Yersinia ruckeri YRB periplasmic binding protein YiuA selectively recognizes a Fe(III)-mono-catecholate siderophore

The marine pathogen Yersinia ruckeri synthesizes the tricatecholate siderophore ruckerbactin, Rb, (DHB-LArg-LSer)3, to acquire iron during infection. Its biosynthetic gene cluster encodes a single periplasmic binding protein, RupB, which surprisingly does not bind Fe(III)-Rb nor the Fe(III) complexes of its hydrolysis products, the di- and mono-catecholate siderophores RbDC and RbMC, with biologically relevant affinities. Instead, the periplasmic binding protein YiuA, encoded in a different region of the chromosome, binds the 1:2 Fe(III) complex of the mono-catecholate RbMC, Fe(III)-(RbMC)2. YiuA is the first periplasmic binding protein (PBP) to selectively recognize a mono-catecholate siderophore, the structural basis of which was illuminated through X-ray crystallography of YiuA bound to Fe(III)-(RbMC)2.