AB
-type toxins are a diverse family of protein toxins composed of an enzymatic active (A) subunit and a pentameric delivery (B) subunit.
serovar Typhi's typhoid toxin features two A subunits, CdtB ...and PltA, in complex with the B subunit PltB. Recently, it was shown that
. Typhi encodes a horizontally acquired B subunit, PltC, that also assembles with PltA/CdtB to produce a second form of typhoid toxin.
. Typhi therefore produces two AB
toxins with the same A subunits but distinct B subunits, an evolutionary twist that is unique to typhoid toxin. Here, we show that, remarkably, the
species independently evolved an analogous capacity to produce two typhoid toxins with distinct B subunits.
's alternate B subunit, PltD, is evolutionarily distant from both PltB and PltC and outcompetes PltB to form the predominant toxin. We show that, surprisingly,
elicits similar levels of CdtB-mediated intoxication as
. Typhi during infection of cultured human epithelial cells. This toxicity is exclusively due to the PltB toxin, and strains lacking
produce increased amounts of PltB toxin and exhibit increased toxicity compared to the wild type, suggesting that the acquisition of the PltD subunit potentially made
less virulent toward humans. Collectively, this study unveils a striking example of convergent evolution that highlights the importance of the poorly understood "two-toxin" paradigm for typhoid toxin biology and, more broadly, illustrates how the flexibility of A-B interactions has fueled the evolutionary diversification and expansion of AB
-type toxins.
Typhoid toxin is an important
Typhi virulence factor and an attractive target for therapeutic interventions to combat typhoid fever. The recent discovery of a second version of this toxin has substantial implications for understanding
. Typhi pathogenesis and combating typhoid fever. In this study, we discover that a remarkably similar two-toxin paradigm evolved independently in
, which strongly suggests that this is a critical aspect of typhoid toxin biology. We observe significant parallels between how the two toxins assemble and their capacity to intoxicate host cells during infection in
. Typhi and
, which provides clues to the biological significance of this unusual toxin arrangement. More broadly, AB5 toxins with diverse activities and mechanisms are essential virulence factors for numerous important bacterial pathogens. This study illustrates the capacity for novel A-B interactions to evolve and thus provides insight into how such a diverse arsenal of toxins might have emerged.
Many bacteria contain an ortholog of the Ro autoantigen, a ring-shaped protein that binds noncoding RNAs (ncRNAs) called Y RNAs. In the only studied bacterium, Deinococcus radiodurans, the Ro ...ortholog Rsr functions in heat-stress-induced ribosomal RNA (rRNA) maturation and starvation-induced rRNA decay. However, the mechanism by which this conserved protein and its associated ncRNAs act has been obscure. We report that Rsr and the exoribonuclease polynucleotide phosphorylase (PNPase) form an RNA degradation machine that is scaffolded by Y RNA. Single-particle electron microscopy, followed by docking of atomic models into the reconstruction, suggests that Rsr channels single-stranded RNA into the PNPase cavity. Biochemical assays reveal that Rsr and Y RNA adapt PNPase for effective degradation of structured RNAs. A Ro ortholog and ncRNA also associate with PNPase in Salmonella Typhimurium. Our studies identify another ribonucleoprotein machine and demonstrate that ncRNA, by tethering a protein cofactor, can alter the substrate specificity of an enzyme.
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► A bacterial Ro protein is complexed with ncRNA and the PNPase exoribonuclease ► ncRNA scaffolds this RNA degradation machine ► Single-particle EM suggests that Rsr channels RNA into the PNPase cavity ► Rsr and the ncRNA specialize PNPase for degrading structured RNA
A bacterial Y noncoding RNA is found to tether Ro protein to polynucleotide phosphorylase exoribonuclease, specializing this enzyme for degrading structured RNAs. Thus, a noncoding RNA can alter the substrate specificity of an enzyme by tethering a protein cofactor.
Bacterial toxins with an AB
architecture consist of an active (A) subunit inserted into a ring-like platform comprised of five delivery (B) subunits. Salmonella Typhi, the cause of typhoid fever, ...produces an unusual A
B
toxin known as typhoid toxin. Here, we report that upon infection of human cells, S. Typhi produces two forms of typhoid toxin that have distinct delivery components but share common active subunits. The two typhoid toxins exhibit different trafficking properties, elicit different effects when administered to laboratory animals, and are expressed using different regulatory mechanisms and in response to distinct metabolic cues. Collectively, these results indicate that the evolution of two typhoid toxin variants has conferred functional versatility to this virulence factor. More broadly, this study reveals a new paradigm in toxin biology and suggests that the evolutionary expansion of AB
toxins was likely fueled by the plasticity inherent to their structural design coupled to the functional versatility afforded by the combination of homologous toxin components.
is a diverse species of bacterial pathogens comprised of >2,500 serovars with variable host ranges and virulence properties. Accumulating evidence indicates that two AB
-type toxins, typhoid toxin ...and ArtAB toxin, contribute to the more severe virulence properties of the
strains that encode them. It was recently discovered that there are two distinct types of
-like genetic elements in
: those that encode ArtAB toxins (
elements) and those in which the
gene is degraded and the ArtB homolog, dubbed PltC, serves as an alternative delivery subunit for typhoid toxin (
elements). Here, we take a multifaceted approach to explore the evolutionary diversification of
-like genetic elements in
. We identify 7 subtypes of ArtAB toxins and 4 different PltC sequence groups that are distributed throughout the
genus. Both
and
are encoded within numerous diverse prophages, indicating a central role for phages in their evolutionary diversification. Genetic and structural analyses revealed features that distinguish
elements from
and identified evolutionary adaptations that enable PltC to efficiently engage typhoid toxin A subunits. For both
and
, we find that the sequences of the B subunits are especially variable, particularly amongst amino acid residues that fine tune the chemical environment of their glycan binding pockets. This study provides a framework to delineate the remarkably complex collection of
/
-like genetic elements and provides a window into the mechanisms of evolution for AB
-type toxins.
Typhoid toxin is an A
B
protein toxin and an important virulence factor for the human-adapted bacterial pathogen Salmonella enterica serovar Typhi, the causative agent of typhoid fever. Typhoid toxin ...contains two enzymatic subunits, PltA and CdtB, which dock onto a pentameric delivery platform composed of the protein PltB. It was recently reported that the same enzymatic subunits can assemble with a different delivery platform composed of the protein PltC, forming a distinct version of typhoid toxin. However, the differences in structure and receptor specificity between the PltC and PltB typhoid toxins remain unknown. Here, we determined atomic-level structures of the pentameric PltC subunit, the fully assembled PltC typhoid toxin, and the PltC pentamers in complex with glycan receptors. Biochemical and structural analyses indicate that PltB and PltC are unable to form heteromeric delivery complexes due to electrostatic repulsion at the subunit interface and thus form separate toxins only. We further observed that, despite low sequence similarity between PltB and PltC, they interact with PltA in a similar manner but that PltC exhibits stronger electrostatic interactions with PltA, enabling it to outcompete PltB in toxin assembly. The ligand-bound atomic structures of PltC show an additional glycan binding site not found in PltB and glycan array analysis indicates that PltB and PltC exhibit significant differences in glycan binding specificity. Collectively, this study offers atomic-level insights into how
. Typhi produces two distinct versions of typhoid toxin, thereby generating functional diversity in this key virulence factor.
Typhoid fever is a devastating disease that kills more than 115,000 people every year and is caused by
Typhi. Typhoid toxin, exclusively produced by
. Typhi, was demonstrated to be responsible for the pathogenesis of typhoid fever. Typhoid toxin consists of a pentameric delivery B subunit to transport the catalytic A subunits into the host cell through binding of the glycan receptors. Recent study shows that
. Typhi encodes two homologous delivery B subunits that are able to associate with the same active subunits to produce alternative toxins with distinct functional characteristics. Here, we show that the two delivery subunits can form only homopentameric delivery platforms that compete to associate with typhoid toxin's active subunits and that the two resulting toxins have distinct glycan-binding properties that confer distinct functional traits. These findings highlight the unique assembly and functional diversification of typhoid toxins.
Typhoid fever is a major global health problem and is the result of systemic infections caused by the human-adapted bacterial pathogen Salmonella enterica serovar Typhi (S. Typhi). The pathology ...underlying S. Typhi infections significantly differ from infections caused by broad host range serovars of the same species, which are a common cause of gastroenteritis. Accordingly, identifying S. Typhi genetic factors that impart functionality absent from broad host range serovars offers insights into its unique biology. Here, we used an in-silico approach to explore the function of an uncharacterized 14-gene S. Typhi genomic islet. Our results indicated that this islet was specific to the S. enterica species, where it was encoded by the Typhi and Paratyphi A serovars, but was generally absent from non-typhoidal serovars. Evidence was gathered using comparative genomics and sequence analysis tools, and indicated that this islet was comprised of Type VI secretion system (T6SS) and contact-dependent growth inhibition (CDI) genes, the majority of which appeared to encode orphan immunity proteins that protected against the activities of effectors and toxins absent from the S. Typhi genome. We herein propose that this islet represents an immune system that protects S. Typhi against competing bacteria within the human gut.
Salmonella Typhi is the cause of typhoid fever, a major global health concern. An essential virulence factor of this pathogen is typhoid toxin. In contrast to most AB-type toxins, typhoid toxin is ...exclusively expressed by intracellular bacteria. The regulatory networks that ensure this unique gene expression pattern are unknown. Here, we developed FAST-INSeq, a genome-wide screening approach to identify S. Typhi genes required for typhoid toxin expression within infected cells. We find that typhoid toxin expression is controlled by a silencing and counter-silencing mechanism through the opposing actions of the PhoP/PhoQ two-component regulatory system and the histone-like protein H-NS. The screen also identified bacterial mutants that alter the proportion of intracellular S. Typhi that reside within an intravacuolar environment, which was essential for toxin expression. Collectively, these data describe a regulatory mechanism that allows a bacterial pathogen to exclusively express a virulence factor when located within a specific intracellular compartment.
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•Developed a genetic method to probe the regulation of intracellular typhoid toxin expression•The PhoP/Q two-component system and SlyA activate toxin expression during infection•The histone-like protein H-NS silences toxin expression under non-inducing conditions•Toxin expression requires bacterial localization within Salmonella-containing vacuoles
Typhoid toxin, a central Salmonella Typhi virulence factor, is an unusual AB-type toxin in that it is exclusively expressed by intracellular bacteria. Here, Fowler and Galán develop a genetic screen and unveil the regulatory mechanisms that control typhoid toxin intracellular expression. Toxin expression requires bacterial localization within Salmonella-containing vacuoles.
Highlights • Typhoid toxin is a unique A2 B5 exotoxin and a key S . Typhi virulence factor. • Its biological program differs from related toxins in many important ways. • It is highly adapted to the ...intracellular niche S . Typhi adopts during infection. • It binds Neu5Ac-terminated sialoglycans to enable its uptake into human cells. • It appears to be a distinguishing factor in S . Typhi pathogenesis.
UV light induces the expression of a wide variety of genes. At present, it is unclear how cells sense the extent of DNA damage and alter the expression of UV-induced genes appropriately. UV light ...induces DNA damage that blocks transcription, and the probability that a gene sustains transcription-blocking DNA damage is proportional to locus size and dose of UV light. Using colon carcinoma cells that express a temperature-sensitive variant of p53 and undergo p53-dependent apoptosis after UV irradiation, we found that the number of p53-induced genes identified by oligonucleotide microarray analysis decreased in a UV dose-dependent manner. This was associated with a statistically significant shift in the spectrum of p53-induced genes toward compact genes with fewer and smaller introns. Genes encoding proapoptotic proteins involved in the initiation of the mitochondrial apoptotic cascade were prominent among the compact p53 target genes, whereas genes encoding negative regulators of p53 and the mitochondrial apoptotic pathway were significantly larger. We propose that the shift in spectrum of UV-responsive gene expression caused by passive effects of UV lesions on transcription acts as a molecular dosimeter, ensuring the elimination of cells sustaining irreparable transcription-blocking DNA damage.
The diverse landscape of AB5-type toxins Brown, Paris I.; Ojiakor, Adaobi; Chemello, Antonio J. ...
Engineering Microbiology,
12/2023, Volume:
3, Issue:
4
Journal Article
Peer reviewed
Open access
AB5-type toxins are a group of secreted protein toxins that are central virulence factors for bacterial pathogens such as Shigella dysenteriae, Vibrio cholerae, Bordetella pertussis, and certain ...lineages of pathogenic Escherichia coli and Salmonella enterica. AB5 toxins are composed of an active (A) subunit that manipulates host cell biology in complex with a pentameric binding/delivery (B) subunit that mediates the toxin's entry into host cells and its subsequent intracellular trafficking. Broadly speaking, all known AB5-type toxins adopt similar structural architectures and employ similar mechanisms of binding, entering and trafficking within host cells. Despite this, there is a remarkable amount of diversity amongst AB5-type toxins; this includes different toxin families with unrelated activities, as well as variation within families that can have profound functional consequences. In this review, we discuss the diversity that exists amongst characterized AB5-type toxins, with an emphasis on the genetic and functional variability within AB5 toxin families, how this may have evolved, and its impact on human disease.
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