Phosphate exchange in the pit transport system in Escherichia coli

The Pit system of phosphate transport in Escherichia coli catalyzes a rapid exchange between the external inorganic phosphate and internal phosphate pools, including some ester phosphates which are in rapid equilibrium with the internal Pi pool. Unlike net energized uptake, the Pi exchange proceeds in energy-depleted cells in the presence of uncouplers and is not accompanied by the movement of potassium ions. In the absence of externally added phosphate, the exit of Pi from the cells is insignificant. The apparent Km for external Pi in the exchange reaction is about 7 mM (2 orders of magnitude higher than that of energized uptake), but the maximal velocity is about the same. The exchange is temperature sensitive and is affected by thiol reagents. The combined observations suggest the operation of a facilitator which is part of the Pit system. The exchange is repressed in cells grown on glucose and other phosphotransferase system substrates, but not in cells grown on other carbohydrate sources. The repression can be reversed by the addition of cyclic AMP to the medium.