VirA of <i>Agrobacterium tumefaciens</i> is an intradimer transphosphorylase and can actively block <i>vir</i> gene expression in the absence of phenolic signals

Brencic, Anja; Qi, Xia; Winans, Stephen C.

doi:10.1111/j.1365-2958.2004.04057.x

Cited by 17 publications

(23 citation statements)

References 52 publications

(82 reference statements)

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“…Our observation that overexpression of wild-type virA decreased vir gene expression below levels seen in A348 cells carrying the empty vector indicates that an excess of wild-type VirA interferes with VirG activity. As this manuscript was being prepared, a similar result was reported by Brencic et al (5). The autoregulatory nature of VirA and VirG suggests that normal vir gene expression might require these two proteins to be present in set proportions.…”

Section: Discussionsupporting

confidence: 64%

“…The VirA dimer appears to be a key structural element that is linked to at least two aspects of signal transduction. First, it has been demonstrated for several histidine kinases, including VirA, that autophosphorylation is an intersubunit reaction, whereby a phosphate group is transferred from the catalytic region on one dimer subunit to a conserved histidine on the second subunit (5,6,27,37,45). Second, the stabilizing influence of the leucine zipper has been used in fusions to VirA and Tar to demonstrate that a particular orientation at the linker interface between dimer subunits correlates with signal-transducing activity (11,39).…”

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confidence: 99%

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Intersubunit Complementation of Sugar Signal Transduction in VirA Heterodimers and Posttranslational Regulation of VirA Activity inAgrobacterium tumefaciens

2005

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The VirA/VirG two-component regulatory system of Agrobacterium tumefaciens regulates expression of the virulence (vir) genes that control the infection process leading to crown gall tumor disease on susceptible plants. VirA, a membrane-bound homodimer, initiates vir gene induction by communicating the presence of molecular signals found at the site of a plant wound through phosphorylation of VirG. Inducing signals include phenols, monosaccharides, and acidic pH. While sugars are not essential for gene induction, their presence greatly increases vir gene expression when levels of the essential phenolic signal are low. Reception of the sugar signal depends on a direct interaction between ChvE, a sugar-binding protein, and VirA. Here we show that the sugar signal received in the periplasmic region of one subunit within a VirA heterodimer can enhance the kinase function of the second subunit. However, sugar enhancement of vir gene expression was vector dependent. virA alleles expressed from pSa-derived vectors inhibited signal transduction by endogenous VirA. Inhibition was conditional, depending on the induction medium and the virA allele tested. Moreover, constitutive expression of virG overcame the inhibitory effect of some but not all virA alleles, suggesting that there may be more than one inhibitory mechanism.

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Section: Discussionsupporting

confidence: 64%

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confidence: 99%

Intersubunit Complementation of Sugar Signal Transduction in VirA Heterodimers and Posttranslational Regulation of VirA Activity inAgrobacterium tumefaciens

2005

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“…The presence of biologically relevant dimers has been demonstrated both biophysically and genetically for the Tar chemoreceptor and histidine kinases such as PhoQ, DcuS, NarX, and TorS (20,32,77,79). The evidence for VirA dimers comes by way of genetic analysis using intersubunit complementation (9,66,73) and by chemical cross-linking analysis indicating that VirA exists as a dimer even in the absence of the inducing ligands (57). While these earlier studies affirm that VirA exists as a dimer and that dimerization is critical to VirA function, our investigations demonstrate periplasmic domain dimerization in the absence of tethering via transmembrane and cytoplasmic domains.…”

Section: Discussionmentioning

confidence: 99%

The Integrity of the Periplasmic Domain of the VirA Sensor Kinase Is Critical for Optimal Coordination of the Virulence Signal Response inAgrobacterium tumefaciens

et al. 2011

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The plant pathogen Agrobacterium tumefaciens responds to three main signals at the plant-bacterium interface: phenolics, such as acetosyringone (AS), monosaccharides, and acidic pH (ϳ5.5). These signals are transduced via the chromosomally encoded sugar binding protein ChvE and the Ti plasmid-encoded VirA/VirG two-component regulatory system, resulting in the transcriptional activation of the Ti plasmid virulence genes. Here, we present genetic and physical evidence that the periplasmic domain of VirA dimerizes independently of other parts of the protein, and we examine the effects of several engineered mutations in the periplasmic and transmembrane regions of VirA on vir-inducing capacity as indicated by AS sensitivity and maximal level of vir-inducing activity at saturating AS levels. The data indicate that helix-breaking mutations throughout the periplasmic domain of VirA or mutations that reposition the second transmembrane domain (TM2) of VirA relieve the periplasmic domain's repressive effects on the maximal activity of this kinase in response to phenolics, effects normally relieved only when ChvE, sugars, and low pH are also present. Such relief, however, does not sensitize VirA to low concentrations of phenolics, the other major effect of the ChvE-sugar and low pH signals. We further demonstrate that amino acid residues in a small Trg-like motif in the periplasmic domain of VirA are crucial for transmission of the ChvE-sugar signal to the cytoplasmic domain. These experiments provide evidence that small perturbations in the periplasmic domain of VirA can uncouple sugar-mediated changes in AS sensitivity from the sugar-mediated effects on maximal activity.

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“…Another explanation involves possible changes in the level of VirA and/or VirG. Since it has been shown recently that elevated levels of VirA can inhibit the functioning of VirG (4,43), alterations in the level of VirA might reduce vir gene expression. These possibilities are being explored.…”

Section: Discussionmentioning

confidence: 99%

“…Colonies were streaked onto MM NH 4 plates with and without arabinose. Colonies containing the Atu1392 locus grew on both media (Fig.…”

Section: Vol 187 2005mentioning

confidence: 99%

Citrate Synthase Mutants of Agrobacterium Are Attenuated in Virulence and Display Reduced vir Gene Induction

et al. 2005

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A citrate synthase (CS) deletion mutant of Agrobacterium tumefaciens C58 is highly attenuated in virulence. The identity of the mutant was initially determined from its amino acid sequence, which is 68% identical to Escherichia coli and 77% identical to Brucella melitensis. The mutant lost all CS enzymatic activity, and a cloned CS gene complemented a CS mutation in Sinorhizobium. The CS mutation resulted in a 10-fold reduction in vir gene expression, which likely accounts for the attenuated virulence. When a plasmid containing a constitutive virG [virG(Con)] locus was introduced into this mutant, the level of vir gene induction was restored to nearly wild-type level. Further, the virG(Con)-complemented CS mutant strain induced tumors that were similar in size and number to those induced by the parental strain. The CS mutation resulted in only a minor reduction in growth rate in a glucose-salts medium. Both the CS mutant and the virG(Con)-complemented CS strain displayed similar growth deficiencies in a glucose-salts medium, indicating that the reduced growth rate of the CS mutant could not be responsible for the attenuated virulence. A search of the genome of A. tumefaciens C58 revealed four proteins, encoded on different replicons, with conserved CS motifs. However, only the locus that when mutated resulted in an attenuated phenotype has CS activity. Mutations in the other three loci did not result in attenuated virulence and any loss of CS activity, and none were able to complement the CS mutation in Sinorhizobium. The function of these loci remains unknown.

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VirA of Agrobacterium tumefaciens is an intradimer transphosphorylase and can actively block vir gene expression in the absence of phenolic signals

Cited by 17 publications

References 52 publications

Intersubunit Complementation of Sugar Signal Transduction in VirA Heterodimers and Posttranslational Regulation of VirA Activity inAgrobacterium tumefaciens

Intersubunit Complementation of Sugar Signal Transduction in VirA Heterodimers and Posttranslational Regulation of VirA Activity inAgrobacterium tumefaciens

The Integrity of the Periplasmic Domain of the VirA Sensor Kinase Is Critical for Optimal Coordination of the Virulence Signal Response inAgrobacterium tumefaciens

Citrate Synthase Mutants of Agrobacterium Are Attenuated in Virulence and Display Reduced vir Gene Induction

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