SUMMARY: A second-stage mutant of K12 designated as strain 806 grew faster on d-lyxose than the mutant strain 805 previously described. Both mutants produced constitutively a novel enzyme, d-mannose isomerase, but strain 806 produced twice as much as strain 805. The enzyme could fortuitously convert d-lyxose to d-xylulose, which is a normal intermediate in the d-xylose catabolic pathway. The purified enzyme consisted of four subunits each with a molecular weight of about 40000. In 0·14 m-NaSO, the tetramer dissociated completely into dimers. While the tetramer values for d-mannose and d-lyxose were 80 mm and 300 mm, respectively, the dimer values for these two sugars were both 300 mm. The amino acid composition of the enzyme was also determined.


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