The Gram-negative intracellular bacterium Francisella tularensis is the causative agent of tularemia and is one of the most virulent pathogens known. Due to its potential to be deployed as a bioterroist agent, F. tularensis has been classified as a category A bioweapon by the United States government.
Tularemia often occurs as geographically confined outbreaks in humans and animals. Transmission to humans frequently occurs through the bite of blood-feeding arthropods, such as ticks, biting flies, or mosquitoes. Inhalation and the ingestion of F. tularensis can also cause disease. Historically, F. tularensis attracted attention as a biological weapon and was a subject of military research in the United States, the former Soviet Union, and Japan.
In the post-Cold War era, however, F. tularensis is included among the top six agents showing potential for great adverse public health impact if used as a bioterrorism agent.
F. tularensis exhibits highly conserved genomic sequence among strains of diverse origin. And despite decades of fervent study, the factors that make this bacterium so pathogenic are still not fully understood. But now, scientists at the Duke University School of Medicine have uncovered a cluster of genes called the “Francisella pathogenicity island” emerged that is essential for its virulence. In this study, researchers carried out a battery of structural
The study was carried out by firstly, by using a stress-sensing molecule or “alarmone” called ppGpp, efforts were made to find factors that might interact with ppGpp, such as the protein pathogenicity island gene regulator or PigR, the macrophage growth locus protein A or MglA, and the stringent starvation protein A or SspA.
Lead study author and Duke graduate student Bonnie J. Cuthbert used the x-ray crystallography technique to produce atomic-level three-dimensional structures of each of these proteins, and then assembled them one by one, like the components of a circuit board. Through the technique, MglA and SspA were found to partner up to form a two-part protein that contains a unique binding pocket on its underside for ppGpp. Once this molecule is bound, it recruits PigR and subsequently stabilizes RNA polymerase to this area of the F. tularensis genome, creating a large complex that latches onto the DNA to flip on the pathogenicity genes.
“We have uncovered a totally novel way for controlling virulence,” said senior study author Richard G. Brennan, Ph.D., James B. Duke Professor of Biochemistry and Chair of Biochemistry at Duke University School of Medicine and also an advisor to Cuthbert. “If we could block this binding pocket, then we could stop virulence in F. tularensis. It would be a new way of fighting this bacteria, by disabling it with antivirulence drugs rather than by killing it outright with antibiotics.”
The researchers then created mutations that destroyed the binding pocket for ppGpp. They found that when the alarmone couldn’t bind, pathogenicity couldn’t be activated.
