Mechanosensitive channels allow bacteria to survive rapid increases in turgor pressure. Substantial questions remain as to how these channels sense and respond to mechanical stress. Here we describe a set of mutants with alterations in their MscK channel protein. The mutants were detected fortuitously by their enhanced ability to modify the accumulation of quinolinic acid. Some amino acid changes lie in the putative pore region of MscK, but others affect sequences that lie amino-terminal to the domain aligning with MscS. We demonstrate that the alterations in MscK cause the channel to open more frequently in the absence of excessive mechanical stress. This is manifested in changes in sensitivity to external K(+) by cells expressing the mutant proteins. Single-channel analysis highlighted a range of gating behaviours: activation at lower pressures than the wild type, inability to achieve the fully open state or a modified requirement for K(+). Thus, the dominant uptake phenotype of these mutants may result from a defect in their ability to regulate the gating of MscK. The locations of the substituted residues suggest that the overall gating mechanism of MscK is comparable to that of MscS, but with subtleties introduced by the additional protein sequences in MscK.