The operon of K12 comprises structural genes for the two enzymes, IMP dehydrogenase and GMP synthetase, required for the biosynthesis of GMP from IMP. The specific activities of these enzymes were measured in various purine auxotrophs. and mutants (guanine-specific) were derepressed under conditions of growth limitation by guanine but were repressed by excess guanine. This suggests that formation of the enzymes is normally controlled by a guanine nucleotide. Derepression of the operon in purine-starved mutants depended on the type of mutant and on whether adenine or guanine was provided. A strain (adenine-specific) and strains with early blocks in purine biosynthesis ( and ) did not derepress. or strains [5′-phosphoribosyl-5-aminoimidazole (AIR)-accumulating] derepressed only 4-fold. The operon was repressed in strains [5′-phosphoribosyl-5-amino-4-imidazolecarboxamide (AICAR)-accumulating] grown with limiting guanine or hypoxanthine, but derepressed by growth with limiting adenine. Two mutants ( and ) which can neither synthesize AMP and GMP , nor interconvert them, were isolated. The specific activity of IMP dehydrogenase in one of these strains grown with different concentrations of guanine and adenine revealed that adenine induces the operon whereas guanine represses it. Intracellular purine nucleotide pools were measured in a mutant repressed (guanine-grown) and derepressed (adenine-grown) for IMP dehydrogenase. The guanylate pool was similar under the two growth conditions; however, the adenylate pool of the adenine-grown bacteria was two to three times greater than that of the guanine-grown cells. A dual mechanism for regulating expression of the operon, involving induction by AMP and repression by GMP, is proposed.


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