SUMMARY: Mutants of impaired in the biosynthesis of carbamoyi phosphate were obtained. Genetical, physiological and enzymic studies of these mutants showed the existence in this organism of two independent enzymic systems which catalysed the synthesis of carbamoyl phosphate from HCO , glutamine, ATP and Mg. One system provides carbamoyi phosphate for the arginine pathway, the other plays a similar role for the pyrimidine pathway. Carbamoyl phosphate from one pathway is freely available for the other. The mutations have been mapped in three unlinked loci. Two loci determine the argininespecific carbamoyl phosphate synthesizing system. The third locus corresponds to the pyrimidine-specific system. Mutations in either of the two genes concerned with the arginine pathway lead to a deficiency in the activity of that pathway to synthesize carbamoyl phosphate. Crossing a mutant deficient in one of the two arginine loci with a mutant deficient at the other produces a diploid in which complementation occurs. Also, activity may be partially regained by combining the cell-free extracts of the two single mutants. The physiological significance of the two enzymic systems is established by the study of their regulation. The carbamoyl phosphate synthesizing activity of the arginine pathway was repressed to 10% of its value in minimal medium by addition of excess of arginine to the growth medium. The activity corresponding to the pyrimidine pathway was only slightly repressed by uracil but was subject to feed-back inhibition by uridine-5'-triphosphate. These results are compared with the data available for other micro-organisms.


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