Two chromosomal loci involved in production of exopolysaccharide in Agrobacterium tumefaciens

Dual adhesive unipolar polysaccharides synthesized by overlapping biosynthetic pathways in Agrobacterium tumefaciens

Agrobacterium tumefaciens is a member of the Alphaproteobacteria that pathogenises plants and associates with biotic and abiotic surfaces via a single cellular pole. A. tumefaciens produces the unipolar polysaccharide (UPP) at the site of surface contact. UPP production is normally surface-contact inducible, but elevated levels of the second messenger cyclic diguanylate monophosphate (cdGMP) bypass this requirement. Multiple lines of evidence suggest that the UPP has a central polysaccharide component. Using an A. tumefaciens derivative with elevated cdGMP and mutationally disabled for other dispensable polysaccharides, a series of related genetic screens have identified a large number of genes involved in UPP biosynthesis, most of which are Wzx-Wzy-type polysaccharide biosynthetic components. Extensive analyses of UPP production in these mutants have revealed that the UPP is composed of two genetically, chemically, and spatially discrete forms of polysaccharide, and that each requires a specific Wzy-type polymerase. Other important biosynthetic, processing, and regulatory functions for UPP production are also revealed, some of which are common to both polysaccharides, and a subset of which are specific to each type. Many of the UPP genes identified are conserved among diverse rhizobia, whereas others are more lineage specific.

Exopolysaccharides of Agrobacterium tumefaciens

Agrobacterium exopolysaccharides play a major role in the life of the cell. Exopolysaccharides are required for bacterial growth as a biofilm and they protect the bacteria against environmental stresses. Five of the exopolysaccharides made by A. tumefaciens have been characterized extensively with respect to their structure, synthesis, regulation, and role in the life of the bacteria. These are cyclic-β-(1, 2)-glucan, cellulose, curdlan, succinoglycan, and the unipolar polysaccharide (UPP). This chapter describes the structure, synthesis, regulation, and function of these five exopolysaccharides.

Phenotypic Switching Affecting Chemotaxis, Xanthan Production, and Virulence in Xanthomonas campestris

The chemotaxis towards sucrose and yeast extract of nine strains of Xanthomonas campestris representing pathovars campestris, armoraciae, translucens, vesicatoria, and pelargonii was analyzed by using swarm plates. Unexpectedly, each of these strains formed small or reduced swarms typical of nonmotile or nonchemotactic bacteria. With time, however, chemotactic cells appeared on the swarm plates as blebs of bacteria. These cells were strongly chemotactic and were concomitantly deficient in exopolysaccharide production. The switch from the wild type (exopolysaccharide producing and nonchemotactic) to the swarmer type (exopolysaccharide deficient and chemotactic) appeared irreversible ex planta in bacteriological medium. However, in radish leaves swarmer-type strains of X. campestris pv. campestris were able to revert to the wild type. Swarmer-type derivatives of two X. campestris pv. campestris wild-type isolates showed reduced virulence and growth in the host plants cauliflower and radish. However, exocellular complementation of X. campestris pv. campestris Hrp (nonpathogenic) mutant was achieved by coinoculation with a swarmer-type strain.

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.

Purification of two lectins from a nopalin Agrobacterium tumefaciens strain

Lectins were evidenced on the surface of one Agrobacterium tumefaciens wild strain (82,139) by agglutination test and neoglycoprotein labelling. Bacteria were incubated in the presence of various fluorescein-labelled neoglycoproteins and the binding was assessed by a fluorimetric method. Among the fluorescein-labelled neoglycoproteins tested, the one bearing alpha-D-galactosyl residues was the most efficient. The labelling was optimal at pH 5.0 and naught at pH above 7. The binding was specifically inhibited by homologous fluorescein-free neoglycoproteins. A galactoside-specific lectin was purified to homogeneity by affinity chromatography on agarose-A4 substituted with alpha-D-galactopyranosyl residues. Upon polyacrylamide gel electrophoresis, a single band (M(r) 58,000) was detected. This alpha-D-galactoside-specific lectin agglutinated preferentially human B red blood cells at pH 5.0. Another lectin specific for alpha-L-rhamnoside (M(r) 40,000) not retained on the immobilised galactose was purified by affinity chromatography on alpha-L-rhamnosyl substituted agarose-A4. This L-rhamnoside-specific lectin preferentially agglutinated horse erythrocytes. On the basis of their M(r) and on their sugar specificity, these two lectins are novel lectins with regard to the known sugar-binding proteins present in the Rhizobiaceae family: Agrobacterium, Rhizobium or Bradyrhizobium strains.

pigB determines a diffusible factor needed for extracellular polysaccharide slime and xanthomonadin production in Xanthomonas campestris pv. campestris

Seven xanthomonadin transcriptional units (pigA through pigG) were identified by transposon saturation mutagenesis within an 18.6-kbp portion of the previously identified 25.4-kbp pig region from Xanthomonas campestris pv. campestris (strain B-24). Since marker exchange mutant strains with insertions in one 3.7-kbp portion of pig could not be obtained, mutations in this region may be lethal to the bacterium. Complementation analyses with different insertion mutations further defined and confirmed the seven transcriptional units. Insertional inactivation of one of the transcriptional units, pigB, resulted in greatly reduced levels of both xanthomonadins and extracellular polysaccharide slime, and a pigB-encoding plasmid restored both traits to these strains. pigB mutant strains could also be restored extracellularly by growth adjacent to strains with insertion mutations in any of the other six xanthomonadin transcriptional units, the parent strain (B-24), or strains of five different species of Xanthomonas. Strain B-24 produced a nontransforming diffusible factor (DF), which could be restored to pigB mutants by the pigB-encoding plasmid. Several lines of evidence indicate that DF is a novel bacterial pheromone, different from the known signal molecules of Vibrio, Agrobacterium, Erwinia, Pseudomonas, and Burkholderia spp.

Inhibition of Agrobacterium tumefaciens oncogenicity by the osa gene of pSa

The IncW plasmid pSa originally derived from Shigella flexneri completely inhibits the tumor-inducing ability of Agrobacterium tumefaciens when it is resident in this organism. Oncogenic inhibition is mediated through the expression of the osa gene on pSa. This gene is part of a 3.1-kb DNA segment of pSa that contains four open reading frames revealed by sequencing. Specific deletions and TnCAT insertions within this segment localized the oncogenic inhibitory activity to the last open reading frame, orf-4, designated osa (for oncogenic suppression activity). No promoter exists immediately upstream of the coding sequence of osa since TnCAT insertions or deletions into orf-3 caused the loss of oncogenic inhibition. Deletion analysis showed that the promoter of orf-1 is required for osa transcription. The first three orfs have no role in oncogenic inhibition, since osa alone placed under the control of a constitutive Pkm promoter completely inhibited A. tumefaciens oncogenicity. This inhibition of oncogenicity by osa is not limited to a specific host plant but appears to show broad host specificity. Because the osa-encoded product has close homologies to the fiwA-encoded product of the IncP plasmid RP1, osa may be involved in fertility inhibition that would prevent or reduce the formation of stable mating pairs and T-DNA transfer between A. tumefaciens and plants.

The virD4 gene is required for virulence while virD3 and orf5 are not required for virulence of Agrobacterium tumefaciens

The virD operon of the resident Ti plasmid of Agrobacterium tumefaciens contains loci involved in T-DNA processing and undefined virulence functions. Nucleotide sequence of the entire virD operon of pTiC58 revealed similarities to the virD operon of the root-inducing plasmid pRiA4b and to that of the octopine-type plasmid pTiA6NC. However, comparative sequence data show that virD of pTiC58 is more akin to that of the pRiA4b than to that of the pTiA6NC. T7f10::virD gene fusions were used to generate polypeptides that confirm the presence of four open reading frames virD1, virD2, virD3, and virD4 within virD which have a coding capacity for proteins of 16.1, 49.5, 72.6, and 73.5 kDa, respectively. virD3 therefore encodes a polypeptide 3.4 times larger (72.6 versus 21.3 kDa) than that encoded by virD3 of octopine Ti plasmids. Non-polar virD4 mutants could not be complemented by a distant homologue, TraG protein of plasmid RP4. An independently regulated fifth ORF (orf5) is located immediately downstream of 3' end of virD4 and encodes a polypeptide of 97.4 kDa. The expression of orf5 is dependent on its own promoter and is independent of acetosyringone induction in A. tumefaciens. Recently, it has been shown that virD3 of octopine Ri or Ti plasmids is not required for virulence. In this report, we confirm and extend these findings on a nopaline Ti plasmid by using several virD non-polar mutants that were tested for virulence. virD3 and orf5 non-polar mutants showed no effect on tumorigenicity on 14 different plant species, while virD4 mutants lost their tumorigenicity completely on all these test plants. These data suggest that virD3 and orf5 are not essential for virulence whereas virD4 is absolutely required on a wide range of host plants.

Two-way chemical signaling in Agrobacterium-plant interactions

The discovery in 1977 that Agrobacterium species can transfer a discrete segment of oncogenic DNA (T-DNA) to the genome of host plant cells has stimulated an intense interest in the molecular biology underlying these plant-microbe associations. This attention in turn has resulted in a series of insights about the biology of these organisms that continue to accumulate at an ever-increasing rate. This excitement was due in part to the notion that this unprecedented interkingdom DNA transfer could be exploited to create transgenic plants containing foreign genes of scientific or commercial importance. In the course of these discoveries, Agrobacterium became one of the best available models for studying the molecular interactions between bacteria and higher organisms. One extensively studied aspect of this association concerns the exchange of chemical signals between Agrobacterium spp. and host plants. Agrobacterium spp. can recognize no fewer than five classes of low-molecular-weight compounds released from plants, and other classes probably await discovery. The most widely studied of these are phenolic compounds, which stimulate the transcription of the genes needed for infection. Other compounds include specific monosaccharides and acidic environments which potentiate vir gene induction, acidic polysaccharides which induce one or more chromosomal genes, and a family of compounds called opines which are released from tumorous plant cells to the bacteria as nutrient sources. Agrobacterium spp. in return release a variety of chemical compounds to plants. The best understood is the transferred DNA itself, which contains genes that in various ways upset the balance of phytohormones, ultimately causing neoplastic cell proliferation. In addition to transferring DNA, some Agrobacterium strains directly secrete phytohormones. Finally, at least some strains release a pectinase, which degrades a component of plant cell walls.

A gene cluster required for coordinated biosynthesis of lipopolysaccharide and extracellular polysaccharide also affects virulence of Pseudomonas solanacearum

Bacterial cell surface components can be important determinants of virulence. At least three gene clusters important for extracellular polysaccharide (EPS) biosynthesis have been previously identified in the plant pathogen Pseudomonas solanacearum. We have found that one of these gene clusters, named ops, is also required for lipopolysaccharide (LPS) biosynthesis. Mutations in any complementation unit of this cluster decreased EPS production, prevented the binding of an LPS-specific phage, and altered the mobility of purified LPS in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. However, restoration of LPS biosynthesis alone was not sufficient to restore virulence to the wild-type level, suggesting that EPS is important for pathogenesis.

Location and cloning of the ketal pyruvate transferase gene of Xanthomonas campestris

Genes required for xanthan polysaccharide synthesis (xps) are clustered in a DNA region of 13.5 kb in the chromosome of Xanthomonas campestris. Plasmid pCHC3 containing a 12.4-kb insert of xps genes has been suggested to include a gene involved in the pyruvylation of xanthan gum (N.E. Harding, J.M. Cleary, D.K. Cabañas, I. G. Rosen, and K. S. Kang, J. Bacteriol. 169:2854-2861, 1987). An essential step toward understanding the biosynthesis of xanthan gum and to enable genetic manipulation of xanthan structure is the determination of the biochemical function encoded by the xps genes. On the basis of biochemical characterization of an X. campestris mutant which produces pyruvate-free xanthan gum, complementation studies, and heterologous expression, we have identified the gene coding for the ketal pyruvate transferase (kpt) enzyme. This gene was located on a 1.4-kb BamHI fragment of pCHC3 and cloned in the broad-host-range cloning vector pRK404. An X. campestris kpt mutant was constructed by mini-Mu(Tetr) mutagenesis of the cloned gene and then by recombination of the mutation into the chromosome of the wild-type strain.

Mapping of the ros virulence regulatory gene of A. tumefaciens

Virulence functions associated with the oncogenicity of Agrobacterium tumefaciens are encoded by vir genes contained in six major operons located on the Ti plasmid. The virC and virD operons encode functions responsible for host range and T-intermediate processing. These two operons are regulated positively by the product of virG and negatively by the product of the chromosomal gene ros, which encodes a 15.5 kDa repressor. To determine the location of the ros gene we have constructed A. tumefaciens HFR strains, using transposon Tn5mob to mobilize the ros locus, and used them to map the location of ros relative to auxotrophic loci. Tight linkage was found between ros, his-34 and his-19. A linkage map is presented showing the location of ros relative to other known chromosomal genes associated with virulence functions.

The virC and virD operons of the Agrobacterium Ti plasmid are regulated by the ros chromosomal gene: analysis of the cloned ros gene

The ros chromosomal gene is present in octopine and nopaline strains of Agrobacterium tumefaciens as well as in Rhizobium meliloti. This gene encodes a 15.5-kDa protein that specifically represses the virC and virD operons in the virulence region of the Ti plasmid. The ros gene was cloned from a genomic bank by electroporation and complementation in Agrobacterium cells. Reporter fusion to the ros gene indicates that the level of transcription is controlled in part by autoregulation. A consensus inverted repeat sequence present in the ros promoter and in the virC and virD promoters of pTiC58, pTiA6, and pRiA4b suggests that a specific Ros binding site exists in these promoters. In the virC and virD promoter region, this binding site is within a cluster of vir box consensus sequences in which the VirG protein binds. This suggests possible binding competition between Ros and VirG at the virC and virD promoters. That the Ros protein binds DNA is suggested by the presence of a 'zinc finger' consensus sequence in the protein.

VirD2 gene product from the nopaline plasmid pTiC58 has at least two activities required for virulence

Virulence genes virD1 and virD2 are required for T-DNA processing in Agrobacterium tumefaciens. The regions within virD2 contributing to T-DNA processing and virulence were investigated. Some insertional mutations in virD2 prevented T-DNA border endonucleolytic cleavage and produced an avirulent phenotype. However, a non-polar insertion immediately after bp 684 of the 1344 bp open reading frame of virD2 did not inhibit endonucleolytic cleavage but still caused a loss of virulence. This suggested that in addition to T-DNA border cleaving activity, the VirD2 protein has another virulence function which resides in the C-terminal half of the protein. Comparative nucleotide sequence analyses of virD2 showed that the first 684 bp were 81% homologous to virD2 of an octopine Ti plasmid whereas the remaining 660 bp were only 44% homologous. A plasmid containing the virD region from octopine Ti plasmid could restore both virulence and processing to a nopaline virD2 mutant. No complementation resulted when a nopaline virD2 clone containing a region similar to eukaryotic nuclear envelope transport sequences was deleted from the 3' end. These results suggest that virD1 and only the first half of virD2 are required to encode for the T-DNA processing endonuclease, and that the 3'-half of virD2 encodes a function separate from endonuclease activity that is required for virulence.

A plant-inducible gene of Xanthomonas campestris pv. campestris encodes an exocellular component required for growth in the host and hypersensitivity on nonhosts

Using Tn4431, a transposon that allows transcriptional fusions to a promoterless luciferase (lux) operon, we have isolated a nonpathogenic mutant of Xanthomonas campestris pv. campestris, i.e., JS111, that does not incite any of the black rot symptoms on all tested cruciferous host plants (J. J. Shaw, L. G. Settles, and C. I. Kado, Mol. Plant Microbe Interact. 1:39-45, 1988). In the study reported here, we determined that in contrast to the wild-type strain, JS111 is unable to induce a hypersensitive necrotic response on nonhost plants such as datura, tomato, and cucumber, suggesting that JS111 is a nonpathogenic, nonhypersensitive Hrp mutant. JS111 displayed culture growth rates, exopolysaccharide production, and protease, pectate lysase, cellulase, amylase, and phosphatase activities comparable to those of the wild-type strain. However, the growth of JS111 in host leaves was markedly attenuated. Coinoculation of JS111 with the wild-type strain in cauliflower or radish leaves rescued the growth deficiency of the mutant to normal levels. The locus mutated in JS111 was cloned and named hrpXc, and transcriptional and genetic complementation analyses of the hrpXc locus were conducted. The regulation of hrpXc expression was also investigated in vitro and in planta, using fusions to a lux or chloramphenicol acetyltransferase reporter gene. The hrpXc gene was found to be strongly induced in radish leaves. This is the first report and analysis of a hrp locus from a Xanthomonas species.

Exopolysaccharide production in Rhizobium and its role in invasion

A complex interaction between rhizobia and specific legume plants results in the formation of nitrogen-fixing root nodules. The necessity for signal exchange and a chemically based recognition system between the symbiotic partners has been appreciated for some time, but the details are only gradually being elucidated. The two basic nodule ontogenies exhibit different requirements for Rhizobium exopolysaccharides. These surface exopolysaccharide molecules of Rhizobium are synthesized at a membrane complex, which is regulated by both transcriptional and post-translational controls. The acidic exopolysaccharide probably plays both a passive and an active role during the invasion process.