A Second Transport System for l-Arabinose in Escherichia coli B/r Controlled by the araC Gene

Abstract

Escherichia coli B/r possesses two active transport systems for l-arabinose, both of which are regulated by araC, the regulatory gene for the l-arabinose operon. The system with the higher affinity for l-arabinose has a K_m for initial uptake of l-arabinose of 8.3 × 10⁻⁶m; the system of lower affinity has a K_m of 1.0 × 10⁻⁴m. The two systems can also be distinguished by differences in their response to analogues that act as competitive inhibitors of initial uptake. d-Galactose strongly inhibits l-arabinose uptake by the high affinity system but only weakly inhibits such uptake by the low affinity system. d-Fucose, d-xylose, and β-methyl-l-arabinoside competitively inhibit the uptake of l-arabinose by both systems to approximately the same extent. d-Glucose and l-lyxose do not inhibit either system. On the basis of kinetic evidence and the properties of mutants lacking one or the other of the two systems, it has been concluded that the high affinity uptake system involves the l-arabinose binding protein. Kinetic studies have shown that the K_m for l-arabinose uptake by the high affinity system resembles the K_m for binding of l-arabinose by the binding protein, and both have similar K_i values for the inhibitory substrates tested. A consideration of l-arabinose transport-deficient mutants has demonstrated that the previously described araE mutants lack the low affinity system but retain the high affinity system and have normal levels of the l-arabinose binding protein. Mutants described in this communication which lack only the high affinity system either contain no detectable l-arabinose binding protein or possess an immunologically cross-reacting material that is reduced in its ability to bind l-arabinose. These observations support a role for the binding protein in l-arabinose uptake.