Deletion of gene encoding the nucleoid-associated protein H-NS unmasks hidden regulatory connections in El Tor biotype Vibrio cholerae

H-NS and ToxT Inversely Control Cholera Toxin Production by Binding to Overlapping DNA Sequences

Vibrio cholerae infects human hosts following ingestion of contaminated food or water, resulting in the severe diarrheal disease cholera. The watery diarrhea that is characteristic of the disease is directly caused by the production of cholera toxin (CT). A complex regulatory cascade controls the production of CT and other virulence factors. However, ultimately, a single protein, ToxT, directly binds to virulence gene promoters and activates their transcription. Previously, we identified two ToxT binding sites, or toxboxes, within the cholera toxin promoter (PctxAB). The toxboxes overlap the two promoter-proximal GATTTTT heptad repeats found within PctxAB in classical biotype V. cholerae strain O395. These heptad repeats were previously found to be located within a large DNA region bound by H-NS, a global transcriptional repressor present in Gram-negative bacteria. The current model for the control of PctxAB transcription proposes complete H-NS displacement from the DNA by ToxT, followed by direct activation by ToxT-RNA polymerase (RNAP) contacts. The goal of this study was to determine more precisely where H-NS binds to PctxAB and test the hypothesis that ToxT completely displaces H-NS from the PctxAB promoter before activating transcription. The results suggest that H-NS binds only to the region of PctxAB encompassing the heptad repeats and that ToxT displaces H-NS only from its most promoter-proximal binding sites, calling for a revision of the current model involving H-NS and ToxT at PctxAB. IMPORTANCE H-NS is a global negative regulator of transcription in Gram-negative bacteria, particularly in horizontally acquired genetic islands. Previous work in Vibrio cholerae suggested that H-NS represses the transcription of cholera toxin genes by binding to a large region upstream of its promoter and that the virulence activator ToxT derepresses transcription by removing H-NS from the promoter. Here, new data support a revised model in which ToxT displaces only H-NS bound to the most promoter-proximal DNA sites that overlap the ToxT binding sites, leaving the upstream sites occupied by H-NS. This introduces a higher-resolution mechanism for the antirepression of H-NS in the control of cholera toxin production.

The Vibrio cholerae MARTX toxin silences the inflammatory response to cytoskeletal damage before inducing actin cytoskeleton collapse

Multifunctional autoprocessing repeats-in-toxin (MARTX) toxins are pore-forming bacterial toxins that translocate multiple functionally independent effector domains into a target eukaryotic cell. Vibrio cholerae colonizes intestinal epithelial cells (IECs) and uses a MARTX toxin with three effector domains-an actin cross-linking domain (ACD), a Rho inactivation domain (RID), and an α/β hydrolase domain (ABH)-to suppress innate immunity and enhance colonization. We investigated whether these multiple catalytic enzymes delivered from a single toxin functioned in a coordinated manner to suppress intestinal innate immunity. Using cultured human IECs, we demonstrated that ACD-induced cytoskeletal collapse activated extracellular signal-regulated kinase, p38, and c-Jun amino-terminal kinase mitogen-activated protein kinase (MAPK) signaling to elicit a robust proinflammatory response characterized by the secretion of interleukin-8 (IL-8; also called CXCL8) and the expression of CXCL8, tumor necrosis factor (TNF), and other proinflammatory genes. However, RID and ABH, which are naturally delivered together with ACD, blocked MAPK activation through Rac1 and thus prevented ACD-induced inflammation. RID also abolished IL-8 secretion induced by heat-killed bacteria, TNF, or latrunculin A. Thus, MARTX toxins use enzymatic multifunctionality to silence the host response to bacterial factors and to the damage caused by the toxins. Furthermore, these data show how V. cholerae MARTX toxin suppresses intestinal inflammation and contributes to cholera being classically defined as a noninflammatory diarrheal disease.

A proteome-wide screen to identify transcription factors interacting with the Vibrio cholerae rpoS promoter

We describe a proteomic approach to identify transcription factors binding to a target promoter. The method's usefulness was tested by identifying proteins binding to the Vibrio cholerae rpoS promoter in response to cell density. Proteins identified in this screen included the nucleoid-associated protein Fis and the quorum sensing regulator HapR.

Engineered Bacteria for Cholera Prophylaxis

Over 1.3 billion persons are at risk for cholera globally. Vaccination campaigns are growing, but intervention options providing nearly immediate protection are also needed. Two recent papers in Science Translational Medicine describe probiotic bacteria that reduce intestinal colonization in animal models and show promise for development as prophylaxis agents.