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Abstract Flavin-dependent halogenases (FDHs) are a class of enzymes renowned for their regioselective ability to precisely insert halogen atoms into small aromatic compounds. Halogen incorporation can enhance the physicochemical and biological properties of molecules, making them valuable for agrochemical and pharmaceutical applications. Through bioinformatic mining of bacterial genomes, we discovered and functionally characterized SnFDHal, an efficient tryptophan 5-halogenase fromStreptomyces nourseiNRRL B-1714. This halogenase operates across a broad pH range and exhibits a melting temperature of 46.7 °C at both pH 6 and pH 8, which is comparable to thermophilic halogenases such as Th-Hal and BorH, and notably higher than those of mesophilic counterparts. Steady-state kinetic analysis revealed that SnFDHal displays superior catalytic efficiency for the chlorination of L-tryptophan compared to other FDHs reported to date. Structural modeling of its active site suggests that a conserved bulky phenylalanine residue (F49) promotes halogenation at the C-5 position of L-tryptophan, consistent with experimental findings. The combination of high catalytic efficiency and thermostability positions SnFDHal as a promising biocatalyst for applications in agrochemical and pharmaceutical industries. 

Abstract The duality of nitrate is nowhere better exemplified than in human physiology—a detrimental pollutant but also a protective nutrient—particularly as connected to nitric oxide. Aside from limited insights into nitrate uptake and storage, foundational nitrate biology has lagged. Genetically encoded fluorescent biosensors can address this gap with real‐time imaging, but such technologies for mammalian cell applications remain rare. Here, we designed and engineered a biosensor fusing the green fluorescent protein EGFP and the nitrate recognition domain NreA fromStaphylococcus carnosus. Seven rounds of directed evolution and 15 mutations resulted in NitrOFF. NitrOFF has a high degree of allosteric communication between the domains reflected in a turn‐off intensiometric response (K_d≈ 9 µM). This was further reinforced by X‐ray crystal structures of apo and nitrate‐bound NitrOFF, which revealed a large‐scale conformational rearrangement changing the relative positioning of the domains by 68.4°. This dramatic difference was triggered by the formation of a long helix at the engineered linker connecting the two domains, peeling the β7 strand off the EGFP and thus extinguishing the fluorescence upon nitrate binding. Finally, we highlighted the utility of NitrOFF to monitor exogenous nitrate uptake and modulation in the human embryonic kidney (HEK) 293 cell line.