Leucine transport in Escherichia coli. The resolution of multiple transport systems and their coupling to metabolic energy.

Abstract

The multiple active transport systems mediating L-leucine accumulation in Escherichia coli strain 7 (K12) and ML 308-225 have been examined. In addition to the previously characterized osmotic shock-sensitive LS (L-leucine-specific) and LIV-I (L-leucine; L-isoleucine-, and L-valine-specific) activities, a third system (designated LIV-II) has been detected, confirming a report by Rahmanian et al. (RAHMANIAN, M., CLAUS, D.R., and OXENDER, D. L. (1973) J. Bacteriol. 116, 1258-1266). This third system transports L-leucine, L-isoleucine, and L-valine with a relatively low affinity (apparent transport Km equals 4 muM for L-leucine) and it is resistant to repression by cell growth on L-leucine. Exploitation of these properties and of the differential sensitivity of the three transport activities to inhibition by L-leucine analogues permits estimation of the contribution by each system to the total transport activity under varying conditions. Such experiments show that, unlike systems LS and LIV-I, system LIV-II is resistant to osmotic shock. The L-leucine, L-iosleucine, and L-valine transport activity in membrane vesicles from strain ML 308-225 has the properties of system LIV-II. Although the L-leucine transport activities in strains 7 and ML 308-225 are in all other respects similar, membrane vesicles from strain 7 do not transport L-leucine, L-isoleucine, or L-valine. L-leucine transport under various conditions of energy supply has been measured in strain ML 308-225 and the corresponding Mg-2+-ATP-ASE-DEFICIENT STRAIN, DL-54. These measurements support the view that the osmotic shock-sensitive LS and LIV-I activities depend on the synthesis of ATP, while the osmotic shock-resistant LIV-II activity depends on the energized membrane state generated by electron flow but not on ATP synthesis, per se. This conclusion is not supported by the inhibitory effects of the energy poisons arsenate and 2,4-dinitrophenol, but these compounds may have secondary chemical effects on the transport systems.