The effect of purines on the formation of two enzymes involved in purine biosynthesis.

In many instances the end product of a sequence of biosynthetic reactions inhibits the formation of enzymes responsible for these reactions. Such “negative feedback” effects have been observed in bacteria for amino acids (3-6) and for pyrimidines (7), and the name “enzyme repression” has been coined for them (5). In the case of several amino acids, enzyme repression could be demonstrated by showing that cells cultured in their presence contained much less of the particular enzyme than cells cultured in their absence (3-6). In the case of pyrimidines, it could be shown that pyrimidine-requiring mutants incubated in a medium devoid of pyrimidines could increase their content of some of the enzymes required for pyrimidine biosynthesis to a level many times that found in the prototroph (7). An earlier observation suggested that the formation of enzymes responsible for the biosynthesis of purines is similarly controlled by purines. It was found that in a guanine-requiring mutant of Aerobacter aerogenes, the level of inosine 5’-phosphate dehydrogenase depended on the guanine content of the growth medium; cells grown in a medium containing guanine in excess had about the same level of enzyme as the prototroph, whereas cells grown in a medium containing a limiting amount of guanine had a level of enzyme about 20 to 40 times higher (8). The study of the kinetics of IMP-dehydrogenase formation in this mutant has now revealed that the rate of synthesis of this emzyme is very slow, so long as guanine is present in the medium; rapid synthesis of the enzyme begins only after the exhaustion of the guanine from the medium. This finding suggested that a convenient method for the investigation of the repressive effects of purines on the formation of enzymes responsible for purine biosynthesis would be the comparison of the enzyme levels attained by cultures grown on limiting and excess purine. Two enzymes were chosen for this study: IMP-dehydrogenase (S), which catalyzes the conversion of IMP to xanthosine 5’-phosphate and is essential for the biosynthesis of guanosine 5’-phosphate but not for that of adenosine 5’-phosphate; and