Researchers from the Institute for Research in Biomedicine (IRB Barcelona) and the Institute of Molecular Biology of Barcelona (IBMB-CSIC), located in the Barcelona Science Park, have participated in a study that reveals how the bacterium that causes cholera activates its virulence program and identifies a single amino acid as a critical point of contact. The study, published in the journal Science Advances and carried out in collaboration with EMBL Heidelberg and the University of Detroit Mercy (USA), has mapped the infection process in unprecedented detail, opening up new avenues for future therapies.

Using cryo-electron microscopy, researchers have solved five structures of the V. cholerae activation complex, revealing how virulence gene expression is triggered once the bacterium reaches the human intestine.

Cholera remains a major global public health challenge, with an estimated 1.3 to 4 million cases and tens of thousands of deaths reported worldwide each year. Caused by the bacterium Vibrio cholerae, the disease spreads primarily through contaminated water and food and continues to disproportionately affect regions with limited access to safe sanitation. Conflict, climate impacts and population displacement are driving increasing outbreaks of cholera. In response to the recent global resurgence of cases affecting 43 countries, and associated mortality (in particular, high infant mortality) the WHO (World Health Organization) classified cholera as a Grade 3 emergency, its highest level of alert, in 2023.

ToxR and TcpP are key transcription factors of Vibrio that sense external cues, such as the presence of bile salts and low oxygen levels, in the human small intestine. Once activated, they bind to bacterial DNA to trigger a regulatory cascade, leading to the production of the cholera toxin and the Toxin Co-regulated Pilus, the microscopic anchors the bacteria used to latch onto intestinal walls.

Although these proteins were long recognized as the key regulators of infection, the 3D map of how they physically recruit the cell’s transcription engine, the RNA polymerase (RNAP), remained missing. Now, a study led by IRB Barcelona and the IBMB-CSIC reveals the molecular architecture of this interaction. Utilizing single-particle cryo-electron microscopy (cryo-EM), the researchers show that the recruitment mechanism is not what scientists expected.

“Understanding this interaction at the molecular level gives us a new way to think about how bacterial virulence is controlled,” says Dr. Miquel Coll, former head of the Structural Biology of Protein & Nucleic Acid Complexes and Molecular Machines lab at IRB Barcelona, and professor at the CSIC.

Stabilizing, rather than reshaping, the transcription machinery

While many bacterial regulators are designed to force a shape-change in the polymerase to initiate transcription, this study reveals that ToxR and TcpP do not induce any conformational rearrangement. Instead, they act as molecular anchors, stabilizing a specific part of the enzyme (the alpha-CTD domain) directly onto the DNA. These findings show that virulence gene activation is achieved not by reshaping the transcription machinery, but by stabilizing it in a productive configuration.

The team identified a single amino acid, a phenylalanine, as the critical molecular bridge between the sensor and the polymerase. “If only this amino acid is mutated, the entire activation process fails, making the bacteria harmless,” says Dr. Adrià Alcaide, first author of the study.

Implications for future therapies

Severe cholera can cause life-threatening dehydration in a few hours, especially in children and older adults. Prompt treatment with rehydration therapy and antibiotics can significantly reduce the death rate. The molecular similarity observed in this study between the active sites (where transcription from DNA to RNA occurs) of the RNA polymerase of V. cholerae and E. coli suggests that existing antibiotics that target the bacterial polymerase could be repurposed or optimized to treat cholera.

» Article of reference: Structures of Vibrio cholerae transcription complexes reveal how ToxR and TcpP recruit the RNA polymerase and activate virulence genes. Adrià Alcaide-Jiménez, Albert Canals, Florence Baudin, Cristina Machón, Montserrat Fàbrega-Ferrer, Olga Bantysh, Rosa Pérez-Luque, Brice Murciano, Ali A. Mohammad, Michael J. Rowse, Joseph M. Ferracciolo, Eric S. Krukonis, Christoph W. Müller & Miquel Coll. Science Advances (2026) Doi: 10.1126/sciadv.adx9680

» Link to the news: IRB Barcelona website [+]