To test the hypothesis that the Kch gene of Escherichia coli encodes a potassium channel, we have transformed E coli with an expression vector containing the Kch sequence and observed the effect of over-expression of Kch on E. coli. We found that: (i) overexpression of Kch is toxic to E. coli, but the toxicity could be prevented by supplementing the growth medium with K+, Rb+, and NH4+, but not Na+, consistent with the properties of a potassium selective pore; (ii) Cs+, a blocker of potassium channels, rescues the growth of Kch over-expressing cells; and (iii) when the putative pore-forming region of Kch, containing the signature sequence, was replaced with the corresponding region of the eukaryotic Shaker potassium channel, and the resultant construct expressed in E coli, the cells became critically dependent on K+ supply for survival. These data are consistent with the proposed function of Kch, i.e., K+ conduction. (C) 2002 Elsevier, Science (USA). All rights reserved.