. Natl. Acad. Sci. USA 84:1881-1885, 1987). The T region of an octopine-type Ti plasmid has four border repeats delimiting three T-DNA regions defined as T left (TL), T center (TC), and T right (TR). The six T strands generated upon induction corresponded to the TL, TC, TR, TL + TC, TC + TR, and TL + TC + TR regions, suggesting that the initiation and termination of T-strand synthesis can occur at each of the four borders. Most TL + TC + TR T-strand molecules corresponded to the top T-DNA strand, whereas the other five T strands corresponded to the bottom T-DNA strand. Generation of T strands required the virA, virG, and virD operons. Extra copies of vir genes, harbored on cosmids within derivatives of A. tumefaciens A348, enhanced production of T strands. The presence of right and left border repeats in their native orientation is important for the generation of full-length T strands. When a right border repeat was placed in the opposite orientation, single-stranded T-DNA molecules that corresponded to the top strand were generated. Deletion of overdrive, a sequence that flanks right border repeats and functions as a T-DNA transmission enhancer, reduced the level of T-strand generation. Induction of A. tumefaciens cells by regenerating tobacco protoplasts increased the copy number of the Ti plasmid relative to the bacterial chromosome.

Genetic analysis of the virE operon of the Agrobacterium Ti plasmid pTiA6

The virE operon of the Agrobacterium tumefaciens Ti plasmid pTiA6 encodes at least one trans-acting protein involved in the expression of virulence. Two open reading frames designated virE1 and virE2 code for polypeptides of 7 and 60 kilodaltons (kDa), respectively, that can be visualized after expression in Escherichia coli minicells. To determine which virE sequences are required for virulence, a strain deleted for the entire locus [strain KE1(pTiA6 delta E)] was constructed and tested for the ability to be complemented by subclones with and without site-directed mutations in the virE operon. One subclone containing only virE1 and virE2 as well as upstream promoter sequences was sufficient to restore full virulence on the host plant Kalanchoe daigremontiana. However, some other virulence locus representing a host range determinant appeared to be deleted from strain KE1(pTiA6 delta E), since virE1 and virE2 were not sufficient to fully restore virulence on wounded tomato plants. virE operon constructs with specific lesions in either virE1 or virE2 were impaired for complementation of pTiA6 delta E. Several mutations specific for the promoter-proximal virE1 locus appeared to have a polar effect on expression of the virE2-encoded 60-kDa protein. However, virE2::lacZ fusion constructs suggest that this effect is not at the level of transcription or translation. Collectively, these data indicate that both the 7- and the 60-kDa polypeptides are virulence determinants for the Ti plasmid pTiA6 and suggest that the 60-kDa protein may be less stable in the absence of the 7-kDa protein.

Overdrive is a T-region transfer enhancer which stimulates T-strand production in Agrobacterium tumefaciens

Introduction of a left or right synthetic border repeat together with the overdrive sequence in an octopine Ti-plasmid deletion mutant, lacking the right border, resulted in the complete restoration of the oncogenicity of the mutant strain. However introduction of a border repeat without the overdrive, only restored oncogenicity partially. The overdrive sequence turned out to be able to stimulate the synthetic border mediated T-region transfer, independent of its orientation and position relative to the border repeat. Furthermore the distance between border repeat and overdrive could be enlarged, without a loss of overdrive activity. Here we enlarged the distance between the two sequences up to 6714bp. These results were confirmed by estimating the amount of single stranded T-DNA molecules from induced agrobacteria, containing the various border constructs.

Double-stranded cleavage of T-DNA and generation of single-stranded T-DNA molecules in Escherichia coli by a virD-encoded border-specific endonuclease from Agrobacterium tumefaciens

The virD locus of Agrobacterium tumefaciens Ti plasmid pTiA6 was sequenced. Computer analysis of the sequence indicated five possible open reading frames (ORFs) within this locus. Two additional ORFs were identified distal to this locus. However, only two polypeptides of apparent molecular masses 16 and 56 kilodaltons, the products of ORFs 1 and 2, were detected in Escherichia coli, both in vivo and in an in vitro coupled transcription-translation system. The virD locus was cloned in expression vector pKK223.3 under control of a tac promoter and introduced into an E. coli strain harboring mini-Ti plasmid pAL1050. When induced with isopropyl-beta-D-thiogalactopyranoside, the virD gene products exhibited double-stranded T-DNA border-specific endonuclease activity. Deletion analysis demonstrated that this activity is encoded within the 5'-proximal 1.7-kilobase-pair portion of the virD locus that carries ORF 1 and most of ORF 2. Neither ORF 1 nor ORF 2 independently showed endonuclease activity; complementation studies indicated that the products of ORFs 1 and 2 together have this activity. The expression of this 1.7-kilobase-pair region of the virD locus caused double-stranded cleavage of the T-DNA at or near the borders and generated single-stranded T-DNA molecules with approximately equal frequencies in E. coli.

Molecular characterization of the virD operon from Agrobacterium tumefaciens

The Agrobacterium tumefaciens Ti plasmid virulence (vir) region contains at least six transcriptional units required for the efficient transfer of T-DNA to the plant genome (virA, B, C, D, E, and G). We have reported that two proteins encoded by the 5'portion of the virD operon are required for a site-specific endonuclease activity that nicks the direct repeats which flank the T-DNA. We have presented the nucleotide sequence for this portion of the operon. The nucleotide sequence of the remainder of the virD operon essential for virulence has now been determined. Two additional open reading frames encode proteins of 21.3 and 75.8 kilodaltons (kd). Translational fusions between virD2, virD3, and virD4 proteins and trpE produced fusion proteins of the size predicted from the nucleotide sequence data. We have used antisera directed against the trpE-virD2 fusion protein to detect both native virD2 protein and a virD2-lacZ fusion protein in crude extracts from Agrobacterium.

Absence in monocotyledonous plants of the diffusible plant factors inducing T-DNA circularization and vir gene expression in Agrobacterium

T-DNA circularization is one of the molecular events specifically induced in agrobacterial cells upon their infection of dicotyledonous plant cells. We developed a seedling co-cultivation procedure to determine whether or not monocotyledonous plants have the ability to induce T-DNA circularization and vir gene expression. Co-cultivation of Agrobacterium tumefaciens with seedlings of dicotyledonous plants showed that the circularization event takes place efficiently. The exudates and extracts of the seedlings also effectively induced T-DNA circularization and vir gene expression, indicating that dicotyledonous seedlings contain diffusible factors capable of inducing these molecular events. In contrast, neither T-DNA circularization nor vir gene expression was detectable when Agrobacterium was incubated with seedlings of monocotyledonous plants. Supplementing with acetosyringone, a known inducer of vir gene expression and T-DNA circularization, resulted in the induction of circularization during co-cultivation with moncotyledonous seedlings. These results indicate that the seedlings of monocotyledonous plants have no detectable amounts of diffusible inducers, unlike dicotyledonous seedlings. Therefore, it is unlikely that the vir genes are expressed in Agrobacterium inoculated in monocotyledonous plants. This may be one of the blocks in tumorigenesis of monocotyledonous plant by Agrobacterium.

Characterization of Agrobacterium tumefaciens strains isolated from grapevine tumors in China

Thirteen strains of Agrobacterium tumefaciens isolated from grapevine tumors in northern China were surveyed. These strains varied in their host range properties, although all were tumorigenic on grapevines. Twelve of these strains belonged to Agrobacterium sp. biotype 3, and 11 strains resulted in the synthesis of the opine octopine in tumor tissue. Interestingly, one strain resulted in accumulation of arginine, a previously unrecognized opine, in tumor tissue. Although DNA in most of these strains showed homology to the previously characterized transferred DNA and vir loci, some virulent strains showed little or no homology to these loci. Thus, some of these strains represent widely divergent examples of Agrobacterium sp. The DNA in most strains exhibited little or no homology to a wide-host-range virA locus but did show strong homology to a limited-host-range virA locus. This finding further supports the idea that Agrobacterium strains associated with grapevines may have a specific virA locus.