Dreyfuss, Jacques (Princeton University, Princeton, N.J.), and Arthur B. Pardee. Regulation of sulfate transport in Salmonella typhimurium. J. Bacteriol. 91:2275-2280. 1966.-The transport of sulfate into Salmonella typhimurium is an active process. Intracellular and extracellular sulfate are in rapid equilibrium. Although the entry process requires energy, the exit of sulfate and the exchange of intracellular sulfate are relatively independent of an exogenous energy supply. When a concentrated suspension of cells is first exposed to sulfate, transport is rapid, but approximately 1 min thereafter the properties of the cells change so that net sulfate flow is outwards. This overshoot seems to depend on a high intracellular sulfate concentration and on the energy supply. Transport ability is inhibited in mutants which accumulate the high energy intermediate 3'-phosphoadenosine 5'-phosphosulfate. On the basis of these results, it is suggested that the entry of sulfate is feedback-inhibited by a high-energy compound which is formed in the course of transport. Feedback inhibition, in conjunction with the repression of sulfate transport, provides an effective means of regulating the intracellular concentration. The regulatory mechanism is highly efficient in rapidly growing cells, but can be perceived as an overshoot in dense cell suspensions.