Tumor-inducing (Ti) plasmids of Agrobacterium share extensive regions of DNA homology

Identification of different agrobacterium strains isolated from the same forest nursery

Several Agrobacterium strains isolated from the same forest nursery from 1982 to 1988 were compared by serological, biochemical, and DNA-DNA hybridization methods. Similarities among strains belonging to biovar 2 were observed by indirect immunofluorescence, whereas biovar 1 strains showed serological heterogeneity. Electrophoretic analysis of bacterial envelope-associated proteins showed that few bands appeared in the strains belonging to biovar 1, whereas many proteins appeared in the case of biovar 2 strains. Chromosomal DNA was analyzed with six random C58 chromosomal fragments. None of the six probes hybridized to the DNA of the two biovar 2 strains. One of the probes gave the same hybridization pattern with all biovar 1 strains, whereas the other probes yielded different patterns. The vir regions were closely related in the different pathogenic strains. The T-DNA and replication regions were less conserved and showed some variations among the strains.

Tumor induction by Agrobacterium rhizogenes involves the transfer of plasmid DNA to the plant genome

The DNA from tumors of Nicotiana glauca initiated by strains of Agrobacterium rhizogenes was shown to contain sequences that are homologous to the root-inducing (Ri) plasmid of the bacterium. Two independently established tumor lines contained a similar portion of the Ri-plasmid. The Ri-plasmid also hybridized to DNA fragments from uninfected N. glauca. A cosmid clone of the Ri-plasmid encompassing the region containing the Ri-plasmid sequences that are stably transferred to the plant also hybridized to the Ri-plasmid-related fragments found in uninfected plants. Five of six tumor lines tested produced a tumor-specific compound that is similar to agropine.

Agropine in "null-type" crown gall tumors: Evidence for generality of the opine concept

Agrobacterium Ti (tumor-inducing) plasmids, the causative agents of crown gall disease, fall into four genetic groups based on the patterns of octopine and nopaline synthesis (by crown gall tumors) and catabolism (by Agrobacterium tumefaciens) for which they are responsible. Two classes of Ti plasmids induce tumors that synthesize neither octopine nor nopaline. The existence of these Ti plasmids challenged the view that opines such as octopine and nopaline play a central role in crown gall biology. We now report the occurrence of an opine in tumors induced by one of these classes of Ti plasmids, the "null-type" plasmids typified by pTi Bo542. The opine was purified by biological enrichment based on its utilization by bacteria containing pTi Bo542 but not by bacteria lacking a Ti plasmid. The mass spectrum and biological properties of this opine are identical to those of agropine, an opine recently discovered in octopine-type tumors. We propose that null-type Ti plasmids now be named for their signal opine, agropine-type Ti plasmids.

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.

VirA, the plant-signal receptor, is responsible for the Ti plasmid-specific transfer of DNA to maize by Agrobacterium

Agrobacteria exhibit marked Ti (tumor-inducing)/Ri (root-inducing) plasmid specificity in their interaction with the Gramineae. In this study, we have used the technique of "agroinfection," in which Agrobacterium-mediated delivery of viral genomes into plants is detected by the development of viral disease symptoms, to identify the region of the Ti plasmid which is responsible for the major differences seen in the ability of nopaline- vs. octopine-type Ti plasmids to transfer maize streak virus (MSV) DNA to maize. Introduction of fragments of the C58 (nopaline-type) Ti plasmid into strains containing an octopine-type Ti plasmid showed that a fragment containing the nopaline-type virA locus was able to complement these normally non-agroinfectious strains to high levels of MSV DNA transfer. Octopine-type virA mutant strains that express vir genes at high levels in the absence of the plant inducing compound acetosyringone also efficiently transferred MSV DNA. These findings imply a functional difference between the virA gene products encoded by octopine- and nopaline-type Ti plasmids which has a profound effect on their ability to mediate DNA transfer to maize.

Characterization of the virE locus of Agrobacterium tumefaciens plasmid pTiC58

The virE locus that is responsible for the efficiency of infection by Agrobacterium tumefaciens (T. Hirooka and C. Kado, J. Bacteriol. 168:237-243, 1986) is located next to the right boundary of the virulence (Vir) region of the nopaline plasmid pTiC58. This locus is very similar to the virE locus of octopine type Ti plasmids on the basis of nucleotide and amino acid sequence comparisons as well as genetic complementation analyses. The nucleotide sequence of virE revealed three open reading frames, arranged as an operon, with a potential coding capacity for proteins of 9, 7.1, and 63.5 kilodaltons. The promoter region of virE was analyzed by using gene fusions to promoterless cat and lux genes. Two different promoters were detected, one which operates in A. tumefaciens and one which operates in Escherichia coli. virE is transcribed from left to right toward the T region. In A. tumefaciens, the expression of virE was induced by acetosyringone and required the presence of pTiC58.

Virulence of Agrobacterium tumefaciens Strain A281 on Legumes

This study addresses the basis of host range on legumes of Agrobacterium tumefaciens strain A281, an l,l-succinamopine strain. We tested virulence of T-DNA and vir region constructs from this tumor-inducing (Ti) plasmid with complementary Ti plasmid regions from heterologous nopaline and octopine strains.

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.

Location of the right boundary of the virulence region on Agrobacterium tumefaciens plasmid pTiC58 and a host-specifying gene next to the boundary

The right boundary of the virulence (Vir) region of the nopaline plasmid pTiC58 of Agrobacterium tumefaciens was determined by transposon insertion, cartridge emplacement, and deletion mutagenesis. Genetic complementation with mutant and wild-type alleles led to the identification of the virE locus at the right boundary, which was located about 6 kilobases from the left border of the segment of DNA that is transferred into the plant genome. virE is 2.0 kilobases long and encodes at least one protein of 69 kilodaltons. Various mutations in virE resulted in different truncated lengths of the 69-kilodalton protein. As this protein was increasingly truncated from the carboxy terminus, the host range of A. tumefaciens and the frequency of tumor formation diminished concomitantly. Thus, as one of its functions, the 69-kilodalton protein of virE is probably involved in some aspect of the host range specificity of A. tumefaciens and in infection efficiency.

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.

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.

Conservation of symbiotic nitrogen fixation gene sequences in Rhizobium japonicum and Bradyrhizobium japonicum

Southern hybridization with nif (nitrogen fixation) and nod (nodulation) DNA probes from Rhizobium meliloti against intact plasmid DNA of Rhizobium japonicum and Bradyrhizobium japonicum strains indicated that both nif and nod sequences are on plasmid DNA in most R. japonicum strains. An exception is found with R. japonicum strain USDA194 and all B. japonicum strains where nif and nod sequences are on the chromosome. In R. japonicum strains, with the exception of strain USDA205, both nif and nod sequences are on the same plasmid. In strain USDA205, the nif genes are on a 112-megadalton plasmid, and nod genes are on a 195-megadalton plasmid. Hybridization to EcoRI digests of total DNA to nif and nod probes from R. meliloti show that the nif and nod sequences are conserved in both R. japonicum and B. japonicum strains regardless of the plasmid or chromosomal location of these genes. In addition, nif DNA hybridization patterns were identical among all R. japonicum strains and with most of the B. japonicum strains examined. Similarly, many of the bands that hybridize to the nodulation probe isolated from R. meliloti were found to be common among R. japonicum strains. Under reduced hybridization stringency conditions, strong conservation of nodulation sequences was observed in strains of B. japonicum. We have also found that the plasmid pRjaUSDA193, which possess nif and nod sequences, does not possess sequence homology with any plasmid of USDA194, but is homologous to parts of the chromosome of USDA194. Strain USDA194 is unique, since nif and nod sequences are present on the chromosome instead of on a plasmid as observed with all other strains examined.

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.

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.

T-DNA fragments of hairy root plasmid pRi8196 are distantly related to octopine and nopaline Ti plasmid T-DNA

Agrobacterium Ti (tumor-inducing) and Ri (root-inducing) plasmids transform dicot plant cells by insertion of a specific plasmid sector called T-DNA (transferred DNA) into host plant nuclear DNA. The mannopine -type Ri plasmid pRi8196 contains four BamHI fragments that encompass core T-DNA. We report Southern hybridization studies that show that these four fragments have no strong homology to octopine-, nopaline-, or agropine -type Ti plasmids. We detected and mapped very weak homology regions, most of which are assignable to opine synthase or opine catabolic functions on the Ti plasmid. We found no homology between Ri T-DNA and the region of Ti T-DNA that encodes tumor morphology functions.

Comparison of Ti plasmids from three different biotypes of Agrobacterium tumefaciens isolated from grapevines

Twenty-six plasmids from grapevine isolates of Agrobacterium tumefaciens were analyzed by SmaI fingerprinting and by hybridization of nick-translated DNA to DNA of another plasmid. These experiments established that octopine Ti plasmids are not highly conserved, although octopine Ti plasmids from biotype 1 A. tumefaciens strains appeared to be very similar. Octopine Ti plasmids from biotype 3 strains are more variable in terms of host range and SmaI fingerprints, but share extensive DNA homology. Fingerprints of nopaline Ti plasmids from strains of a given biotype resemble each other but not fingerprints of Ti plasmids from strains of the other two biotypes. The wide host range octopine Ti plasmid from the biotype 3 strain Ag86 shares more DNA homology with narrow host range Ti plasmids, nopaline Ti plasmids, and octopine catabolism plasmids than with the wide host range octopine Ti plasmid from biotype 1 strain 20/1. pTiAg86 does share homology with the portion of pTi20/1 integrated and expressed in plant tumor cells. Since all wide host range Ti plasmids studied contain these sequences, we suggest that natural selection for a wide host range resulted in the presence of the common sequences in distantly related plasmids. The lack of homology between this "common DNA" and limited host range Ti plasmids shows that the DNA sequences per se are not required for tumorigenesis.

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.

Studies on the structure of cointegrates between octopine and nopaline Ti-plasmids and their tumour-inducing properties

Stable cointegrates between incRh-1 octopine (Ach5) and nopaline (C58) Ti-plasmids, present in ten independently isolated Agrobacterium tumefaciens strains, showed identical restriction endonuclease patterns. Each cointegration event had taken place in the common sequence between the T-regions of both Ti-plasmids. This illustrates a high preference for this region when used in the formation of cointegrates. Four crown gall tissues, obtained after transformation of Nicotiana tabacum cells by one of the mutants, were analysed by using Southern blot analysis for their T-DNA structure. The borders of T-DNA frequently appeared to differ from T-DNA borders previously detected in tumour tissues that had been induced by Agrobacterium strain C58 or Ach5. Therefore, it was concluded that possibly a less stringent mechanism exists for the integration into plant DNA of T-DNA, derived from a composite (octopine/nopaline) T-region than for integration of T-DNA from a normal (octopine or nopaline) T-region.

Relationship between Nif plasmids of fast-growing Rhizobium species and Ti plasmids of Agrobacterium tumefaciens

By use of the Southern blot hybridization technique the extent of DNA homology was determined between the Nif plasmid of a number of fast-growing Rhizobium species and Ti plasmids of the octopine (pTiAch5) and nopaline (pTiC58) type. DNA sequences common to these plasmids were located on functional maps of the Ti plasmids. No homology between Nif plasmids and the T region of Ti plasmids was detected.

DNA sequence homology in Rhizobium meliloti plasmids

Plasmids were recovered by an alkaline procedure from six symbiotically effective strains of Rhizobium meliloti of diverse geographical origin, reported to harbour only one middle-size large plasmid (ranging from 89 to 143 Megadaltons). Each purified plasmid was digested with eight restriction endonucleases; cleavage patterns were very complex: only KpnI and XbaI gave a limited number of bands. Fingerprints were very different, whatever the restriction enzyme or the geographical origin of the strains. However, Southern DNA-DNA hybridizations revealed that the plasmids showed homologous sequences having a high thermal stability. We gave evidence that some of these sequences are common to all the plasmids of R. meliloti. The biological function of these common sequences is unknown. Hybridization with cloned nitrogen fixation (nif) genes from Klebsiella pneumoniae had demonstrated that nif genes were not located on the middle -- size plasmids of R. meliloti studied in this paper.

Grown gall plant tumors of abnormal morphology, induced by Agrobacterium tumefaciens carrying mutated octopine Ti plasmids; analysis of T-DNA functions

Ti plasmid mutants derived from Agrobacterium tumefaciens strain Ach5 that induce tumors of abnormal morphology have been analyzed. On tobacco, A. tumefaciens mutant strain LBA4060 induces tumors that specifically give rise to shoots. Shoots continue to grow from in vitro cultured bacteria-free tumor tissue derived from such tumors. The mutant character is shown to be correlated with the insertion of an A. tumefaciens IS element, IS60, into the left arm of the T-region of the octopine Ti plasmid. Evidence is presented showing that IS60 is transferred into the plant cell DNA as part of the T-DNA. A second Ti plasmid insertion mutant A. tumefaciens strain LBA4210, with a Tn904 transposon in the center of the T-region, induces tumors that specifically exhibit a root development on tobacco plants. T-DNA has been detected in sterile amorphous crown-gall tissue derived from these tumors. The transposon Tn904 insertion was shown to result a changed "core" T-DNA. Abnormal tumor morphologies induced by these mutant strains have been observed also on Kalanchoë stems. On tomato plants the mutants induce small unorganized tumors while on Nicotiana rustica unorganized tumors, nearly equal in size to those caused by the wild-type strain have been induced. LBA4060 was shown to be avirulent on Kalanchoë leaves and LBA4210 was weakly virulent. Infection of Kalanchoë leaves or tomato plants with a mixture of separately grown cultures of both mutants resulted in the formation of more or less normal tumors. The exposure of a tomato plant to naphthalene acetic acid (NAA), a synthetic auxin, during development of tumors induced by LBA4060 stimulated tumor formation. Tumor growth induced by LBA4210 was found to be stimulated by kinetin.

Relationship between the limited and wide host range octopine-type Ti plasmids of Agrobacterium tumefaciens

The relationship between the limited host range octopine Ti plasmids and the wide host range octopine Ti plasmids pTiB6806 and pTiA6 was studied. The limited host range Ti plasmids shared extensive deoxyribonucleic acid homology; pTiAg63 and pTiAg162 were essentially completely homologous with pTiAg158 while pTiAg57 shared approximately 64% homology with pTiAg158. In contrast, the limited host range octopine Ti plasmids only shared 6 to 15% homology with the wide host range octopine Ti plasmid pTiB6806. Thus, limited and wide host range octopine Ti plasmids comprise distinct families of plasmids. The deoxyribonucleic acid homology shared between the limited host range Ti plasmids and pTiB6806, however, was distributed over some 50% of pTiB6806, suggesting that both families of plasmids evolved from a common progenitor plasmid. The limited host range Ti plasmids showed relatively strong homology with pTiB6806 HpaI fragment 7, a region which codes for octopine utilization by the bacterium, but showed only weak homology with pTiB6806 HpaI fragment 12, a region required for virulence. In addition, homology between the limited host range octopine Ti plasmids and the "common deoxyribonucleic acid," sequences shown to have a central role in plant cell transformation, was barely detectable when stringent hybridization conditions were used. We therefore conclude that a highly conserved version of the common deoxyribonucleic acid is not required for crown gall tumorigenesis on all plant species.

Further insight on the transferred-DNA of octopine crown gall

Six octopine tumour lines incited by pTiB6S3, pTiAch5 and pTiA6 on tobacco, Arabidopsis and Petunia were studied by the Southern blotting hybridisation technique in order to define accurately the dimensions of the segments of plasmid origin transferred to the tumourous cell and their organisation in the plant genome. Emphasis has been put on the comparison between octopine and nopaline T-DNAs and on the lines presented here compared with those studied previously (Thomashow et al. 1980). The length of the transferred DNA segment does not depend on the plasmids used, nor on the host plants. The octopine T-DNA organisation in the cell nucleus is significantly different from that of nopaline T-DNAs: tandem arrangements of T-DNA segments could not be detected and the T-DNA itself is much shorter. The tumour lines described here can be compared to some extent with those studied by another group (Thomashow et al. 1980) by the same technique. However, some differences were observed. The transferred DNA was seen as a unique stretch of about 11 kb present only once per cell. No amplification of any part was noticed in any of these six lines. Examination of the restriction patterns presented by the boundary fragments of the T-DNA in these lines suggested that some of them were of common origin.

Relationship of plasmids responsible for hairy root and crown gall tumorigenicity

Three strains of Agrobacterium rhizogenes were examined for plasmids. Strains 15834 and A4 contained essentially identical large plasmids, pAr15834c and pArA4c, respectively (approximately 260 x 10(6) daltons). These plasmids can dissociate to two smaller plasmid species. Strain TR105 contained only a single plasmid, which was homologous with the dissociation product of pAr15834c, pAr15834b. Plasmid pAr15834c shared little overall sequence homology with other Ti plasmids. One region of conserved homology between pAr15834c and a region of the octopine type plasmid pTiB6806 which contains oncogenicity functions was detected. Lower levels of homology were detected with sequences which are distributed throughout 65% of pTiB6806. Homology with the so-called common deoxyribonucleic acid in the integrated plasmid deoxyribonucleic acid region was detected only after lowering the stringency of hybridization (Tm, -41 degrees C). Furthermore, the A. rhizogenes plasmid is compatible with other Ti plasmids. Therefore, the results suggest that the virulence plasmids of A. rhizogenes are functionally similar to other Ti plasmids, yet have diverged sufficiently from an ancestral Ti plasmid that they now represent a distinct plasmid type based on homology, compatibility, and virulence.

Agrobacterium tumefaciens mutants affected in crown gall tumorigenesis and octopine catabolism

Mutants of Agrobacterium tumefaciens which affect virulence or the ability to catabolize octopine were isolated after Tn5-induced mutagenesis. Of 8,900 colonies tested, 7 mutants with Tn5 insertions in a specific region of other Ti plasmid unable to catabolize octopine were isolated. Thirty-seven mutants affected in tumorigenesis resulted from insertions in the Ti plasmid and the Agrobacterium chromosome. Of these mutations, 12 were chromosomal and 25 mapped on the plasmid. Twenty-three mapped within a 20-megadalton region, which is distinct from the Ti plasmid sequences found stably integrated into the plant cell genome T-deoxyribonucleic acid). Included in these were mutants that were either a virulent or produced tumors with unusual morphologies. Three mutants contained insertions in the T-deoxyribonucleic acid. These three mutants incited tumors which synthesized octopine but had an altered morphology due to either extensive proliferation of shoots or roots from the tumor callus. Three additional mutants not caused by Tn5 contained mutations in the Ti plasmid.

Tumor formation and rhizogenicity of Agrobacterium rhizogenes carrying Ti plasmids

Octopine Ti plasmids were introduced into different Agrobacterium rhizogenes strains. Transconjugants became able to utilize octopine and to induce aberrant tumors, the surface of which was all covered wioth roots of different size and morphology. The plasmids endogenous in A. rhizogenes were shown to be compatible with incRH-1 Ti plasmids.

Characterization of Tn904 insertions in octopine Ti plasmid mutants of Agrobacterium tumefaciens

Seven Tn904 insertion mutants of pTi Ach5 affecting Agrobacterium tumefaciens virulence were studied. The mutant character was shown to be plasmid borne. Four of these mutants were avirulent and carried an insertion in restriction endonuclease HpaI fragment 12, a 3.3-megadalton fragment, which therefore appears to be a Ti plasmid region essential for virulence. Two mutants were attenuated in virulence. The inserts mapped close to HpaI fragment 12. One mutant giving rise to small tumors with excessive adventitious root formation on Kalanchoe daigremontiana carried an insertion in the right side of the common sequence in the deoxyribonucleic acid of the Ti plasmid detected in crown gall tumors. The insertion behavior of Tn904 was studied by analyzing 11 independently isolated and randomly chosen mutants. The Tn904 inserts did not affect oncogenicity, tumor morphology, bacterial transfer functions, octopine catabolism functions, or vital parts of the Ti plasmid, such as the origin of replication. Most of the Tn904 inserts were concentrated in a small part of the map. The size of additional deoxyribonucleic acid as a result of Tn904 inserts varied between 5 and 15 megadaltons. In two cases a Ti plasmid was found with two Tn904 insertions at different positions.

Localization of the replication control region on the physical map of the octopine Ti plasmid

A series of small plasmids has been derived from the octopine plasmid pTi-B6 by in vitro manipulation. The smallest plasmid that is able to replicate in Agrobacterium tumefaciens contains the ampicillin-resistance determinant from Tn1, coupled to a piece of DNA that is homologous to HpaI fragment number 11 of the octopine Ti plasmid pTi-Ach 5. The incompatibility functions are also specified by this region.

Expression of Ti-plasmid DNA in E. coli: comparison of homologous fragments cloned from Ti plasmids of Agrobacterium strains C58 and Ach5

Fragments of two Ti plasmids from Agrobacterium tumefaciens were cloned and studied for cross-hybridization. Homologous fragments were further investigated for areas of strong homology and mapped with various restriction enzymes. The fragments were inserted in two directions and the recombinant plasmid was brought into E. coli strains producing minicells. About five proteins were found to be coded by those fragments of the Ti plasmids.

Interactions between octopine and nopaline plasmids in Agrobacterium tumefaciens

Transfer of octopine Ti plasmids to strains already carrying an octopine Ti plasmid was found to occur at the same (high) frequency as transfer to Ti plasmid lacking recipients, showing that resident Ti plasmids do not exhibit entry exclusion towards incoming Ti plasmids. The resident octopine Ti plasmid was lost by the recipient after the entrance of the incoming Ti plasmid, which is indicative of the incompatibility between the Ti plasmids. Octopine Ti plasmids were found to become established only infrequently in recipients with a nopaline Ti plasmid and, vice versa, nopaline Ti plasmids were only rarely established in recipients with an octopine Ti plasmid. Rare clones in which the incoming octopine (nopaline) Ti plasmid had been established despite the presence of a nopaline (octopine) Ti plasmid appeared to harbor cointegrates consisting of the entire incoming Ti plasmid and the entire resident Ti plasmid. The integration event invariably had occurred in a region of the plasmids that is highly conserved in evolution and that is essential for oncogenicity. These results show that octopine and nopaline Ti plasmids cannot be maintained as separate replicons by one and the same cell. Therefore, be definition, these plasmids belong to the same incompatibility group, which has been names inc Rh-1. Agrobacterial non-Ti octopine and nopaline plasmids were found to belong to another incompatibility group. The tumorigenic properties of strains harboring two different Ti plasmids, in a cointegrate structure, were indicative of the virulence genes of both of them being expressed. The agrobacterial non-Ti octopine and nopaline plasmids did not influence the virulence properties encoded by the Ti plasmid.

Genetic and biochemical studies on the suppression of and a recovery from the tumorous state in higher plants

Four neoplastic diseases of plants: crown gall, which is caused by Ti plasmid DNA; Black's wound tumor disease by an RNA virus; the Kostoff genetic tumors by chromosomal imbalance; and habituation, which results from a spontaneous activation of select biosynthetic systems, have been analyzed and compared. It has been found that both the development of a capacity for autonomous growth and the nature of the heritable cellular change that underlies tumorigenesis are similar in the four instances. All develop a capacity for autonomous growth as a result of the persistent activation of select biosynthetic systems, the products of which are concerned with cell growth and division. That the persistent activation of these biosynthetic systems does not involve heritable changes of an irreversible type is indicated by the finding that a reversal of the neoplastic state occurred in three of the test systems. Since the tumor cells in these instances were found to remain totipotent the results suggest that whether the normal or tumor phenotype is expressed is determined by how the genetic information is regulated in a cell. Regulation appears to be accomplished in part through positive feedback control mechanisms. Foreign genetic information could act either in a regulatory manner to persistently activate normal biosynthetic systems or it could code for one or more essential but normally limiting substance(s) and thus replace a substance(s) that in the case of the Kostoff tumors or habituation is specified by host cell genes, or it could do both. In either case, the foreign genetic information can be regulated in much the same manner as are the host cell genes to give rise to either the normal or tumor phenotype.

Some thoughts about genetics, differentiation, and malignancy

This article deals with three related questions: (1) whether malignancy is determined by genetic or epigenetic mechanisms; (2) whether epigenetic mechanisms, as conventionally defined, actually exist; (3) what criteria are appropriate for defining dominance or recessiveness of the malignant state in cell fusion experiments.

Negative control of octopine degradation and transfer genes of octopine Ti plasmids in Agrobacterium tumefaciens

The regulatory system that controls the expression of the Ti plasmid-borne octopine degradation (uad) and transfer (tra) genes in Agrobacterium tumefaciens was studied. A deletion mutant derived from the cointegrate plasmid R702::Ti-B6S3 was isolated, which was compatible with a wild-type Ti plasmid and which had retained the uad genes. By means of this mutant plasmid pAL116, it was possible to make cells diploid for the uad genes. pAL116 was introduced into Rec- strains that contained different types of regulation mutants for the uad and tra genes. The repression pattern that was found in this complementation analysis indicated that the uad and tra operons are controlled by a common repressor system. Several results indicated that there may be additional transcriptional relations between both operons. The corresponding genes of the non-tumorigenic octopine plasmid pAt-AG60 appeared to be controlled by a repressor related to that of the octopine Ti plasmid.

Transcription of Ti plasmid-derived sequences in three octopine-type crown gall tumor lines

Total RNA isolated from three octopine-type crown gall lines contains sequences homologous to specific regions of the tumor-inducing (Ti) plasmid of Agrobacterium tumefaciens strain 15955. A comparison of transcripts in these three tumor lines suggests that tumor cells transcribe various sequences within a sector of plasmid DNA of 13 x 10(6) daltons and that transcription may not be uniform across the plasmid derived sequences (T-DNA). Transcription of T-DNA by octopine-type tumors occurs at four major sites. The levels of transcription occurring at three of these sites appear to vary considerably among the three tumor lines investigated. Part of this variability may reflect differences in the organization and copy number of T-DNA. One of the transcription sites maps within a region of DNA with common sequence homology with all Ti plasmids. Varying amounts of transcript homologous to this region of T-DNA are present in all three tumor lines. It is suggested that transcription of these conserved sequences in the plant may have significance regarding the mechanism of tumorigenesis.