The Regulation of Purine Utilization in Bacteria

Abstract Initial rates of adenine uptake by Escherichia coli resting cells were found to correlate closely with adenine phosphoribosyltransferase activity of cell homogenates when measured under optimal conditions. Cultural conditions, inhibitors, and subjecting the cells to osmotic altered both uptake and phosphoribosyltransferase activities pari passu. At appropriate concentrations virtually all of the adenine in the medium was taken up into the cells even under conditions in which no growth occurred and macromolecular synthesis was negligible. In energy-depleted cells glucose, ribose, or P-ribose-PP markedly stimulated uptake of adenine while several other sugar phosphates gave much less or no stimulation. P-ribose-PP also stimulated uptake of several additional purines and pyrimidines. For P-ribose-PP-stimulated adenine uptake the kinetics with respect to both P-ribose-PP and adenine concentrations was similar to those observed for adenine phosphoribosyltransferase. Iodo-acetamide (0.4 mm) caused virtually complete inhibition of the glucose- or ribose-stimulated uptake of adenine but did not affect the P-ribose-PP-stimulated uptake. Labeled adenine taken up was found intracellularly predominantly as AMP, IMP, and GMP while free adenine accounted for 0 to 2% of the total radioactivity taken up. Nucleotide inhibitors of adenine phosphoribosyltransferase activity were also found to be inhibitors of adenine uptake; free purines (which do not inhibit adenine phosphoribosyltransferase) also inhibited uptake, most probably acting after conversion to nucleotides. Several phosphoribosyltransferases are apparently located in the pericytoplasmic space since variable amounts (up to 70%) are released into the shock fluid when cells are subjected to osmotic by the method of Nossal and Heppel (J. Biol. Chem., 241, 3055 (1966)). This release is accompanied by a parallel loss in purine uptake capacity. Although all purine and pyrimidine uptake systems can be stimulated to a greater or lesser extent by P-ribose-PP, they are distinct and may be governed by completely different mechanisms.