Like other siderophores, petrobactin is secreted by an animal pathogenic bacterium. B. anthracis uses petrobactin to acquire iron from its host. Interestingly, while the 3,4-catecholate ends of petrobactin do not improve iron(III) affinity relative to hydroxamate ends, they speed up iron removal from human diferric transferrin. Petrobactin in its ferric and iron-free forms is bound selectively by YclQ (an isogenic disruption mutant in the transporter encoded by the yclNOPQ operon in Bacillus subtilis), as is petrobactin's precursor protocatechuic acid and the ferric petrobactin photoproduct. The yclNOPQ operon is required for the utlization of petrobactin and yclNOPQ orthologs likely contribute to the pathogenicity of Bacilli.