Evidence for a protonmotive force related regulatory system in Escherichia coli and its effects on lactose transport.

Strains of Escherichia coli with mutations in the eup (energy-uncoupled phenotype) locus do not grow on nonfermentable carbon sources, have reduced growth yields on limiting glucose, are insensitive to colicins A and K, exhibit resistance to aminoglycoside antibiotics, and are defective in protonmotive force coupled active transport. eup mutations do not result in lowered protonmotive force. Here we show that deenergization of a eup+ strain results in the appearance of a new low KT, low Vmax form of the lactose carrier; in a strain deleted of the eup locus, deenergization does not evoke the low KT, low Vmax form of the lactose carrier. Cells bearing a eup point mutation and exhibiting the Eup- phenotype possess the low KT, low Vmax form of the lactose carrier even when energized. In addition to affecting the kinetic parameters of the lactose carrier, the eup point mutation also reduces the KT and Vmax of the proline carrier. On the basis of these findings, we suggest that the normal eup gene product mediates a novel regulation of lactose carrier function following deenergization. The defect in proline and lactose transport caused by eup point mutations may stem from an altered eup product aberrantly mediating the regulation under energized conditions. Finally, the pleiotropy associated with eup point mutations may be indicative of those protonmotive force driven functions that are subject to eup regulation.