The Effects of Bases and Nucleosides on the Intracellular Contents of Nucleotides and 5‐Phosphoribosyl 1‐pyrophosphate in <i>Escherichia coli</i>

Abstract

It has been reported that supplying preformed purine and pyrimidine bases and nucleosides to growing cells of Escherichia coli and other organisms results in the preferential utilization of these precursors for nucleotide biosynthesis, but the factors responsible for inhibition of the de novo pathways in vivo are not known. We have surveyed the effects of addition of purine and pyrimidine bases and nucleosides on the intracellular contents of nucleotides and 5‐phosphoribosyl 1‐pyrophosphate ( P ‐Rib‐ PP ) of Escherichia coli in an attempt to determine what some of these factors might be. The results show that initially adenine is converted mainly to ATP, guanine mainly to GTP and uracil mainly to UTP, while hypoxanthine is converted to both ATP and GTP. The corresponding nucleosides have similar effects except for adenosine, whose effects are similar to those of hypoxanthine and inosine. All purine precursors, except for guanosine, deplete the intracellular content of P ‐Rib‐ PP to 10% or less of its normal value. This finding indicates that the limitation in the concentration of P ‐Rib‐ PP for the first reaction of the de novo pathway is an important factor in the inhibition of that pathway when a preformed purine is available to support a salvage pathway of purine nucleotide biosynthesis. Our results also suggest that the decreased availability of P ‐Rib‐ PP may also cause immediate transient decreases observed in the levels of CTP and UTP, after the addition of a preformed purine precursor. Uracil as a pyrimidine precursor causes only a slight depletion of P ‐Rib‐ PP while uridine causes an initial increase in the P ‐Rib‐ PP content. It is concluded, therefore, that an increase in feedback inhibition, caused by elevated concentrations of uridine nucleotides, may be a major factor causing the inhibition of de novo biosynthesis of pyrimidine nucleotides when preformed pyrimidines are available, or that the inhibition may not be fully effective until repression and growth have diluted out the enzymes for the de novo biosynthesis.