Régulation du métabolisme des hexuronates chez Escherichia coli K12

Kinetics of induction of the five enzymes catalyzing hexuronate ( d ‐galacturonate and d ‐glucuronate) to 2‐keto‐3‐deoxy‐ d ‐gluconate catabolism in Escherichia coli K12 have been studied. In wild type organism, two patterns of hexuronate system induction have been identified. The first is limited, observed with galacturonate or tagaturonate and characterized by only inducing the three galacturonate degradative enzymes: uronic isomerase, altronate: NAD + oxidoreductase and altronic hydrolyase. The second one is general, induced by glucuronate or fructuronate and characterized by induction of the five enzymes of the system: uronic isomerase, altronate : NAD + oxidoreductase and hydrolyase, mannonate : NAD + oxidoreductase and hydrolyase. In both cases, differential rates of synthesis of enzymes have been measured when the nature or the concentration of the inducer is varied. Induction effects of d ‐mannonic amide have also been examined: this compound induces all the hexuronate system under conditions of gratuity. Hexuronate system induction was subjected to catabolite repression. Induction occurred without significant delay and differential rates of synthesis remained constant during two or three doublings of cell mass after addition of the inducer. The process is extremely specific: metabolites of the hexuronate pathway and some very closely related substances only showed inducing power. Using a set of single mutants carrying mutations in structural genes of enzyme of the hexuronate system, it has been possible to establish that limited and general inductions were indirect processes. In the former situation, tagaturonate acted as true inducer, in the latter case, fructuronate and perhaps mannonate acted as well. Uronic isomerase and altronic hydrolase on the one hand, mannonate: NAD + oxidoreductase and mannonic hydrolyase on the other hand were coordinately induced. These results agree very well with independent genetic studies and strongly suggest the existence of two corresponding operons. In agreement with the isolated location of its structural gene, the induction of altronate: NAD + oxidoreductase was completely decoordinated with the induction of all four other enzymes of the system, which indicates the existence of a corresponding independent operon. Physiological properties of various regulatory mutants suggest an unique regulatory mechanism for controlling differential induction of hexuronate system.