Discovery of an iron-regulated citrate synthase in Staphylococcus aureus

Bacteria need to scavenge iron from their environment, and this is no less important for bacterial pathogens while attempting to survive in the mammalian host. One key strategy is the synthesis of small iron chelators known as siderophores. The study of siderophore biosynthesis systems over the past several years has shed light on novel enzymology and, as such, has identified new therapeutic targets. Staphylococcus aureus, a noted human and animal pathogen, produces two citrate-based siderophores, termed staphyloferrin A and staphyloferrin B. The iron-regulated gene cluster for the biosynthesis of staphyloferrin B, sbnA-I, contains several yet uncharacterized genes. Here, we report on the identification of an enzyme, SbnG, which is annotated in the genome sequence as a metal-dependent class II aldolase. In contrast to this prediction, we report that, instead, SbnG has evolved to catalyze metal-independent citrate synthase activity using oxaloacetate and acetyl-CoA as substrates. We describe an in vitro assay to synthesize biologically active staphyloferrin B from purified enzymes and substrates, and identify several SbnG inhibitors, including metals such as calcium and magnesium.