Repression of biosynthetic enzyme synthesis in is incomplete even when the bacteria are growing in a nutritionally complex environment. The synthesis of four of the enzymes of the arginine biosynthetic pathway (-acetyl - - glutamokinase/-acetylglutamate-γ-semialdehyde dehydrogenase, ornithine carbamoyltransferase and acetylornithine-β-transaminase) could be repressed and derepressed, but the maximum difference observed between repressed and derepressed levels for any enzyme of the pathway was only 5-fold (for orni-thine carbamoyltransferase). No repression of five enzymes of the pyrimidine biosynthetic pathway (aspartate carbamoyltransferase, dihydro-orotase, dihydro-orotate dehydrogenase, orotidine-5′-phosphate pyrophosphorylase and orotidine-5′-phosphate decarboxylase) could be detected on addition of pyrimidines to minimal asparagine cultures of A90, but a 1.5-to 2-fold degree of derepression was found following pyrimidine starvation of pyrimidine auxotrophic mutants of A90. Aspartate carbamoyltransferase in crude extracts of A90 was inhibited by (in order of efficiency) pyrophosphate, CTP, UTP and ATP, at limiting but not at saturating concentrations of carbamoyl phosphate.


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