Escherichia coli accumulates K by means of multiple uptake systems of which Kup is the major transport system at acidic pH. In cells grown under fermentative conditions at pH 5.5, K influx by a wild-type strain upon hyper-osmotic stress at pH 5.5 was accompanied by a marked decrease in H efflux, with a 1:1 ratio of K to H fluxes. This was observed with cells treated with N,NP-dicyclohexylcarbodiimide. Similar results with a mutant defective in Kdp and TrkA but with a functional Kup system but not in a mutant defective in Kdp and Kup but having an active TrkA system suggest that Kup operates as a H ^K -symporter. ß