THE IMPORTANCE OF THE BINDING-PROTEIN-DEPENDENT MGI SYSTEM TO THE TRANSPORT OF GLUCOSE IN ESCHERICHIA-COLI GROWING ON LOW SUGAR CONCENTRATIONS

Glucose limitation in chemostats derepressed the binding-protein-depen dent Mgl transport system, which is strongly repressed during growth i n batch culture with high glucose levels. The limitation-induced Mgl a ctivity was higher than that of batch cultures ''fully induced'' for t he Mgl system after growth on glycerol plus fucose. Mgl- strains were impaired compared to Mgl+ bacteria in removing glucose from sugar-limi ted chemostats and were outcompeted in mixed continuous culture on lim iting glucose. The influence of Mgl was not observed on growth with li miting maltose or non-carbohydrates, and thus was specific for glucose , a known substrate of the Mgl system. In the absence of the two gluco se-specific membrane components of the phosphoenolpyruvate:sugar phosp hotransferase system, non-PTS-dependent growth on glucose was observed in continuous culture, but only under sugar-limited conditions derepr essing the Mgl system and not in glucose-rich batches or continuous cu lture. Hence growth of Escherichia coli on glucose at micromolar conce ntrations involves a significant contribution of a binding-protein-dep endent transport system. The participation of multiple transporters in glucose transport can account for the complex non-hyperbolic dependen ce of growth-rate on glucose concentration and for discrepancies in st udies attempting to describe growth on glucose purely in terms of phos photransferase kinetics.