Rameau dégradatif commun des hexuronates chez Escherichia coli K12

After the recent investigation on the biochemical properties of the 2‐keto‐3‐deoxy‐glucono‐kinase and 2‐keto‐3‐deoxy‐6‐phospho‐gluconate aldolase in Escherichia coli K12, we report in this paper the induction mechanism of these two enzymes and the genetic control of a new enzymatic activity: the 2‐keto‐3‐deoxy‐gluconate transport system. The d ‐glucuronate and d ‐galacturonate raise the high basal levels of the base and aldolase on glycerol by a factor of 5 and 3–4, respectively, the kinase and aldolase being enzymes of the common degradative pathway of d ‐glucuronate and d ‐galacturonate. The differential rates of synthesis of these two enzymes are constant from the addition of the galacturonate. Under a variety of conditions (different inducers and substrates of growth) kinase and aldolase have been found to be synthesized non coordinately. The strongest decoordination is carried out by gluconate: this compound which is a good inducer of the aldolase, not only is unable to induce the kinase but also represses it. Moreover, the aldolase seems to be less sensitive to the catabolic repression than the kinase. By means of appropriate negative mutants we have established that the two hexuronates and two keto‐uronates do not induce directly kinase and aldolase, but by sequential conversion into 2‐keto‐3‐deoxy‐gluconate. Furthermore, the fact that the hexuronates induce the kinase in aldolase‐negative mutants ( kdg A ) and besides, “over” induce the aldolase in kinase negative mutants ( kdg K ) strongly suggests that 2‐keto‐3‐deoxy‐gluconate is a true inducer for both enzymes. In addition, the gluconate which still induces the aldolase in an edd strain suggests that, this compound and/or the 6‐phospho‐gluconate are/is also inducer(s) of this enzyme. Moreover, we have isolated spontaneous mutants able to utilize the 2‐keto‐3‐deoxy‐gluconate as a unique carbon source. Two classes have been identified. The mutants of these classes which arise at high frequency (10 −5 to 10 −6 ) are either simultaneously constitutive for a specific 2‐keto‐3‐deoxy‐gluconate transport system, kinase and aldolase (class kdg R c )or only constitutive for the transport system (class kdg P c ). The 2‐keto‐3‐deoxy‐gluconate as exogenous substrate, which cannot induce its transport system in the wild type, behaves like a non‐inducer substrate. So the strains kdg R c and kdg P c are respectively i − and O c lac ‐like mutants. These findings and the fact that we have found thermosensitive mutants mapping in the kdg R locus (34.5 min) and which are simultaneously derepressed for the permease, kinase and aldolase at 42°C but not at 28°C, strongly suggest that the synthesis of the three sequential enzymes degrading the 2‐keto‐3‐deoxy‐gluconate is negatively controlled by a common regulator gene product. The decoordinated syntheses of these three enzymes are in agreement with the scattering of the corresponding operons on the chromosome: transport system operon (76.5min), kinase operon (69 min), aldolase operon (34 min).