SUMMARY: Mechanisms whereby and grow on L-histidinol as sole source of carbon and nitrogen have been examined.

Polyacrylamide gel electrophoresis (PAGE) showed two distinct histidinol dehydrogenase (HDH) bands in partially purified extracts: HDH(B) was always present but increased up to twofold by growth with histidine; HDH(I), additionally present only in histidinol-grown organisms, which had up to 10 times the total and specific HDH activity of organisms grown on salts+glucose medium. HDH biosynthesis was not repressed by histidine, glucose+ammonia or glutamate. HDH(I) could have evolved from HDH(B) by gene duplication and modification, with the resulting gene acquiring a new, inducible control mechanism.

A mutant, , of grew on histidinol as sole source of carbon and nitrogen, which the wild-type did only poorly. Only one HDH band was found on PAGE of partially purified extracts of either mutant or wild-type, however grown, though histidinol-grown showed a more intense band and 60 times the HDH activity of glucose+salts-grown organisms. Histidine, glucose+ammonia or glutamate abolished this HDH increase. Here the adaptation seemed to consist of an additional control mechanism giving rise to high levels of the normal HDH when histidinol was sole source of carbon and nitrogen.


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