Small molecule produced by <i>Photorhabdus</i> interferes with ubiquinone biosynthesis in Gram-negative bacteria

The spread of resistant pathogens has led to the antimicrobial resistance crisis, and the need for new compounds acting against Gram-negative pathogens is especially acute. From a screen of <i>Photorhabdus</i> symbionts of nematodes, we identified 3,6-dihydroxy-1,2-benzisoxazole (DHB) that acts against a range of Gram-negative bacteria, including <i>Escherichia coli</i>, <i>Enterobacter cloacae</i>, <i>Klebsiella pneumoniae</i>, and <i>Acinetobacter baumannii</i>. DHB had previously been isolated from other bacterial species, but its mechanism of action remained unknown. We show that DHB is unique among antimicrobials, with dual action as an inhibitor of an important enzyme, UbiA, in the biosynthesis pathway of ubiquinone and as a prodrug. DHB is a mimic of the natural substrate, and UbiA modifies it into a toxic product, contributing to the antimicrobial action of this unusual antibiotic. We also uncover the mechanism of DHB selectivity, which depends on a particular fold of the UbiA enzyme.