NONPHYSIOLOGICAL EXPRESSION OF UHPT DOES NOT LEAD TO UNCONTROLLED LEAKAGE OF SUGAR PHOSPHATES OUT OF ESCHERICHIA-COLI-CELLS

De-regulated expression of uhpT under control of the tac promoter, by increasing concentrations of isopropyl-thio-beta-D-galactoside, progre ssively inhibited the growth rate of Escherichia call cells, to such a n extent that growth was fully inhibited at 1 mM of the inducer. Signi ficantly, addition of glucose 6-phosphate to the growth medium of the cells did not protect against this inhibition. Furthermore, efflux of sugar phosphates from the cells did not take place under these conditi ons, unless protonophoric uncouplers were added. We therefore conclude that the regulation of uhpT expression, i.e. via extracellular induct ion through a two-component system, did not evolve in order to prevent loss of essential metabolites from the cytoplasm under conditions whe n extracellular sugar phosphates are not available.