Scientists Discover Natural “Off Switch” for Nitrogen-Fixing Enzyme
16 October 2026, St. Louis, Missouri: Researchers at Saint Louis University have resolved the three dimensional structure of a long misunderstood piece of molecular machinery behind biological nitrogen fixation, a finding that overturns decades of textbook assumptions about how the process is regulated and that could eventually inform efforts to reduce agriculture’s dependence on synthetic fertiliser.
The study, accepted for publication in the journal Nature, was led by Rajnandani Kashyap, a postdoctoral fellow at SLU’s School of Medicine, working with principal investigator Edwin Antony, a professor of biochemistry and molecular biology at the university. Using cryo electron microscopy, the team produced a detailed 3D model of nitrogenase, the enzyme that converts atmospheric nitrogen into ammonia, the chemical building block that underpins both natural soil fertility and industrial fertiliser production. The model was built from an ancient methane producing microbe rather than the common bacteria that have long served as the standard reference organism for studying the enzyme.
What the team found challenges the accepted model directly. Rather than existing as a simple, always available catalyst, the enzyme in this organism forms a larger supercomplex in which three catalytic units are physically locked in an inactive state by six copies of a regulatory protein pair. That regulatory pair blocks a docking site the enzyme needs to function. The researchers found that specific cellular energy and nutrient signals can cause the regulatory proteins to release their grip, switching nitrogen fixation back on. In effect, the organism carries a built in, reversible off switch for one of biology’s most energy intensive chemical reactions, a feature not captured by the bacterial models scientists have relied on for decades.
The structural work relied on a cryo electron microscope at the Washington University Center for Cellular Imaging, built under a 15 year partnership between SLU and Washington University in St Louis that was established in 2019. SLU contributed $2.5 million of the instrument’s $5 million cost from its Doisy Fund. According to Enrico Di Cera, chair of SLU’s Department of Biochemistry and Molecular Biology, that investment has since helped generate more than $22 million in federal grants along with a steady stream of publications and new faculty hires, a point he described as an example of how shared scientific infrastructure can pay for itself many times over. The nitrogenase project itself was funded by the US Department of Energy, and the University of Arkansas contributed by engineering the microbe and producing the protein samples used in the imaging work.
A discovery with a long road to the farm
Nitrogen fixation sits at the centre of global food production because most of the world’s fertiliser nitrogen is still made through the energy intensive Haber Bosch industrial process, which converts natural gas and atmospheric nitrogen into ammonia at high temperature and pressure. That process alone is estimated to consume a significant share of global natural gas supply and contributes meaningfully to agriculture’s carbon footprint. Biological nitrogen fixation, the natural process this study examines, achieves the same chemical conversion at ordinary temperatures and pressures inside microorganisms, which is why understanding its regulation in finer detail has long interested scientists working on more sustainable approaches to crop nutrition.
The SLU led team is careful to frame its work as basic science rather than an imminent agricultural product. The paper describes a regulatory mechanism in a non plant associated microbe, and translating that insight into engineered, nitrogen fixing cereal crops or more efficient biological fertilisers would require years of further research into whether, and how, similar regulatory switches operate in the bacteria that already associate with crop roots. Researchers involved in the work have nonetheless pointed to biotechnology applications in more sustainable agriculture as one of the eventual payoffs of mapping nitrogenase regulation this precisely, alongside potential uses in green energy production.
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