RT Journal Article SR Electronic T1 A single extracellular loop of FoxA controls ligand specificity, uptake, and signaling in Pseudomonas aeruginosa JF bioRxiv FD Cold Spring Harbor Laboratory SP 2022.11.18.517105 DO 10.1101/2022.11.18.517105 A1 Derek C. K. Chan A1 Inokentijs Josts A1 Kalinka Koteva A1 Gerard D. Wright A1 Henning Tidow A1 Lori L. Burrows YR 2022 UL http://biorxiv.org/content/early/2022/11/18/2022.11.18.517105.abstract AB The outer membrane of Gram-negative bacteria prevents many antibiotics from reaching intracellular targets. However, some antimicrobials can take advantage of iron import transporters to cross this barrier. We showed previously that the thiopeptide antibiotic, thiocillin, exploits the nocardamine (ferrioxamine E) xenosiderophore transporter, FoxA, of the opportunistic pathogen Pseudomonas aeruginosa for uptake. Here we show that FoxA also transports the xenosiderophore bisucaberin and describe at 2.5 Å resolution the first crystal structure of bisucaberin bound to FoxA. Bisucaberin is distinct from other siderophores because it forms a 3:2 rather than 1:1 siderophore-iron complex. Mutations in a single extracellular loop of FoxA differentially affected nocardamine, thiocillin, and bisucaberin binding, uptake, and signal transduction. These results show that in addition to modulating ligand binding, the extracellular loops of siderophore transporters are of fundamental importance for controlling ligand uptake and its regulatory consequences, which has implications for the development of siderophore-antibiotic conjugates to treat difficult infections.Competing Interest StatementThe authors have declared no competing interest.