scholarly journals An Unusual Mechanism of Isopeptide Bond Formation Attaches the Collagenlike Glycoprotein BclA to the Exosporium of Bacillus anthracis

mBio ◽  
2011 ◽  
Vol 2 (3) ◽  
Author(s):  
Li Tan ◽  
Mei Li ◽  
Charles L. Turnbough
Keyword(s):  
Bacillus Anthracis ◽  
Bond Formation ◽  
Basal Layer ◽  
Side Chain ◽  
Cross Linking ◽  
Amino Group ◽  
Content Type ◽  
Isopeptide Bond ◽  
Isopeptide Bonds

ABSTRACTThe outermost exosporium layer of spores ofBacillus anthracis, the causative agent of anthrax, is comprised of a basal layer and an external hairlike nap. The nap includes filaments composed of trimers of the collagenlike glycoprotein BclA. Essentially all BclA trimers are tightly attached to the spore in a process requiring the basal layer protein BxpB (also called ExsFA). Both BclA and BxpB are incorporated into stable, high-molecular-mass complexes, suggesting that BclA is attached directly to BxpB. The 38-residue amino-terminal domain of BclA, which is normally proteolytically cleaved between residues 19 and 20, is necessary and sufficient for basal layer attachment. In this study, we demonstrate that BclA attachment occurs through the formation of isopeptide bonds between the free amino group of BclA residue A20 and a side chain carboxyl group of an acidic residue of BxpB. Ten of the 13 acidic residues of BxpB can participate in isopeptide bond formation, and at least three BclA polypeptide chains can be attached to a single molecule of BxpB. We also demonstrate that similar cross-linking occursin vitrobetween purified recombinant BclA and BxpB, indicating that the reaction is spontaneous. The mechanism of BclA attachment, specifically, the formation of a reactive amino group by proteolytic cleavage and the promiscuous selection of side chain carboxyl groups of internal acidic residues, appears to be different from other known mechanisms for protein cross-linking through isopeptide bonds. Analogous mechanisms appear to be involved in the cross-linking of other spore proteins and could be found in unrelated organisms.IMPORTANCEIsopeptide bonds are protein modifications found throughout nature in which amide linkages are formed between functional groups of two amino acids, with at least one of the functional groups provided by an amino acid side chain. Isopeptide bonds generate cross-links within and between proteins that are necessary for proper protein structure and function. In this study, we discovered that BclA, the dominant structural protein of the external nap ofBacillus anthracisspores, is attached to the underlying exosporium basal layer protein BxpB via isopeptide bonds formed through a mechanism fundamentally different from previously described mechanisms of isopeptide bond formation. The most unusual features of this mechanism are the generation of a reactive amino group by proteolytic cleavage and promiscuous selection of acidic side chains. This mechanism, which apparently relies only on short peptide sequences in protein substrates, could be a general mechanismin vivoand adapted for protein cross-linkingin vitro.

scholarly journals In vitro reconstitution of sortase-catalyzed pilus polymerization reveals structural elements involved in pilin cross-linking

2018 ◽  
Vol 115 (24) ◽  
pp. E5477-E5486 ◽  
Author(s):  
Chungyu Chang ◽  
Brendan R. Amer ◽  
Jerzy Osipiuk ◽  
Scott A. McConnell ◽  
I-Hsiu Huang ◽  
...  
Keyword(s):  
Cross Linking ◽  
Structural Elements ◽  
Gram Positive Bacteria ◽  
Activating Mutations ◽  
In Vitro Reconstitution ◽  
Isopeptide Bonds ◽  
Signature Sequence ◽  
Class C ◽  
Enzyme Intermediate

Covalently cross-linked pilus polymers displayed on the cell surface of Gram-positive bacteria are assembled by class C sortase enzymes. These pilus-specific transpeptidases located on the bacterial membrane catalyze a two-step protein ligation reaction, first cleaving the LPXTG motif of one pilin protomer to form an acyl-enzyme intermediate and then joining the terminal Thr to the nucleophilic Lys residue residing within the pilin motif of another pilin protomer. To date, the determinants of class C enzymes that uniquely enable them to construct pili remain unknown. Here, informed by high-resolution crystal structures of corynebacterial pilus-specific sortase (SrtA) and utilizing a structural variant of the enzyme (SrtA2M), whose catalytic pocket has been unmasked by activating mutations, we successfully reconstituted in vitro polymerization of the cognate major pilin (SpaA). Mass spectrometry, electron microscopy, and biochemical experiments authenticated that SrtA2Msynthesizes pilus fibers with correct Lys–Thr isopeptide bonds linking individual pilins via a thioacyl intermediate. Structural modeling of the SpaA–SrtA–SpaA polymerization intermediate depicts SrtA2Msandwiched between the N- and C-terminal domains of SpaA harboring the reactive pilin and LPXTG motifs, respectively. Remarkably, the model uncovered a conserved TP(Y/L)XIN(S/T)H signature sequence following the catalytic Cys, in which the alanine substitutions abrogated cross-linking activity but not cleavage of LPXTG. These insights and our evidence that SrtA2Mcan terminate pilus polymerization by joining the terminal pilin SpaB to SpaA and catalyze ligation of isolated SpaA domains in vitro provide a facile and versatile platform for protein engineering and bio-conjugation that has major implications for biotechnology.

scholarly journals Bacterial pseudokinase catalyzes protein polyglutamylation to inhibit the SidE-family ubiquitin ligases

Science ◽  
2019 ◽  
Vol 364 (6442) ◽  
pp. 787-792 ◽  
Author(s):  
Miles H. Black ◽  
Adam Osinski ◽  
Marcin Gradowski ◽  
Kelly A. Servage ◽  
Krzysztof Pawłowski ◽  
...  
Keyword(s):  
Protein Kinase ◽  
Host Cell ◽  
Crystal Structures ◽  
Active Site ◽  
Bond Formation ◽  
Ubiquitin Ligases ◽  
Carboxyl Group ◽  
Amino Group ◽  
Isopeptide Bond

Enzymes with a protein kinase fold transfer phosphate from adenosine 5′-triphosphate (ATP) to substrates in a process known as phosphorylation. Here, we show that the Legionella meta-effector SidJ adopts a protein kinase fold, yet unexpectedly catalyzes protein polyglutamylation. SidJ is activated by host-cell calmodulin to polyglutamylate the SidE family of ubiquitin (Ub) ligases. Crystal structures of the SidJ-calmodulin complex reveal a protein kinase fold that catalyzes ATP-dependent isopeptide bond formation between the amino group of free glutamate and the γ-carboxyl group of an active-site glutamate in SidE. We show that SidJ polyglutamylation of SidE, and the consequent inactivation of Ub ligase activity, is required for successful Legionella replication in a viable eukaryotic host cell.

scholarly journals Structural Basis of a Thiol-Disulfide Oxidoreductase in the Hedgehog-Forming ActinobacteriumCorynebacterium matruchotii

2018 ◽  
Vol 200 (9) ◽  
pp. e00783-17 ◽  
Author(s):  
Truc Thanh Luong ◽  
Reyhaneh Tirgar ◽  
Melissa E. Reardon-Robinson ◽  
Andrzej Joachimiak ◽  
Jerzy Osipiuk ◽  
...  
Keyword(s):  
Disulfide Bond ◽  
Gene Deletion ◽  
Bond Formation ◽  
Toxin Production ◽  
Content Type ◽  
Cellular Processes ◽  
Disulfide Oxidoreductase ◽  
New Gene ◽  
Resolution Structure

ABSTRACTThe actinobacteriumCorynebacterium matruchotiihas been implicated in nucleation of oral microbial consortia leading to biofilm formation. Due to the lack of genetic tools, little is known about basic cellular processes, including protein secretion and folding, in this organism. We report here a survey of theC. matruchotiigenome, which encodes a large number of exported proteins containing paired cysteine residues, and identified an oxidoreductase that is highly homologous to theCorynebacterium diphtheriaethiol-disulfide oxidoreductase MdbA (MdbACd). Crystallization studies uncovered that the 1.2-Å resolution structure ofC. matruchotiiMdbA (MdbACm) possesses two conserved features found in actinobacterial MdbA enzymes, a thioredoxin-like fold and an extended α-helical domain. By reconstituting the disulfide bond-forming machinein vitro, we demonstrated that MdbACmcatalyzes disulfide bond formation within the actinobacterial pilin FimA. A new gene deletion method supported thatmdbAis essential inC. matruchotii. Remarkably, heterologous expression of MdbACmin theC. diphtheriaeΔmdbAmutant rescued its known defects in cell growth and morphology, toxin production, and pilus assembly, and this thiol-disulfide oxidoreductase activity required the catalytic motif CXXC. Altogether, the results suggest that MdbACmis a major thiol-disulfide oxidoreductase, which likely mediates posttranslocational protein folding inC. matruchotiiby a mechanism that is conserved inActinobacteria.IMPORTANCEThe actinobacteriumCorynebacterium matruchotiihas been implicated in the development of oral biofilms or dental plaque; however, little is known about the basic cellular processes in this organism. We report here a high-resolution structure of aC. matruchotiioxidoreductase that is highly homologous to theCorynebacterium diphtheriaethiol-disulfide oxidoreductase MdbA. By biochemical analysis, we demonstrated thatC. matruchotiiMdbA catalyzes disulfide bond formationin vitro. Furthermore, a new gene deletion method revealed that deletion ofmdbAis lethal inC. matruchotii. Remarkably,C. matruchotiiMdbA can replaceC. diphtheriaeMdbA to maintain normal cell growth and morphology, toxin production, and pilus assembly. Overall, our studies support the hypothesis thatC. matruchotiiutilizes MdbA as a major oxidoreductase to catalyze oxidative protein folding.

scholarly journals Differential Effects of Linezolid and Ciprofloxacin on Toxin Production by Bacillus anthracis in anIn VitroPharmacodynamic System

2011 ◽  
Vol 56 (1) ◽  
pp. 513-517 ◽  
Author(s):  
Arnold Louie ◽  
Brian D. VanScoy ◽  
Henry S. Heine ◽  
Weiguo Liu ◽  
Terry Abshire ◽  
...  
Keyword(s):  
Bacillus Anthracis ◽  
Bacterial Population ◽  
Total Population ◽  
Toxin Production ◽  
Pharmacodynamic Model ◽  
Protein Synthesis Inhibitor ◽  
Synthesis Inhibitor ◽  
Content Type ◽  
Linezolid Therapy

ABSTRACTBacillus anthraciscauses anthrax. Ciprofloxacin is a gold standard for the treatment of anthrax. Previously, using the non-toxin-producing ΔSterne strain ofB. anthracis, we demonstrated that linezolid was equivalent to ciprofloxacin for reducing the total (vegetative and spore) bacterial population. With ciprofloxacin therapy, the total population consisted of spores. With linezolid therapy, the population consisted primarily of vegetative bacteria. Linezolid is a protein synthesis inhibitor, while ciprofloxacin is not. Since toxins are produced only by vegetativeB. anthracis, the effect of linezolid and ciprofloxacin on toxin production is of interest. The effect of simulated clinical regimens of ciprofloxacin and linezolid on the vegetative and spore populations and on toxin production was examined in anin vitropharmacodynamic model over 15 days by using the toxin-producing Sterne strain ofB. anthracis. Ciprofloxacin and linezolid reduced the total Sterne population at similar rates. With ciprofloxacin therapy, the total Sterne population consisted of spores. With linezolid therapy, >90% of the population was vegetativeB. anthracis. With ciprofloxacin therapy, toxin was first detectable at 3 h and remained detectable for at least 5 h. Toxin was never detected with linezolid therapy. Ciprofloxacin and linezolid reduced the total Sterne population at similar rates. However, theB. anthracispopulation was primarily spores with ciprofloxacin therapy and was primarily vegetative bacteria with linezolid therapy. Toxin production was detected for at least 5 h with ciprofloxacin therapy but was never detected with linezolid treatment. Linezolid may have an advantage over ciprofloxacin for the treatment ofB. anthracisinfections.

scholarly journals Transglutaminase 5 is regulated by guanine–adenine nucleotides1

2004 ◽  
Vol 381 (1) ◽  
pp. 313-319 ◽  
Author(s):  
Eleonora CANDI ◽  
Andrea PARADISI ◽  
Alessandro TERRINONI ◽  
Valentina PIETRONI ◽  
Sergio ODDI ◽  
...  
Keyword(s):  
Amino Acid ◽  
Three Dimensional ◽  
Binding Pocket ◽  
Cross Linking ◽  
Bifunctional Enzyme ◽  
Amino Acid Sequence Alignment ◽  
Gtp Binding ◽  
Epidermal Tissue ◽  
Isopeptide Bonds

Transglutaminases (TGases) are Ca2+-dependent enzymes capable of catalysing transamidation of glutamine residues to form intermolecular isopeptide bonds. Nine distinct TGases have been described in mammals, and two of them (types 2 and 3) are regulated by GTP/ATP. TGase2 hydrolyses GTP and is therefore a bifunctional enzyme. In the present study, we report that TGase5 is also regulated by nucleotides. We have identified the putative TGase5 GTP-binding pocket by comparative amino acid sequence alignment and homology-derived three-dimensional modelling. GTP and ATP inhibit TGase5 cross-linking activity in vitro, and Ca2+ is capable of completely reversing this inhibition. In addition, TGase5 mRNA is not restricted to epidermal tissue, but is also present in different adult and foetal tissues, suggesting a role for TGase5 outside the epidermis. These results reveal the reciprocal actions of Ca2+ and nucleotides with respect to TGase5 activity. Taken together, these results indicate that TGases are a complex family of enzymes regulated by calcium, with at least three of them, namely TGase2, TGase3 and TGase5, also being regulated by ATP and GTP.

scholarly journals Characterization of the Enzymes Encoded by the Anthrose Biosynthetic Operon of Bacillus anthracis

2010 ◽  
Vol 192 (19) ◽  
pp. 5053-5062 ◽  
Author(s):  
Shengli Dong ◽  
Sylvia A. McPherson ◽  
Yun Wang ◽  
Mei Li ◽  
Pengfei Wang ◽  
...  
Keyword(s):  
Bacillus Anthracis ◽  
Biosynthetic Pathway ◽  
Basal Layer ◽  
Side Chain ◽  
Side Chains ◽  
Aminotransferase Activity ◽  
Acyltransferase Activity ◽  
Hydratase Activity ◽  
Etiological Agents

ABSTRACT Bacillus anthracis spores, the etiological agents of anthrax, possess a loosely fitting outer layer called the exosporium that is composed of a basal layer and an external hairlike nap. The filaments of the nap are formed by trimers of the collagenlike glycoprotein BclA. Multiple pentasaccharide and trisaccharide side chains are O linked to BclA. The nonreducing terminal residue of the pentasaccharide side chain is the unusual sugar anthrose. A plausible biosynthetic pathway for anthrose biosynthesis has been proposed, and an antABCD operon encoding four putative anthrose biosynthetic enzymes has been identified. In this study, we genetically and biochemically characterized the activities of these enzymes. We also used mutant B. anthracis strains to determine the effects on BclA glycosylation of individually inactivating the genes of the anthrose operon. The inactivation of antA resulted in the appearance of BclA pentasaccharides containing anthrose analogs possessing shorter side chains linked to the amino group of the sugar. The inactivation of antB resulted in BclA being replaced with only trisaccharides, suggesting that the enzyme encoded by the gene is a dTDP-β-l-rhamnose α-1,3-l-rhamnosyl transferase that attaches the fourth residue of the pentasaccharide side chain. The inactivation of antC and antD resulted in the disappearance of BclA pentasaccharides and the appearance of a tetrasaccharide lacking anthrose. These phenotypes are entirely consistent with the proposed roles for the antABCD-encoded enzymes in anthrose biosynthesis. Purified AntA was then shown to exhibit β-methylcrotonyl-coenzyme A (CoA) hydratase activity, as we predicted. Similarly, we confirmed that purified AntC had aminotransferase activity and that purified AntD displayed N-acyltransferase activity.

scholarly journals Evidence that Oxidative Stress InducesspxA2Transcription in Bacillus anthracis Sterne through a Mechanism Requiring SpxA1 and Positive Autoregulation

2016 ◽  
Vol 198 (21) ◽  
pp. 2902-2913 ◽  
Author(s):  
Skye Barendt ◽  
Cierra Birch ◽  
Lea Mbengi ◽  
Peter Zuber
Keyword(s):  
Oxidative Stress ◽  
Bacillus Anthracis ◽  
Transcription Initiation ◽  
Bacterial Species ◽  
Negative Control ◽  
Mobility Shift ◽  
Content Type ◽  
Oxidative Protein Damage

ABSTRACTBacillus anthracispossesses two paralogs of the transcriptional regulator, Spx. SpxA1 and SpxA2 interact with RNA polymerase (RNAP) to activate the transcription of genes implicated in the prevention and alleviation of oxidative protein damage. ThespxA2gene is highly upregulated in infected macrophages, but how this is achieved is unknown. Previous studies have shown that thespxA2gene was under negative control by the Rrf2 family repressor protein, SaiR, whose activity is sensitive to oxidative stress. These studies also suggested thatspxA2was under positive autoregulation. In the present study, we show byin vivoandin vitroanalyses thatspxA2is under direct autoregulation but is also dependent on the SpxA1 paralogous protein. The deletion of eitherspxA1orspxA2reduced the diamide-inducible expression of anspxA2-lacZconstruct.In vitrotranscription reactions using purifiedB. anthracisRNAP showed that SpxA1 and SpxA2 protein stimulates transcription from a DNA fragment containing thespxA2promoter. Ectopically positionedspxA2-lacZfusion requires both SpxA1 and SpxA2 for expression, but the requirement for SpxA1 is partially overcome whensaiRis deleted. Electrophoretic mobility shift assays showed that SpxA1 and SpxA2 enhance the affinity of RNAP forspxA2promoter DNA and that this activity is sensitive to reductant. We hypothesize that the previously observed upregulation ofspxA2in the oxidative environment of the macrophage is at least partly due to SpxA1-mediated SaiR repressor inactivation and the positive autoregulation ofspxA2transcription.IMPORTANCERegulators of transcription initiation are known to govern the expression of genes required for virulence in pathogenic bacterial species. Members of the Spx family of transcription factors function in control of genes required for virulence and viability in low-GC Gram-positive bacteria. InBacillus anthracis, thespxA2gene is highly induced in infected macrophages, which suggests an important role in the control of virulence gene expression during the anthrax disease state. We provide evidence that elevated concentrations of oxidized, active SpxA2 result from an autoregulatory positive-feedback loop drivingspxA2transcription.

In vitro digestibility and pasting properties of epichlorohydrin modified cassava starch

2016 ◽  
Vol 46 (4) ◽  
pp. 517-528 ◽  
Author(s):  
Marta Aurelia Horianski ◽  
Juan Manuel Peralta ◽  
Luis Alberto Brumovsky
Keyword(s):  
Reaction Time ◽  
Resistant Starch ◽  
Reaction Times ◽  
Cassava Starch ◽  
Pasting Properties ◽  
Cross Linking ◽  
In Vitro Digestibility ◽  
Total Dietary Fiber ◽  
Content Type

Purpose The purpose of this study was to analyze the influence of epichlorohydrin (ECH) concentration and reaction time on the food-grade resistant starch production and its pasting properties by using native cassava starch of Misiones-Argentina origin. Design/methodology/approach Cassava starch was modified using ECH (0.30 and 0.15 per cent) during 4 or 8 h. Digestibility was evaluated by determining resistant starch as total dietary fiber. Pasting properties and the cross-linking degree were studied using a micro-viscoamylograph (Brabender). Findings Resistant starch content was not influenced by ECH concentration and reaction time. Cross-linking was detected at higher reaction times (8 h) and ECH concentrations (0.30 per cent), where a decrease in viscosity peaks by more than 80 per cent was observed. Both pasting temperature and breakdown were increased, whereas a decrease in retrogradation was detected. Practical implications Starches can be suitable for different food applications. This is because of the ability to modify its pasting properties and the invariability of the in vitro digestibility of cassava starch as a result of using ECH (at concentrations approved by local and regional legislation) and reaction times of 4 and 8 h. Originality/value Information related to the modification of cassava starch using ECH is scarce or not available nowadays in literature.

scholarly journals Petrobactin Protects against Oxidative Stress and Enhances Sporulation Efficiency inBacillus anthracisSterne

mBio ◽  
2018 ◽  
Vol 9 (6) ◽  
Author(s):  
Ada K. Hagan ◽  
Yael M. Plotnick ◽  
Ryan E. Dingle ◽  
Zachary I. Mendel ◽  
Stephen R. Cendrowski ◽  
...  
Keyword(s):  
Oxidative Stress ◽  
Bacillus Anthracis ◽  
Iron Acquisition ◽  
Sporulation Medium ◽  
Term Survival ◽  
Content Type ◽  
Stress Protection ◽  
Dormant Spore ◽  
Iron Iron

ABSTRACTBacillus anthracisis a Gram-positive bacillus that under conditions of environmental stress, such as low nutrients, can convert from a vegetative bacillus to a highly durable spore that enables long-term survival. The sporulation process is regulated by a sequential cascade of dedicated transcription factors but requires key nutrients to complete, one of which is iron. Iron acquisition by the iron-scavenging siderophore petrobactin is required for vegetative growth ofB. anthracisunder iron-depleted conditions and in the host. However, the extent to which petrobactin is involved in spore formation is unknown. This work shows that efficientin vitrosporulation ofB. anthracisrequires petrobactin, that the petrobactin biosynthesis operon (asbAto-F) is induced prior to sporulation, and that the siderophore itself associates with spores. Petrobactin is also required for oxidative stress protection during late-stage growth and for wild-type levels of sporulation in sporulation medium. Sporulation in bovine blood was found to be petrobactin dependent. Collectively, thein vitrocontributions of petrobactin to sporulation as well as growth imply that petrobactin may be required forB. anthracistransmission via the spore during natural infections, in addition to its key known functions during active anthrax infections.IMPORTANCEBacillus anthraciscauses the disease anthrax, which is transmitted via its dormant, spore phase. However, conversion from bacillus to spore is a complex, energetically costly process that requires many nutrients, including iron.B. anthracisrequires the siderophore petrobactin to scavenge iron from host environments. We show that, in the Sterne strain, petrobactin is required for efficient sporulation, even when ample iron is available. The petrobactin biosynthesis operon is expressed during sporulation, and petrobactin is biosynthesized during growth in high-iron sporulation medium, but instead of being exported, the petrobactin remains intracellular to protect against oxidative stress and improve sporulation. It is also required for full growth and sporulation in blood (bovine), an essential step for anthrax transmission between mammalian hosts.

scholarly journals Bacillus anthracis Virulence Regulator AtxA Binds Specifically to the pagA Promoter Region

2019 ◽  
Vol 201 (23) ◽  
Author(s):  
Rita M. McCall ◽  
Mary E. Sievers ◽  
Rasem Fattah ◽  
Rodolfo Ghirlando ◽  
Andrei P. Pomerantsev ◽  
...  
Keyword(s):  
Gene Regulation ◽  
Bacillus Anthracis ◽  
Promoter Region ◽  
Virulence Gene ◽  
Anthrax Toxin ◽  
Content Type ◽  
Polymerase Binding

ABSTRACT Anthrax toxin activator (AtxA) is the master virulence gene regulator of Bacillus anthracis. It regulates genes on the chromosome as well as the pXO1 and pXO2 plasmids. It is not clear how AtxA regulates these genes, and direct binding of AtxA to its targets has not been shown. It has been previously suggested that AtxA and other proteins in the Mga/AtxA global transcriptional regulators family bind to the curvature of their DNA targets, although this has never been experimentally proven. Using electrophoretic mobility shift assays, we demonstrate that AtxA binds directly to the promoter region of pagA upstream of the RNA polymerase binding site. We also demonstrate that in vitro, CO2 appears to have no role in AtxA binding. However, phosphomimetic and phosphoablative substitutions in the phosphotransferase system (PTS) regulation domains (PRDs) do appear to influence AtxA binding and pagA regulation. In silico, in vitro, and in vivo analyses demonstrate that one of two hypothesized stem-loops located upstream of the RNA polymerase binding site in the pagA promoter region is important for AtxA binding in vitro and pagA regulation in vivo. Our study clarifies the mechanism by which AtxA interacts with one of its targets. IMPORTANCE Anthrax toxin activator (AtxA) regulates the major virulence genes in Bacillus anthracis. The bacterium produces the anthrax toxins, and understanding the mechanism of toxin production may facilitate the development of therapeutics for B. anthracis infection. Since the discovery of AtxA 25 years ago, the mechanism by which it regulates its targets has largely remained a mystery. Here, we provide evidence that AtxA binds to the promoter region of the pagA gene encoding the main central protective antigen (PA) component of the anthrax toxin. These data suggest that AtxA binding plays a direct role in gene regulation. Our work also assists in clarifying the role of CO2 in AtxA’s gene regulation and provides more evidence for the role of AtxA phosphorylation in virulence gene regulation.

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