Studies of the kinetics of analog and amino acid incorporation into metabolic pools and proteins of yeast and E. coli have shown that at least two processes are involved in the selection of the natural amino acid in preference to the analog. These selection steps occur in the formation of the metabolic pools, the first occurring in the concentrating system of the cell, which accumulates exogenous amino acids or analogs into the cell at levels exceeding external concentrations. In the yeast, the final distinction between analog and amino acid occurs at the time of entry into a second metabolic pool (internal pool). In this pool conversion of amino acids to the required end products occurs. Once incorporated into this pool no further selection occurs; the ratio of analog to amino acid in this pool and in protein is the same. * C Medium: . g NHaC1, 0 g Na,zHPO 4, 3 g KH~PO~, 3 g NaCI, o.ol g Mg as Mg(12, o.o20 g S as NaaS(),, 1oo i111 IO ยฐ' o maltose or fructose, and 0oo ml distilled lI2().
* p fluorophenylalanine has been shown by MUNIER AND C()HEX 9 tO replace ollly phenylalanine in the proteins of 1'2. coli.