Characterization of the Lactose Transport System in Citrobacter freundii

Abstract

The lactose transport system of Citrobacter freundii was characterized. Both the lactose transport system and β-galactosidase were induced with either lactose or isopropyl-β-D-thiogalactopyranoside (IPTG), the latter being the better inducer. The K_m values for methyl-β-D-thiogalactopyranoside (TMG) transport and lactose transport were 0.61 mM and 1.1 mM, respectively, and the V_max values were 53 nmol/min/mg cell protein and 12 nmol/min/mg cell protein, respectively. Thus, TMG is a better substrate than lactose. Thiodigalactopyranoside (TDG) was a very potent competitive inhibitor. Neither Na⁺ nor Li⁺ had a significant effect on the TMG transport or the lactose transport. Proton/substrate cotransport (symport) via this system was observed.