| Early studies of D-gluconate catabolism in Escherichia coli revealed two similar systems thought to be involved with gluconate catabolism. The main system, GntI, is well characterized and known to be the primary pathway for gluconate catabolism. The subsidiary system, GntII, shares genetic similarities to the GntII system and was predicted to encode a secondary gluconate pathway. The experiments presented in this dissertation reveal that the GntII system is a novel pathway for the catabolism of L-idonate in E. coli and describe the biochemistry, physiology, gene organization, and mechanisms of transcription regulation. The overall process of L-idonate metabolism is as follows: L-idonate is transported into the cell by IdnT and is oxidized by IdnD to the intermediate 5-ketogluconate (5KG), which is subsequently reduced by IdnO to gluconate, then phosphorylated by IdnK. An intermediate of the pathway, D-gluconate, serves to induce the edd-eda operon encoding the Entner-Doudoroff pathway, which further catabolizes the 6-phosphogluconate. The enzymes of the L-idonate pathway are encoded by the IdnR regulon, comprised of two operons: idnDOTR and divergently transcribed idnK. This regulon is induced by L-idonate or 5KG, with both operons being expressed at similar levels. The regulatory region located between idnD and idnK contains the transcriptional start sites, promoters, and single binding sites for IdnR and cAMP-CRP, as well as an UP element sequence. Transcription occurs through a CRP-dependent Class III mechanism. The regulators for the L-idonate and D-gluconate pathways, IdnR and GntR bind to the same palindromic sequence, providing the foundation for cross talk. Discrimination between the IdnR and GntR regulons is achieved through preferential binding of GntR to operators of the gluconate regulon and IdnR to the operator of the idonate regulon through interactions influenced by their respective inducers. Thus, a GntI system mutation is complemented by the GntII pathway enzymes through the intracellular accumulation of the inducer for L-idonate catabolism. |
