Quantitative aspects of active transport by the lactose transport system of Escherichia coli

Accumulation by the lactose transport system of Escherichia coli has been measured in cells induced so as to contain increasing numbers of membrane carriers. Carrier activity was assayed both by the rate of o-nitrophenyl-β-d-galactopyranoside entry and the initial rate of accumulation of methyl-1-thio-gb-d-galactopyranoside. At the steady-state cells with a low number of carriers accumulated considerable amounts of substrate when compared to the fully induced control. This is consistent with the hypothesis that there are two distinct routes of both entry and of exit: a carrier mediated pathway and a diffusion component. When these two factors are evaluated quantitatively they account for the observed relationship between the number of carriers and the steady-state accumulation achieved.