Relationships between Intracellular Contents of Nucleotides and 5‐Phosphoribosyl 1‐Pyrophosphate in Escherichia coli

The intracellular contents of nucleotides and 5‐phosphoribosyl 1‐pyrophosphate ( P ‐Rib‐ PP ) in cells of Escherichia coli were quantitatively determined under various conditions of growth. When the growth rate was limited by the carbon source available in the growth medium the intracellular content of each of ATP, CTP, GTP, UTP, ADP and GDP was proportional to the growth rate. The intracellular content of P ‐Rib‐ PP showed a similar, though stronger dependence, suggesting that at the slower growth rates, the availability of P ‐Rib‐ PP could become limiting for nucleotide biosynthesis. Several treatments which are known to specifically stimulate the rate of net RNA synthesis in E. coli had a common effect in causing a transient depletion of intracellular ribonucleoside triphosphates. For each treatment, the depletion of triphosphates coincided with transiently elevated levels of P ‐Rib‐ PP , despite increases in the ADP:ATP ratio, an effect which might be expected to inhibit P ‐Rib‐ PP synthetase activity in E. coli (Atkinson and Fall [15], Klungsøyr et al. [16]). Explanations of this apparent anomaly might be made on the basis of the properties of P ‐Rib‐ PP synthetase from Salmonella typhimurium , (Switzer, [22], Switzer and Sogin, [21]). One explanation is that the enzyme is regulated by total concentrations of nucleotides so that decreases in nucleoside triphosphate concentrations result in stimulation of the reaction. Another is that the decrease in nucleoside triphosphate concentrations releases from chelation a pulse of free Mg 2+ which stimulates the synthetase. And thirdly, the concentration of the substrate, ribose 5‐phosphate, might be initially high enough to exert its inhibitory effect on the enzyme, so that stimulation of the reaction can occur if the substrate concentration falls.