TRANSMEMBRANE EFFECTS OF BETA-GALACTOSIDES ON THIOMETHYL-BETA-GALACTOSIDE TRANSPORT IN ESCHERICHIA COLI

1. 1. The transport rate in Escherichia coli (ML strain) of thiomethyl-β-[14C]galactoside was measured in the presence and absence of thiomethyl-β-galactoside (TMG), o-nitrophenyl-β-galactoside (ONPG), or thio-β-digalactoside (TDG) on the side of the cell membrane toward which [14C]TMG transport was occurring. 2. 2. Efflux rate of [14C]TMG was markedly stimulated by the presence of non-radioactive TMG in the external medium, whereas preloading these cells with non-radioactive TMG caused little effect on the influx rate of low concentrations of [14C]TMG and only a small stimulation of influx rate at high concentrations. 3. 3. ONPG in the external medium caused a smaller stimulation of [14C]TMG efflux rate than did TMG. Preloading the cells with ONPG inhibited influx rate of low concentrations of [14C]TMG, but had little effect on this rate for high concentrations. 4. 4. TDG in the external medium caused only a slight stimulation of [14C]TMG efflux rate, whereas preloading with TDG causes a marked inhibition of the influx rate of all concentrations of [14C]TMG. 5. 5. These results are consistent with the hypothesis that TMG undergoes exchange diffusion with [14C]TMG, ONPG has little exchange diffusion, and TDG has an inhibitory exchange" effect; but that superimposed on these exchange effects are the effects of an external partial diffusion barrier. © 1969."