Molecular mechanism of plasmid-borne resistance to sulfonamides

Abstract

The sulfonamides (sulfas) are the oldest class of synthetic antibacterial that target the essential, conserved dihydropteroate synthase (DHPS) enzyme, encoded by folP, through chemical mimicry of its substrate p-aminobenzoic acid (pABA). Resistance has complicated their clinical utility and is widespread in pathogenic species. Resistance is mediated by acquisition of sul genes on mobile genetic elements, which code for the so-called Sul enzymes that are divergent DHPS enzymes with intrinsic sulfa-insensitivity. Even decades after the discovery of this resistance mechanism, its molecular details have not been understood. In this study, we elucidate the molecular basis for intrinsic resistance of Sul enzymes using x-ray crystallography, enzymology, mutagenesis, intrinsic tryptophan fluorescence, antibiotic susceptibility of a contemporary {Delta}folP strain, and adaptive laboratory evolution of folP. We show that the active sites of Sul enzymes possess a modified pABA-interaction region based on insertion of a Phe-Gly sequence. This insertion is necessary for discrimination between pABA and sulfonamides, more than 1000-fold loss in binding affinity of sulfas to Sul enzymes, and robust pan-sulfonamide resistance. We detect no fitness cost due to this active site modification, as it does not compromise the rate of dihydropteroate biosynthesis and complements the thymidine-auxotrophy of an E. coli folP deletion strain. Lab-evolved sulfa-resistance folP recapitulated this mechanism through the same active site insertion. Finally, we show that this insertion and a nearby loop confer increased active site flexibility of Sul enzymes relative to DHPS. These results provide a molecular foundation for revisiting DHPS-targeted antibacterials to evade resistance.