- A purine auxotroph deficient in phosphoribosylpyrophosphate amidotransferase and phosphoribosylpyrophosphate aminotransferase activities with normal activity of ribose-5-phosphate aminotransferase. Holmes, E.W., King, G.L., Leyva, A., Singer, S.C. Proc. Natl. Acad. Sci. U.S.A. (1976)