Periplasmic Binding Protein YiuA Enables <i>Yersinia pestis</i> to Scavenge Fe(III)-Catechol Siderophores

<i>Yersinia pestis</i>, the pathogen causing plague, requires iron to grow. <i>Y. pestis</i> employs several uptake pathways for iron, including the siderophore yersiniabactin, as well as hemin and inorganic iron. The <i>Y. pestis</i> iron assimilation repertoire further harbors the uncharacterized YiuRABC pathway, presumed to transport an unidentified Fe(III)-siderophore(s). Through intrinsic fluorescence quenching of the periplasmic binding protein YiuA, we discovered that YiuA displays high affinity toward Fe(III) complexes of the hydrolysis products of enterobactin, Fe(III)-[di(DHB-^LSer)] and Fe(III)-[DHB-^LSer]₂, with K_d values of 27.6 ± 4.2 nM and 28.2 ± 6.9 nM, respectively, as well as the bis-catechol siderophore butanochelin, with K_d 0.76 ± 0.17 nM. By comparison, YiuA has a much weaker affinity for intact Fe(III)-enterobactin, K_d 444.7 ± 20.6 nM. Electronic circular dichroism spectroscopy reveals YiuA has a strong preference for binding Λ configured Fe(III)-siderophores, which can be achieved with the Fe(III) bis-catechol complexes but not Fe(III)-enterobactin.