CHARACTERIZATION OF THE LACTOSE TRANSPORT-SYSTEM IN CITROBACTER-FREUNDII

The lactose transport system of Citrobacter freundii was characterized. Both the lactose transport system and beta-galactosidase were induced with either lactose or isopropyl-beta-D-thiogalactopyranoside (IPTG), the latter being the better inducer. The K. values for methyl-beta-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.