ANTAGONISM BETWEEN SUBSTRATE AND REPRESSOR IN CONTROLLING THE FORMATION OF A BIOSYNTHETIC ENZYME

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Abstract
The effect of ornithine on the synthesis of ornithine-transcarbamylase has been studied in strains (R+) in which the synthesis of this enzyme is repressed by arginine and in strains (R-) in which it is not repressed. To permit independent control of arginine and ornithine concentration, double mutants blocked both before and after this enzyme were used. Furthermore, the chemostat was used to maintain arginine levels that resulted in enzyme concentrations intermediate between complete repression and derepression. Ornithine reversed the repressive effect of arginine in an intermediate concentration range; however, it was without effect under conditions of complete repression or derepression. The nature of the chemostat experiment is self-correcting for arginine-ornithine competition for entrance into the cell. It therefore appears that ornithine competes directly with arginine at some site within the cell. An analogy exists between the repressor-inducer relationship in the cases of ornithine transcarbamylase, a biosynthetic (so-called "constitutive") enzyme, and beta-galactosidase, a degradative (so-called "inducible") enzyme.