Mutants of Aerobacter aerogenes Capable of Utilizing Xylitol as a Novel Carbon

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Abstract
Wild-type A. aerogenes 1033 is unable to utilize xylitol. A succession of mutants was isolated capable of growth on this compound (0.2%) at progressively faster rates. Whereas the ability to utilize xylitol was achieved in the 1st-stage mutant (X1) by constitutive production of ribitol dehydrogenase (for which xylitol is a substrate but not an inducer), the basis for enhanced utilization of xylitol in the 2nd-stage mutant (X2) was an alteration of ribitol dehydrogenase. This enzyme was purified from the various mutants. The apparent Km for xylitol was 0.12 M with X2 enzyme and 0.29 M with X1 enzyme: The X2 enzyme was also less heat stable and, at 0.05 M substrate concentration, had a higher ratio of activity with xylitol compared to ribitol than did the Xl enzyme. The 3rd mutant (X3), with an evenfaster growth rate on xylitol, produced a ribitol dehydrogenase indistinguishable physically or kinetically from that o fX2. However, X3 produced constitutively an active transport system which accepts xylitol. The usual function of this system is apparently for the transport of D-arabitol since the latter is not only a substrate but also an inducer of the transport system in parental strains of X3. The sequence of mutations described illustrates how genes belonging to different metabolic systems can be mobilized to serve a new biochemical pathway.

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