The hexuronate metabolism in Escherichia coli is regulated by two related transcription factors from the FadR subfamily of the GntR family, UxuR and ExuR. UxuR controls the D-glucuronate metabolism, while ExuR represses genes involved in the metabolism of all hexuronates. We use a comparative genomics approach to reconstruct the hexuronate metabolic pathways and transcriptional regulons in gammaproteobacteria. We demonstrate differences in the binding motifs of UxuR and ExuR, identify new candidate members of the UxuR/ExuR regulons, and describe the links between the UxuR/ExuR regulons and the adjacent regulons UidR, KdgR, and YjjM. We provide experimental evidence that two predicted members of the UxuR regulon, yjjM and yjjN, are the subject of complex regulation by this transcription factor in E. coli.D-Glucuronate and D-galacturonate can each serve as a sole carbon source for the growth of Escherichia coli. They are utilized via the Ashwell catabolic pathway (27). Hexuronates enter the cell using two different transport systems, GntP and ExuT (from the GntP and MFS families of transporters, respectively), which have dual specificity, importing D-tagaturonate/D-fructuronate and D-galacturonate/D-glucuronate, respectively. In addition, -D-glucuronides can be utilized to form D-glucuronate using the UidB transporter and the UidA glucuronidase ( Fig. 1).D-Galacturonate and D-glucuronate are converted to the same metabolite, 2-keto-3-deoxy-D-gluconate, via parallel pathways, which involve one enzyme common for both pathways, D-glucuronate/D-galacturonate isomerase (UxaC), and two pairs of enzymes, D-mannonate and D-altronate hydrolases (UxuA and UxaA, respectively) and oxidoreductases (UxuB and UxaB, respectively), in each pathway (Fig. 1). The UxuB and UxaB proteins of E. coli are homologous (identity, 26%; positives, 42%; coverage, 95%) but may be confidently distinguished by the genome context analysis. The UxuA and UxaA demonstrate no discernible homology.The hexuronate metabolism in E. coli is regulated by two related transcription factors (TFs) from the FadR subfamily of the GntR family, UxuR and ExuR, whose amino acid sequences are 46% identical. UxuR controls the D-glucuronate metabolism by repressing uxuAB, uidABC, gntP, and its own gene uxuR (3,7,17,(24)(25)(26)(27). ExuR negatively controls most of the genes involved in the metabolism of both D-galacturonate and D-glucuronate, including exuT, uxaCA, uxaB, uxuAB, and exuR (6,22,[24][25][26][27].Understanding the regulation of the uidABC genes may have practical applications. Indeed, inhibiting -glucuronidase produced by the gut flora can prevent the intestinal metabolism of the anticancer drug irinotecan, thereby diminishing life-threatening toxicity and conceivably allowing dose escalation that will enhance the drug's efficacy (33). It is also known that bacterial -glucuronidase inhibitors can prevent a side effect of the common colon cancer chemotherapeutic CPT-11, severe diarrhea caused by -glucuronidases from symbiotic bacteria that reactivate t...