Binding-protein-dependent glucose transport by Agrobacterium radiobacter grown in glucose-limited continuous culture

Agrobacterium radiobacter NCIB 11883 was grown in glucose-limited continuous culture at low dilution rate. Whole cells transported glucose using an energy-dependent mechanism which exhibited an accumulation ratio greater than 2000. Three major periplasmic proteins were purified and their potential role as glucose-binding proteins (GBP) were investigated using equilibrium dialysis. Two of these, GBP1 (Mr 36,500) and GBP2 (Mr 33,500), bound D-glucose with high affinity (KD 0.23 and 0.07 microM respectively), whereas the third protein (Mr 30,500) showed no binding ability. Competition experiments using various analogues showed that those which differed from glucose at C-6 (e.g. 6-chloro-6-deoxy-D-glucose and 6-deoxy-D-glucose) variably decreased the binding of glucose to both GBP1 and GBP2, whereas those which differed at C-4 (e.g. D-galactose) were only effective with GBP1. The rate of glucose uptake and the concentration of the glucose-binding proteins increased in parallel during prolonged growth under glucose-limitation due to the emergence of new strains in which GBP1 (e.g. strain AR18) or GBP2 (e.g. strain AR9), but not both, was hyperproduced and accounted for at least 27% of the total cell protein. It is concluded that A. radiobacter synthesizes two distinct periplasmic binding proteins which are involved in glucose transport, and that these proteins are maximally derepressed during growth under glucose limitation.