Construction and application of R prime plasmids, carrying different segments of an octopine Ti plasmid from Agrobacterium tumefaciens, for complementation of vir genes

Efficient vir gene induction in Agrobacterium tumefaciens requires virA, virG, and vir box from the same Ti plasmid

The vir genes of octopine, nopaline, and L,L-succinamopine Ti plasmids exhibit structural and functional similarities. However, we observed differences in the interactions between octopine and nopaline vir components. The induction of an octopine virE(A6)::lacZ fusion (pSM358cd) was 2.3-fold higher in an octopine strain (A348) than in a nopaline strain (C58). Supplementation of the octopine virG(A6) in a nopaline strain with pSM358 did not completely restore virE(A6) induction. However, addition of the octopine virA(A6) to the above strain increased virE(A6) induction to a level almost comparable to that in octopine strains. In a reciprocal analysis, the induction of a nopaline virE(C58)::cat fusion (pUCD1553) was two- to threefold higher in nopaline (C58 and T37) strains than in octopine (A348 and Ach5) and L,L-succinamopine (A281) strains. Supplementation of nopaline virA(C58) and virG(C58) in an octopine strain (A348) harboring pUCD1553 increased induction levels of virE(C58)::cat fusion to a level comparable to that in a nopaline strain (C58). Our results suggest that octopine and L,L-succinamopine VirG proteins induce the octopine virE(A6) more efficiently than they do the nopaline virE(C58). Conversely, the nopaline VirG protein induces the nopaline virE(C58) more efficiently than it does the octopine virE(A6). The ability of Bo542 virG to bring about supervirulence in tobacco is observed for an octopine vir helper (LBA4404) but not for a nopaline vir helper (PMP90). Our analyses reveal that quantitative differences exist in the interactions between VirG and vir boxes of different Ti plasmids. Efficient vir gene induction in octopine and nopaline strains requires virA, virG, and vir boxes from the respective Ti plasmids.

Specificity of signal molecules in the activation of Agrobacterium virulence gene expression

The activation of the Agrobacterium virulence system is known to be induced by certain phenolic compounds. We have tested the vir-inducing ability of fifty compounds, by using a virB-lacZ gene fusion, and analysed the relationship between structure and activity of these compounds. In this way we have identified several new vir-inducers: coniferylalcohol, 3,5-dimethoxy-4-hydroxybenzene, homovanillic acid, ferulic acid, 3-ethoxy-4-hydroxybenzaldehyde and guaiacol, all of which are compounds with strong or moderate activity and four compounds with weak vir-inducing activity. In view of the specificity of vir-inducers, our data extended observations of others and enabled us to define the specific structural features of a vir-inducer molecule. In addition we show here that induction of the octopine Ti vir-genes is (i) optimal at 29 degrees C and totally abolished at 37 degrees C, and (ii) strongly inhibited at low concentrations of sodium chloride. The implications for plant transformation are discussed.

Analysis of the complete nucleotide sequence of the Agrobacterium tumefaciens virB operon

The complete nucleotide sequence of the virB locus, from the octopine Ti plasmid of Agrobacterium tumefaciens strain 15955, has been determined. In the large virB-operon (9600 nucleotides) we have identified eleven open reading frames, designated virB1 to virB11. From DNA sequence analysis it is proposed that nearly all VirB products, i.e. VirB1 to VirB9, are secreted or membrane associated proteins. Interestingly, both a membrane protein (VirB4) and a potential cytoplasmic protein (VirB11) contain the consensus amino acid sequence of ATP-binding proteins. In view of the conjugative T-DNA transfer model, the VirB proteins are suggested to act at the bacterial surface and there play an important role in directing T-DNA transfer to plant cells.

Molecular characterization of the virulence gene virA of the Agrobacterium tumefaciens octopine Ti plasmid

The virulence loci play an essential role in tumor formation by Agrobacterium tumefaciens. Induction of vir gene expression by plant signal molecules is solely dependent on the virulence loci virA and virG. This study focused on the virA locus of the octopine type Ti plasmid pTi15955. The nucleic acid sequence of a 5.7-kilobase fragment encompassing virA was determined. Genetic analysis of this region revealed that virA contains one open reading frame coding for a protein of 91 639 daltons. Immunodetection with antibodies raised against a 35-kDa VirA fusion protein produced in E. coli identified the VirA product in wild-type Agrobacterium cells. Moreover, it is shown that the VirA protein is located in the cytoplasmic membrane fraction of Agrobacterium. These data confirm the proposed regulatory function of VirA whereby VirA acts as a membrane sensor protein to identify plant signal molecules in the environment. The proposed sensory function of VirA strikingly resembles the function of the chemotaxis receptor proteins of E. coli.

Molecular characterization of the virulence gene virA of the Agrobacterium tumefaciens octopine Ti plasmid

The virulence loci play an essential role in tumor formation by Agrobacterium tumefaciens. Induction of vir gene expression by plant signal molecules is solely dependent on the virulence loci virA and virG. This study focused on the virA locus of the octopine type Ti plasmid pTi15955. The nucleic acid sequence of a 5.7-kilobase fragment encompassing virA was determined. Genetic analysis of this region revealed that virA contains one open reading frame coding for a protein of 91 639 daltons. Immunodetection with antibodies raised against a 35-kDa VirA fusion protein produced in E. coli identified by the VirA product in wild-type Agrobacterium cells. Moreover, it is shown that the VirA protein is located in the cytoplasmic membrane fraction of Agrobacterium. These data confirm the proposed regulatory function of VirA whereby VirA acts as a membrane sensor protein to identify plant signal molecules in the environment. The proposed sensory function of VirA strikingly resembles the function of the chemotaxis receptor proteins of E. coli.

Nucleotide sequence of the virulence gene virG of the Agrobacterium tumefaciens octopine Ti plasmid: significant homology between virG and the regulatory genes ompR, phoB and dye of E. coli

The entire nucleotide sequence of the virG locus, from the octopine Ti plasmid of Agrobacterium tumefaciens strain 15955, has been determined. The virG gene is 801 nucleotides in length and has one open reading frame which encodes a protein of Mr 29,995. The virG gene is involved in the transcriptional activation of the Ti plasmid vir-loci, which occurs after induction by specific compounds present in plant exudate. Sequence analysis of the Agrobacterium virG protein showed significant homology with the Escherichia coli ompR, phoB and dye proteins, which are all positive regulatory genes for genes encoding envelope proteins. These results suggest that the virG gene encodes a positive regulatory protein which can activate vir gene expression.

The hypervirulence of Agrobacterium tumefaciens A281 is encoded in a region of pTiBo542 outside of T-DNA

We used a binary-vector strategy to study the hypervirulence of Agrobacterium tumefaciens A281, an L,L-succinamopine strain. Strain A281 is hypervirulent on several solanaceous plants. We constructed plasmids (pCS65 and pCS277) carrying either the transferred DNA (T-DNA) or the remainder of the tumor-inducing (Ti) plasmid (pEHA101) from this strain and tested each of these constructs in trans with complementary regions from heterologous Ti plasmids. Hypervirulence on tobacco could be reconstructed in a bipartite strain with the L,L-succinamopine T-DNA and the vir region on separate plasmids. pEHA101 was able to complement octopine T-DNA to hypervirulence on tobacco and tomato plants. Nopaline T-DNA was complemented better on tomato plants by pEHA101 than it was by its own nopaline vir region, but not to hypervirulence. L,L-Succinamopine T-DNA could not be complemented to hypervirulence on tobacco and tomato plants with either heterologous vir region. From these results we suggest that the hypervirulence of strain A281 is due to non-T-DNA sequences on the Ti plasmid.

Conservation of IS66 homologue of octopine Ti plasmid DNA in Rhizobium fredii plasmid DNA

DNA sequences homologous to the T-DNA region of the octopine Ti plasmid from Agrobacterium tumefaciens are found in various fast-growing Rhizobium fredii strains. The largest fragment (BamHI fragment 2) at the right-boundary region of the 'core' T-DNA hybridizes to more than one plasmid present in R. fredii. However, one smaller fragment (EcoRI fragment 19a) adjacent to the 'core' T-DNA shows homology only with the plasmid carrying the symbiotic nitrogen-fixation genes (pSym). Hybridization data obtained with digested R. fredii USDA193 pSym DNA suggests that the homology is mainly with two HindIII fragments, 1.7 kb and 8.8 kb in size, of the plasmid. The 1.7 kb HindIII fragment also hybridizes to two regions of the virulence plasmid of A. tumefaciens, pAL1819, a deletion plasmid derived from the octopine Ti plasmid, pTiAch5. Hybridization studies with an insertion element IS66 from A. tumefaciens indicate that the 1.7 kb HindIII fragment of R. fredii plasmid, homologous to the T-DNA and the virulence region of Ti plasmid, is itself an IS66 homologue.

Physical and functional map of supervirulent Agrobacterium tumefaciens tumor-inducing plasmid pTiBo542

Agrobacterium tumefaciens strains carrying pTiBo542 induce large, fast-appearing tumors and have an unusually wide host range. A clone bank was made from this 250-kilobase plasmid in a wide-host-range vector, and restriction maps were determined for BamHI and SalI. The virulence genes, transferred DNA genes, plasmid incompatibility region, and a region that inhibits growth of certain A. tumefaciens strains were localized. The six virulence genes and two tms genes were highly homologous to the genes of pTiA6, but the tmr gene was not. Mutations in each of the six vir loci of pTiA6 were complemented by clones from the vir region of pTiBo542.

Initial interactions of Agrobacterium tumefaciens with plant host cells

Infections of wounded dicotyledonous plants by Agrobacterium tumefaciens result in the formation of crown gall tumors. The initial step in tumor formation is the site-specific attachment of the bacteria to the host cells. The mechanism of recognition and attachment in this interaction has been studied in detail. Current information on the nature of the bacterial binding sites, the nature of the host receptors, the role of bacterial cellulose fibrils, and the genetics of bacterial attachment will be summarized, and a model for the attachment of Agrobacterium to host cells will be presented.

The role of bacterial attachment in the transformation of cell-wall-regenerating tobacco protoplasts by Agrobacterium tumefaciens

The presence of a newly formed primary cell wall was shown to be required for attachment and subsequent transformation of tobacco leaf protoplasts by Agrobacterium tumefaciens in cocultivation experiments. In these experiments both protoplasts at different stages after their isolation and cell-wall inhibitors were used. The specificity of Agrobacterium attachment was shown by using other kinds of bacteria that did not attach. By diminishing the concentration of divalent cations using ethylenediaminetetraacetic acid, neither attachment nor transformation was found; however, when more specifically the Ca(2+)concentration was lowered by ethylene glycol-bis (β-aminoethyl ether)-N,N,N',N'-tetraacetic acid, both phenomena occurred. Commercial lectins had no effect on binding, but this observation does not exclude the involvement of other lectins. Protoplasts isolated from various crown-gall callus tissues also developed binding sites, but when they were at the stage of dividing cells, attachment of agrobacteria was no longer observed. In this respect, cells from protoplasts of normal tobacco leaves behaved differently. Even 16 d after protoplast isolation, the dividing cells were still able to bind A. tumefaciens, while transformation was not detected. For transformation of 3-d-old tobacco protoplasts, a minimal co-cultivation period of 24 h was required, while optimal attachment took place within 5 h. It is concluded that the primary cell wall was sufficiently well formed that certain functional receptor molecules were available for attachment of Agrobacterium as the first step of a multistep process leading to the transformation of cells. The expression of bacterial functions required for attachment, moreover, was independent of the presence of Ti-plasmid.

Structure and expression of DNA transferred to tobacco via transformation of protoplasts with Ti-plasmid DNA: co-transfer of T-DNA and non T-DNA sequences

The T-DNA structure and organization in tissues obtained via transformation of tobacco protoplasts with Ti-plasmid DNA was found to be completely different from the T-DNA introduced via Agrobacterium tumefaciens. It is often fragmented. Overlapping copies of T-DNA, having various sizes, as well as separated fragments of T-DNA were detected. The border sequences of 23 basepairs (bp), flanking the T-region in the Ti-plasmid as direct repeats are not used as preferred sequences for integration. Similar results were obtained with a T-region clone lacking one of the TL-borders. This clone, which carried the cytokinin locus and only the right border sequence of TL and the left border sequence of TR, still had the capacity to transform protoplasts. Also the Vir-region of the Ti-plasmid is not required for integration of foreign DNA via DNA transformation. This is demonstrated by the results with the T-region clone mentioned and by the transforming capacity of a Ti-plasmid carrying a mutated Vir-region. Nevertheless, in a number of Ti-plasmid DNA transformants Vir-region fragments were found to be stably integrated. Furthermore, it has been established that co-transformation can occur with plant cells. Besides the detection of Ti-plasmid fragments from outside the T-region also DNA sequences originating from two DNA sources, which were both independently present in transformation experiments, have been found in some DNA transformants, e.g. calf thymus DNA, which was used as carrier DNA. No expression of the co-transferred DNA was observed. In total three phenotypical classes of DNA transformants were isolated. Although the T-DNA was often scrambled, polyA(+) mRNA studies indicated that the different phenotypes studied can be explained by the presence of active T-DNA genes with known functions.

Identification of pTiC58 plasmid-encoded proteins for virulence in Agrobacterium tumefaciens

Analyses were made of the host-dependent-variation (hdv) locus of the virulence (vir) region of the pTiC58 plasmid of Agrobacterium tumefaciens. The hdv locus is comprised of at least four genes that encode polypeptides of 13, 15, 29, and 28 kDa. Insertion of transposon Tn5 in the first gene abolishes the expression of all four genes in vitro and in vivo. Nucleotide sequence analysis of the hdv locus revealed four open reading frames tandemly arranged with spacer sequences having no promoter-like sequences and lacking the ability to bind A. tumefaciens RNA polymerase. These studies suggest that the hdv locus is comprised of at least four genes arranged in an operon in the vir region. The protein products of these genes are likely to function in some aspect of the host-range determination of A. tumefaciens.

Plant-inducible virulence promoter of the Agrobacterium tumefaciens Ti plasmid

Agrobacterium tumefaciens is the causative agent of crown gall, a plant tumour that can arise on most species of dicotyledonous plants. The tumour-inducing capacity of the bacterium requires the presence of a large plasmid, designated the Ti plasmid, which itself contains two regions essential for tumour formation-the T(umour)-region and the Vir(ulence)-region. The T-region is transferred to plant cells by an unknown mechanism, and becomes stably integrated into the plant genome. The Vir-region has been identified by transposon mutagenesis, but the DNA of this region has never been detected in tumour lines. However, trans-complementation of Vir mutants indicates that genes of the Vir-region are functional in the bacterium. Moreover, the Vir- and T-regions can be physically separated in A. tumefaciens without loss of tumour-inducing capacity. Seven loci, designated virA-F and virO (refs 17, 20-22), have been identified in the Vir-region of the octopine Ti plasmid, but their functions are unknown. As virC mutants in the octopine-type plasmid pTiB6 are invariably avirulent in tests on various plant species, this gene seems to be essential for virulence and we are studying it in detail. We report here that the promoter of virC shows no detectable activity in A. tumefaciens and Escherichia coli K-12 grown in standard medium, but that its activity is induced by a plant product.

Design and development of amplifiable broad-host-range cloning vectors: analysis of the vir region of Agrobacterium tumefaciens plasmid pTiC58

The construction of a set of new plasmids that are suitable as general cloning vectors in Escherichia coli and Agrobacterium tumefaciens is described. Plasmid pUCD2 is amplifiable in E. coli, replicates in a wide range of gram-negative hosts and contains a number of useful restriction endonuclear cleavage sites and antibiotic resistance genes. This includes unique sites for KpnI, SacI, SacII, PstI, ClaI, SalI, EcoRV, and PvuII and the genes for resistance to kanamycin, tetracycline, ampicillin, and spectinomycin/streptomycin. Derivatives of pUCD2 include pUCD4, which has a unique XbaI site and the cosmid pUCD5, which also contains a unique EcoRI site. Two smaller plasmids pUCD9P and pUCD9X, contain many of the same unique sites as pUCD2 and pUCD4, but carry only the pBR322 replication origin and therefore do not display the extensive host-range of pSa. These plasmids were used to isolate and manipulate fragments of the A. tumefaciens pTiC58 plasmid in both E. coli and A. tumefaciens. Fragments from the virulence (vir) region of pTiC58 inserted immediately upstream of the spectinomycin resistance gene of pUCD2 resulted in spectinomycin resistance levels that varied greatly depending on the particular fragment and its orientation of insertion. Using this property we find that a major portion of the vir region of pTiC58 is transcribed in A. tumefaciens and E. coli from left to right toward the T region.

trans-Acting virulence functions of the octopine Ti plasmid from Agrobacterium tumefaciens

All Ti plasmid-encoded virulence functions that were studied act in trans. An octopine Ti plasmid-specific vir operon, called vir-O, located on an EcoRI restriction fragment has been characterized. Sequences with promoter activity in Escherichia coli were identified for a second vir operon, called vir-C, which was located close to the position of vir-O.

A comparison of virulence determinants in an octopine Ti plasmid, a nopaline Ti plasmid, and an Ri plasmid by complementation analysis of Agrobacterium tumefaciens mutants

Transposon-insertion mutants with vir- Ti plasmids were characterized and then used in complementation experiments. One of the mutants (LBA 1517) had a mutation in a newly discovered vir locus called virF. The virF mutation led to a strongly diminished virulence on tomato and tobacco, but not on certain other plant species. Also a mutant (LBA 1505) was isolated with a mutation somewhere in the bacterial genome but outside the octopine Ti plasmid that caused a restriction in host range for tumor induction. Introduction of a nopaline Ti plasmid or an Ri plasmid into LBA 1505 did not restore normal virulence, showing that the vir gene affected in LBA 1505 determines a factor which is essential for normal tumor induction both by different types of Ti plasmids and by the Ri plasmid. The introduction of R primes containing part or all of the octopine Ti plasmid virulence region led to a restoration of virulence in strains with a vir- nopaline Ti plasmid. Also the transfer of an Ri plasmid to a large number of different vir- octopine or nopaline Ti plasmid mutants rendered these strains virulent. These results indicate that the octopine Ti plasmid, the nopaline Ti plasmid, and the Ri plasmid each have a similar virulence system which can mediate the transfer of T-DNA to plant cells from different types of Ti or Ri plasmids. In complementation experiments between vir- octopine Ti plasmid mutations and vir- nopaline Ti plasmid mutations it was found that equivalent functions are determined by the areas of DNA homology in the virulence regions of these two types of Ti plasmids. The previously defined octopine Ti plasmid virC locus appeared to consist of two different loci. One of these loci was found to be in a region of the octopine Ti plasmid which does not share DNA homology with the nopaline Ti plasmid, and was therefore called virO (octopine Ti plasmid specific). For the other locus the name virC was retained. Whereas mutations in the virC locus were avirulent on all plant species tested, mutations in virO were avirulent on tomato and pea, but virulent on sunflower and Nicotiana rustica. VirO- mutants produced rooty tumors on Kalanchoë tubiflora.

Genetic complementation of Agrobacterium tumefaciens Ti plasmid mutants in the virulence region

Mutants with Tn5 insertions in the vir region of the Agrobacterium tumefaciens TiC58 plasmid are unable to form crown-gall tumors. Complementation tests of these vir region mutants were carried out by constructing merodiploids in a recombination-deficient strain. Each merodiploid possessed a mutant TiC58 plasmid and a recombinant plasmid containing either the homologous wild-type DNA region or the homologous region containing a second Tn5 insertion. The analysis identified six complementation groups. Mutations in one of these complementation groups were not complemented in trans and represent a cis-dominant locus. The mutation in one complementation group showed variation in host range.

Introduction of genetic material into plant cells

The tumor-inducing (Ti) plasmid of the soil microorganism Agrobacterium tumefaciens is the agent of crown gall disease in dicotyledonous plants. The Ti plasmid contains two regions that are essential for the production of transformed cells. One of these regions, termed transfer DNA, induces tumor formation and is found in all established plant tumor lines; the other, termed the virulence region, is essential for the formation but not the maintenance of tumors. Transfer DNA, which transfers to the plant genomes in a somewhat predictable manner, can be increased in size by the insertion of foreign DNA without its transferring ability being affected. The tumor-causing genes can be removed so that they no longer interfere with normal plant growth and differentiation. This modified Ti plasmid can thus be used as a vector for the transfer of foreign genes into plants.

Nucleotide sequence of the T-DNA region from theA grobacterium tumefaciens octopine Ti plasmid pTi15955

The complete nucleotide sequence of the transferred region (T-DNA) of an octopine tumor inducing (Ti) plasmid fromAgrobacterium tumefaciens (pTi15955) has been determined. A total of 24 595 nucleotides extending approximately 900 bases to either side of the outermost, T-DNA boundaries was sequenced. Computer analysis of the sequenced portion of the Ti plasmid revealed that recognition sites for 72 restriction endonucleases are present in the DNA sequence at least once; no site forEcoK exists in this DNA sequence. Two imperfect 24 base repeats border the T-DNA sequence; the left starts at position 909 and the right ends at position 23 782, giving the T-DNA region a total length, of 22 874 nucleotides. Another two similar 24 base repeats lie within T-DNA and divide it, into three distinct domains: T-left (TL-DNA) 13 175 bp of apparently eukaryotic origin; T-center (TC-DNA) 1816 bp of prokaryotic origin; and T-right (TR-DNA) 7 883 bp of eukaryotic origin. The T-DNA contains nine reported transcripts, however, 26 open reading frames longer than 300 bases that start with an ATG initiation codon were found. Fourteen open reading frames are bounded by putative eukaryotic promoters, ribosome binding sites, and poly(A) addition sites and occur only in TL-and TR-DNAs. No open reading frames showing eukaryotic promoter sequences are located within the TC-DNA.

Nucleotide sequence of the Agrobacterium tumefaciens octopine Ti plasmid-encoded tmr gene

The nucleotide sequence of the tmr gene, encoded by the octopine Ti plasmid from Agrobacterium tumefaciens (pTiAch5), was determined. The T-DNA, which encompasses this gene, is involved in tumor formation and maintenance, and probably mediates the cytokinin-independent growth of transformed plant cells. The nucleotide sequence of the tmr gene displays a continuous open reading frame specifying a polypeptide chain of 240 amino acids. The 5'- terminus of the polyadenylated tmr mRNA isolated from octopine tobacco tumor cell lines was determined by nuclease S1 mapping. The nucleotide sequence 5'-TATAAAA-3', which sequence is identical to the canonical "TATA" box, was found 29 nucleotides upstream from the major initiation site for RNA synthesis. Two potential polyadenylation signals 5'-AATAAA-3' were found at 207 and 275 nucleotides downstream from the TAG stopcodon of the tmr gene. A comparison was made of nucleotide stretches, involved in transcription control of T-DNA genes.

Expression of bacterial genes in plant cells

Chimeric bacterial genes conferring resistance to aminoglycoside antibiotics have been inserted into the Agrobacterium tumefaciens tumor-inducing (Ti) plasmid and introduced into plant cells by in vitro transformation techniques. The chimeric genes contain the nopaline synthase 5' and 3' regulatory regions joined to the genes for neomycin phosphotransferase type I or type II. The chimeric genes were cloned into an intermediate vector, pMON120, and inserted into pTiB6S3 by recombination and then introduced into petunia and tobacco cells by cocultivating A. tumefaciens cells with protoplast-derived cells. Southern hybridization was used to confirm the presence of the chimeric genes in the transformed plant tissues. Expression of the chimeric genes was determined by the ability of the transformed cells to proliferate on medium containing normally inhibitory levels of kanamycin (50 micrograms/ml) or other aminoglycoside antibiotics. Plant cells transformed by wild-type pTiB6S3 or derivatives carrying the bacterial neomycin phosphotransferase genes with their own promoters failed to grow under these conditions. The significance of these results for plant genetic engineering is discussed.

Non-oncogenic T-region mutants ofAgrobacterium tumefaciens do transfer, T-DNA into plant cells

A new procedure for site-directed mutagenesis has been applied to the shooting and rooting loci of T-DNA of an octopine Ti-plasmid ofAgrobacterium tumefaciens. Mutants have been obtained which induced tumours that either developed shoots or produced more roots than normally observed. Double mutations, in which both types of T-DNA loci were affected, resulted in non-oncogenic strains. Indications have been obtained, showing that T-DNA coded oncogenic functions can be eliminated without affecting T-DNA transfer into plant cells.

Site-directed mutagenesis in Escherichia coli of a stable R772::Ti cointegrate plasmid from Agrobacterium tumefaciens

The host range of an octopine Ti plasmid is limited to Rhizobiaceae. This has been extended also to Escherichia coli in the form of a stable cointegrate with the wide-host-range plasmid R772. Its structure was studied by constructing a physical map of R772 and of the R772::pTiB6 cointegrate. An insertion sequence present in R772, called IS70, turned out to be involved in cointegrate formation. We found one intact copy of IS70 and a small segment of IS70, respectively, at the junctions of R772 and Ti DNA. The absence of a complete second copy of IS70 is a likely explanation for the stability of the cointegrate plasmid. A procedure for site-directed mutagenesis of this cointegrate plasmid in E. coli is described. The effect of mutations in the Ti plasmid part can be studied subsequently by transferring the cointegrate into Agrobacterium tumefaciens. The advantage of this procedure for Ti plasmids over other methods used at present is discussed.

Mutational analysis of the virulence region of an Agrobacterium tumefaciens Ti plasmid

Forty-nine Tn3 and Tn5 transposition insertion mutations were introduced into the virulence region of the pTiA6NC plasmid of Agrobacterium tumefaciens. Five Tn5 transposition mutations from an earlier study (D. Garfinkel and E. Nester, J. Bacteriol. 144:732-743, 1980) were also mapped more accurately. These mutations defined five separate loci within the virulence region. Two Tn3 insertions into one of these loci, virA, result in a strain which is only weakly virulent; however, a Tn5 insertion into this locus eliminates virulence. One Tn5 insertion into another locus, virC, results in a strain which is weakly virulent. Two additional Tn5 insertions into this locus eliminate virulence. Insertions into the remaining three loci eliminate virulence entirely.

Units of genetic expression in the virulence region of a plant tumor-inducing plasmid of Agrobacterium tumefaciens

The effect of a large number of Tn3 insertions in the vir region of the Ti plasmid pTiA6NC on the virulence of Agrobacterium was determined. The Vir- insertions were mapped in three of the five loci that have been defined previously. Merodiploid Rec- strains carrying one insertion mutation on the Ti plasmid and another insertion mutation (or the homologous wild-type region) on a compatible plasmid were constructed and used in complementation tests for virulence in test plants. This analysis has revealed that there are ten units of gene expression, presumably transcription units in the vir region. Mutation in one of these units is confirmed to be dominant while those in all others are recessive. Co-infection of test plants with pairs of insertion mutants did not restore virulence.