Identification and Characterization of SirA, an Iron-Regulated Protein from Staphylococcus aureus

Iron is an essential nutrient required by virtually all bacteria (30). Because the concentration of free iron in tissue has been estimated to be as low as 10−12 μM (3), most pathogenic bacteria have developed elaborate mechanisms to obtain iron at concentrations sufficient for growth. One of these mechanisms involves the production of low-molecular-weight chelators, collectively called siderophores. Once complexed with iron, siderophores are transported into cells by specific receptors. Both siderophores and their receptors have been well characterized for gram-negative organisms, but their production and utilization in gram-positive bacteria is less well understood. The production of siderophores by staphylococcal strains has recently been demonstrated. Staphylococcus hyicus has been shown to produce at least two siderophores, staphyloferrins A and B (8, 17). Both staphyloferrins A and B were also found to be produced by certain strains of Staphylococcus aureus. Recently, Courcol et al. (5) identified a third siderophore produced by S. aureus called aureochelin. Modun et al. (18) described the binding of human transferrin by intact S. aureus cells and identified a 42-kDa protein that is presumably responsible for this binding. In addition, several proteins of unknown function have also been shown to be regulated by the available iron concentration. These include proteins with apparent molecular masses of 120, 88, 57, 35, and 33 kDa (5) and of 36 and 39 kDa (15), all of which are repressed by high iron concentrations. We report here the molecular cloning and characterization of a locus (called sir, for staphylococcal iron regulated) from S. aureus with homology to the siderophore acquisition locus cbr of Erwinia chrysanthemi. We examined the expression of the product of the first gene in this locus, sirA, under iron-limiting conditions and describe a regulatory region upstream of the gene which is similar to ferric uptake regulator (Fur) boxes of gram-negative bacteria. We also provide evidence that production of this protein correlates with accumulation of siderophore activity in supernatants of S. aureus cultures grown under iron-limited conditions. These results suggest that SirA may be a membrane-associated siderophore-binding protein and may serve as an effective target for antimicrobial agents or vaccines. (A preliminary account of this work was presented at the 98th General Meeting of the American Society for Microbiology [12].)