B10, a mutant strain of B, which exhibited a requirement for histidine + uracil under certain conditions of growth, became pyrimidine-independent when grown in a simple medium supplemented with these two substances. It was demonstrated that this change from uracil-dependence to non-dependence was not due to the selection of a competent back-mutant, but to the formation of the enzyme dihydro-orotic acid dehydrogenase, which is lacking in uracil-requiring organisms. A study of the enzyme content of mutant and wild type at various stages of growth demonstrated that a high enzyme-forming capacity is associated with young organisms harvested from cultures in the late lag or early logarithmic period.


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