A Binding Site for Sulfate and Its Relation to Sulfate Transport into Salmonella typhimurium

Abstract

Mutants of Salmonella typhimurium classified by nutritional and enzymatic tests as transport-negative for sulfate are capable of combining with small amounts of this anion (up to 104 molecules per bacterium). The following results support the suggestion that the bacteria possess highly specific sulfate binding sites near the cell surface. 1. Under growth conditions sulfate combines with the bacteria according to a typical adsorption isotherm, which reaches half-saturation at 0.004 mm. 2. Various anions structurally similar to sulfate are specific inhibitors of binding and transport. 3. Inability of the mutants to grow on sulfate or thiosulfate suggests that the anions cannot enter the cells, but are bound near the cell surface. 4. The cells lose their ability to bind sulfate after they are converted to spheroplasts or are osmotically shocked. With the use of a new assay for the measurement of binding to soluble entities, the binding ability lost from the cells is found in solution. 5. The properties of the binding system suggest that it might be a part of the active transport system for sulfate. The surface location of the binding material as well as the following results are consistent with this hypothesis. Both binding activity and active transport are repressed by the growth of bacteria on cysteine as a sulfur source. Both functions are similarly (but not identically) inhibited by various anions. Both functions are lost, in proportion, upon osmotic shock of transport-positive cells. Both functions are simultaneously lost in a class of chromate-resistant mutants, and are regained upon transduction with PLT-22 phage or reversion.