The subsidiary GntII system for gluconate metabolism in Escherichia coli: Alternative induction of the gntV gene
Two systems are involved in the transport and phosphorylation of gluconate in Escherichia coli. GntI, the main system, consists of high and low-affinity gluconate transporters and a thermoresistant gluconokinase for its phosphorylation. The corresponding genes, gntT, gntU and gntK at 76.5 min, are induced by gluconate. GntII, the subsidiary system, includes IdnT and GntV, which duplicate activities of transport and phosphorylation of gluconate, respectively. Gene gntV at 96.8 min is divergently transcribed from the idnDOTR operon involved in L-idonate metabolism. These genetic elements are induced by the substrate or 5-keto-D-gluconate. Because gntV is also induced in cells grown in gluconate, it was of interest to investigate its expression in this condition. E. coli gntK, idnOokan mutants were constructed to study this question. These idnO kan-cassete inserted mutants, unable to convert gluconate to 5-keto-D-gluconate, permitted examining gntV expression in the absence of this inducer and demonstrating that it is not required when the cells grow in gluconate. The results suggest that E. coli gntV gene is alternatively induced by 5-keto-D-gluconate or gluconate in cells cultivated either in idonate or gluconate. In this way, the control of gntV expression would seem to be involved in the efficient utilization of these substrates.