SEQUENCE-ANALYSIS OF THE GNTII (SUBSIDIARY) SYSTEM FOR GLUCONATE METABOLISM REVEALS A NOVEL PATHWAY FOR L-IDONIC ACID CATABOLISM IN ESCHERICHIA-COLI

The presence of two systems in Escherichia coli for gluconate transpor t and phosphorylation is puzzling. The main system, GntI, is web chara cterized, while the subsidiary system, GntII, is poorly understood. Ge nomic sequence analysis of the region known to contain genes of the Gn tII system led to a hypothesis,which,vas tested biochemically and conf irmed: the GntII system encodes a pathway for catabolism of L-idonic a cid in which D-gluconate is an intermediate. The genes have been named accordingly: the idnK gene, encoding a thermosensitive gluconate kina se, is monocistronic and transcribed divergently from the idnD-idnO-id nT-idnR operon, which encodes L-idonate 5-dehydrogenase, 5-keto-D-gluc onate 5-reductase, an L-idonate transporter, and an L-idonate regulato ry protein, respectively. The metabolic sequence is as follows: IdnT a llows uptake of L-idonate; IdnD catalyzes a reversible oxidation of L- idonate to form 5-ketogluconate; IdnO catalyzes a reversible reduction of 5-ketogluconate to form D-gluconate; IdnK catalyzes an ATP-depende nt phosphorylation of D-gluconate to form 6-phosphogluconate, which is metabolized further via the Entner-Doudoroff pathway; and IdnR appear s to act as a positive regulator of the IdnR regulon, with L-idonate o r 5-ketogluconate serving as the true inducer of the pathway. The L-id onate 5-dehydrogenase and 5-keto-D-gluconate 5-reductase reactions wer e characterized both chemically and biochemically by using crude cell extracts, and it was firmly established that these two enzymes allow f or the redox-coupled interconversion of L-idonate and D-gluconate,ia t he intermediate 5-ketogluconate. E. coli K-12 strains are able to util ize L-idonate as the sole carbon and energy source, and as predicted, the ability of idnD, idnK idnR, and edd mutants to grow on L-idonate i s altered.