Upstream sequence-dependent suppression and AtxA-dependent activation of protective antigens in Bacillus anthracis

The Transcription Activator AtxA from Bacillus Anthracis was Employed for Developing a Tight-Control, High-Level, Modulable, and Stationary-Phase Specific Transcription Activity in Escherichia Coli

The strong transcriptional activity of the virulent gene pagA in Bacillus anthracis has been proven to be anthrax toxin activator (AtxA)-regulated. However, the obscure pagA transcription mechanism hinders practical applications of this strong promoter. In this study, a 509-bp DNA fragment [termed 509sequence, (−508)-(+1) relative to the P2 transcription start site] was cloned upstream of rbs-GFPuv as pTOL02B to elucidate the AtxA-regulated transcription. The 509sequence was dissected into the −10 sequence, −35 sequence, AT_rich tract, SLI/SLII and upstream site. In conjunction with the heterologous co-expression of AtxA (under the control of the T7 promoter), the −10 sequence (TATACT) was sufficient for the AtxA-regulated transcription. Integration of pTOL02F + pTOLAtxA as pTOL03F showed that the AtxA-regulated transcription exhibited a strong specific fluorescence intensity/common analytical chemistry term (OD₆₀₀) of 40 597 ± 446 and an induction/repression ratio of 122. An improved induction/repression ratio of 276 was achieved by cultivating Escherichia coli/pTOL03F in M9 minimal medium. The newly developed promoter system termed P_AtxA consists of AtxA, the −10 sequence and Escherichia RNA polymerase. These three elements synergistically and cooperatively formed a previously undiscovered transcription system, which exhibited a tight-control, high-level, modulable and stationary-phase-specific transcription. The P_AtxA was used for phaCAB expression for the stationary-phase polyhydroxybutyrate production, and the results showed that a PHB yield, content and titer of 0.20 ± 0.27 g/g-glucose, 68 ± 11% and 1.5 ± 0.4 g/l can be obtained. The positive inducible P_AtxA, in contrast to negative inducible, should be a useful tool to diversify the gene information flow in synthetic biology.

Graphical Abstract

PRD-Containing Virulence Regulators (PCVRs) in Pathogenic Bacteria

Bacterial pathogens rely on a complex network of regulatory proteins to adapt to hostile and nutrient-limiting host environments. The phosphoenolpyruvate phosphotransferase system (PTS) is a conserved pathway in bacteria that couples transport of sugars with phosphorylation to monitor host carbohydrate availability. A family of structurally homologous PTS-regulatory-domain-containing virulence regulators (PCVRs) has been recognized in divergent bacterial pathogens, including Streptococcus pyogenes Mga and Bacillus anthracis AtxA. These paradigm PCVRs undergo phosphorylation, potentially via the PTS, which impacts their dimerization and their activity. Recent work with predicted PCVRs from Streptococcus pneumoniae (MgaSpn) and Enterococcus faecalis (MafR) suggest they interact with DNA like nucleoid-associating proteins. Yet, Mga binds to promoter sequences as a homo-dimeric transcription factor, suggesting a bi-modal interaction with DNA. High-resolution crystal structures of 3 PCVRs have validated the domain structure, but also raised additional questions such as how ubiquitous are PCVRs, is PTS-mediated histidine phosphorylation via potential PCVRs widespread, do specific sugars signal through PCVRs, and do PCVRs interact with DNA both as transcription factors and nucleoid-associating proteins? Here, we will review known and putative PCVRs based on key domain and functional characteristics and consider their roles as both transcription factors and possibly chromatin-structuring proteins.

Direct Regulons of AtxA, the Master Virulence Regulator of Bacillus anthracis

AtxA, the master virulence regulator of Bacillus anthracis, regulates the expression of three toxins and genes for capsule formation that are required for the pathogenicity of B. anthracis. Recent transcriptome analyses showed that AtxA affects a large number of genes on the chromosome and plasmids, suggesting a role as a global regulator. However, information on genes directly regulated by AtxA is scarce. In this work, we conducted genome-wide analyses and cataloged the binding sites of AtxA in vivo and transcription start sites on the B. anthracis genome. By integrating these results, we detected eight genes as direct regulons of AtxA. These consisted of five protein-coding genes, including two of the three toxin genes, and three genes encoding the small RNAs XrrA and XrrB and a newly discovered 95-nucleotide small RNA, XrrC. Transcriptomes from single-knockout mutants of these small RNAs revealed changes in the transcription levels of genes related to the aerobic electron transport chain, heme biosynthesis, and amino acid metabolism, suggesting their function for the control of cell physiology. These results reveal the first layer of the gene regulatory network for the pathogenicity of B. anthracis and provide a data set for the further study of the genomics and genetics of B. anthracis.

IMPORTANCEBacillus anthracis is the Gram-positive bacterial species that causes anthrax. Anthrax is still prevalent in countries mainly in Asia and Africa, where it causes economic damage and remains a public health issue. The mechanism of pathogenicity is mainly explained by the three toxin proteins expressed from the pXO1 plasmid and by proteins involved in capsule formation expressed from the pXO2 plasmid. AtxA is a protein expressed from the pXO1 plasmid that is known to upregulate genes involved in toxin production and capsule formation and is thus considered the master virulence regulator of B. anthracis. Therefore, understanding the detailed mechanism of gene regulation is important for the control of anthrax. The significance of this work lies in the identification of genes that are directly regulated by AtxA via genome-wide analyses. The results reveal the first layer of the gene regulatory network for the pathogenicity of B. anthracis and provide useful resources for a further understanding of B. anthracis.

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

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.