Springer Online Journal Archives 1860-2000
Summary Two classes of negative mutants in the hexuronate system of Escherichia coli K 12 were isolated and characterized. Strains of the first class were specifically deficient in the enzyme uronic isomerase. They did not grow on galacturonate or glucuronate, the substrates of isomerase, but very well developped on the products, tagaturonate or fructuronate. In these mutants, keto-but not aldo-hexuronates could still induce other enzymes of the hexuronate system. The corresponding mutations mapped in the uxa C locus and were localized by sexual crosses and P1 transduction near min 60, between tol C and arg G markers. The uxa C locus was very closely linked to the uxa A locus, the structural gene for altronic hydrolyase. Biochemical characterizations of uxa C mutants using temperature sensitive revertants was achieved by means of thermal inactivation and kinetic studies. Results indicated that the uxa C locus was the structural gene for uronic isomerase. Mutant strains of the second class were unable to develop on galacturonate or tagaturonate but normally grew on glucuronate. They were specifically deficient in the enzyme altronic oxidoreductase which is the following functional enzyme, after isomerase, in galacturonate catabolism. The corresponding mutations were localized by interrupted and non-interrupted conjugations. They all mapped in the uxa B locus near 45 min. Biochemical studies of altronic oxidoreductase residual activities in uxa B mutants strongly suggested that the uxa B locus was the structural gene for altronic oxidoreductase. A direct method for the estimation of uronic isomerase using aldonic oxidoreductases as coupling enzymes was described and routinely applied to characterize hexuronate system mutants.
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