Control of Carbamyl Phosphate Synthesis in Salmonella typhimurium

Carbamyl phosphate synthetase from Salmonella typhimurium LT-2 has been purified 320-fold; the modulation of enzyme activity by substrate concentration, effectors, and the effect of temperature on this modulation have been studied. Uridine phosphate inhibits the enzymic reaction; ornithine reverses this inhibition and also activates the enzyme in the absence of UMP by decreasing the apparent Km for ATP. The stimulation by ornithine is greater at low temperature, whereas the inhibitory effect of UMP is equal at high and low temperatures. Studies with various strains blocked by mutation in steps which interconvert cytosineand uracil-containing compounds have established that the pyrimidine repressor of the synthesis of carbamyl phosphate synthetase is a cytosine-containing compound.