SUMMARY

An unusual Escherichia coli K I ~ mutant for carbamyl phosphate synthetase is described. The mutation was generated by bacteriophage MUI insertion and left a 5 residual activity of the enzyme using either ammonia or glutamine as donors. The mutation is recessive to the wild-type allele and maps at or near the pyrA gene, but the mutant requires only arginine and not uracil for growth. By a second block in the pyrB gene it was possible to shift the accumulated carbamyl phosphate to arginine biosynthesis. The K, values and the levels of ornithine activa-tion and inhibition by UMP were normal in the mutant enzyme