Duke researchers create molecular map of F. tularensis to learn how to disable a potential bioweapon

Researchers at Duke University recently developed a three-dimensional map of the complex molecular circuitry of Francisella tularensis, the bacteria that causes tularemia, in order to better understand how the pathogen becomes virulent.

“Now we have the coordinates for stopping one of the most infectious agents known to man,” Maria A. Schumacher, senior author of the study and Duke University professor, said. “By having all of these pieces, and understanding how they fit together, we can design new drugs that can shut down virulence.”

Currently named as one of the six most concerning bioterrorism agents along with anthrax, botulism, viral hemorrhagic fever, smallpox and plague by the U.S. Centers for Disease Control and Prevention (CDC), F. tularensis is an extremely durable organism that has the ability to infect a variety of hosts including humans, rabbits and mosquitoes and can survive for weeks at a time in dead or decaying organisms.

According to the World Health Organization, 110 pounds of F. tularensis dispersed over a city of five million people could cause approximately 250,000 cases of severe illness and 19,000 deaths.

Despite decades of research, little is still known about the pathogen and even less is known about a cluster of genes known as “Francisella pathogenicity island” which is essential for the bacteria’s virulence.

To gain a better understanding of the pathogen, the researchers first conducted a number of structural, biochemical and cellular studies in order to define the molecular characteristics that turn pathogenicity genes on and off.

Specifically, the researchers suspected the alarmone known as ppGpp, a stress-sensing molecule that responds to stressful situations by promoting survival in bacteria, might be a cause. From that starting point, they began looking at factors that might interact with that alarmone, such as PigR, a protein pathogenicity island gene regulator, the macrophage growth locus proteins A and MgIA, and the starvation proteins A and SspA.

Utilizing a technique called x-ray crystallography, the researchers produced atomic-level three-dimensional structures of each protein and assembled them one by one, similar to a circuit board.

They found that MgIA and SspA combine to produce a two-part protein that contains a binding pocket on its underside for ppGpp. From there, it “recruits” PigR which stabilizes RNA polymerase to that area of the F. tularensis genome and creates a large complex that binds to the DNA to turn on the pathogenicity genes.

The researchers then developed mutations they destroyed the binding pocket for ppGpp, which disallowed the pathogenicity to be activated.

“We have uncovered a totally novel way for controlling virulence,” Rochard Brennan, professor of biochemistry at Duke, said. “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.”

Research for the study was published in detail in a recent issue of the journal Genes and Development.