April 20, 2012
Upon starvation, groups of up to 100,000 cells of the social bacterium Myxococcus xanthus cooperate to build spore-bearing fruiting bodies (green, false color). A fruiting body is shown growing on an agar surface (brown). [Image by Supriya Kadam and Juergen Berger, Max Planck Institute]
WOOSTER, Ohio — A recent discovery by two scientists at The College of Wooster has led to new insight about what happens when genes “jump” from one organism to another. The process, known as Horizontal Gene Transfer (HGT), is an intriguing phenomena in which genes essentially jump from one species to another, sometimes crossing remarkable physiological barriers between the donor and the recipient.
This study, conducted by Dean Fraga and Mark Snider, focused on phosphagen kinases, protein that are involved in maintaining energy stores in animal cells. Phosphagen kinases were previously thought to be exclusive to eukaryotes — organisms made up of large cells with complex intracellular structures. In 2008, Fraga and Snider were the first to identify and characterize phosphagen kinases in bacteria — organisms which are much smaller and simpler than eukaryotes. Their most recent work has added to the story of these ‘jumping’ genes by determining what role one of the transplanted genes plays in the soil bacterium Myxococcus xanthus.
Myxococcus xanthus is a common form of soil bacterium with an uncommon motility, according to Fraga. “It’s been likened to a wolf pack that hunts other bacteria, breaks them down, and then ‘eats’ up the leftovers,” he says. “With that kind of lifestyle, it’s not hard to imagine that the bacteria would pick up some DNA along the way. The incorporated DNA is usually broken down as a food source but on rare occasions some genes can be incorporated into a genome. Once part of the genome, the newly transplanted gene either finds a ‘job’ so to speak or is lost over time through mutations. So, the first question we had was, since the gene had clearly ‘survived’, what advantage does the transplanted gene provide the bacteria?”
Fraga and Snider, along with a collaborator, Mitch Singer (UC-Davis) and a host of student researchers, set out to answer that question by “knocking out” the gene in the bacteria to see how the organism is affected. “What we found was very interesting,” says Fraga. “The first thing we learned was that the absence of the phosphagen kinase disrupts the organism’s ability to respond to stress — not much, but a measurable amount. Not terribly exciting and not unexpected since responding to stress requires energy and phosphagen kinases help cells manage energy. What was more exciting, and previously undocumented for a phosphagen kinase, was that the mutants without the phosphagen kinase were unable to undergo a developmental pathway that Myxococcus undergo when faced with starvation. The goal of this developmental pathway is to produce spores that allow the colony, or wolf pack, to survive difficult times, such as when food is scarce.”
Fraga, who studies molecular evolution, says the findings, which will appear in an upcoming issue of the Journal of Bacteriology, are significant in that they may help scientists to determine how genes evolve and acquire new roles over time. “Essentially this gene has been transplanted into a new ‘environment’ and forced to adapt or be lost,” he says. “By continuing to explore this in detail, we hope to gain more insight into how genes change and acquire new functions, and whether there is some hidden flexibility in protein structure that might allow a protein to adapt to a new environment.”
Ultimately, Fraga and Snider hope to determine how the transplanted gene has adapted to its new environment and what specific role it plays in regulating this complex developmental process in Myxococcus. “That’s the million dollar question,” says Fraga. “A seemingly random event has eventually resulted in the transplanted gene becoming an essential part of a complex and highly regulated process. How did that happen?”
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