Enterotoxigenic Escherichia coli causes diarrhea by producing several virulence factors including heat-labile enterotoxin (LT). LT is maximally expressed at 37degreesC. The histone-like nucleoid structuring protein (H-NS) appears to inhibit LT expression by binding to a downstream regulatory element (DRE) at low temperatures. An hns(+) stop E. coli strain, X7026, carrying an LT-beta-galactosidase translational fusion plasmid (pLT-lac) was shown to be responsive to varying amounts of sodium chloride (NaCl) as well as sucrose or lithium chloride. Maximal responsiveness to the various osmolytes was obtained with cells grown at 37degreesC under microaerophilic conditions. Temperature-osmotic upshift experiments demonstrate LT expression is thermo-osmoregulated. pLT-lac was tested in an hns strain or its congenic hns(+) strain for its response to NaCl. LT expression is elevated in the hns strain regardless of NaCl concentration and retains its osmoresponsiveness. The response of the DRE deletion plasmid (pLT-lacDeltaNC) to NaCl is similar to that of the undeleted plasmid.