The regulation of purine utilization in bacteria. V. Inhibition of purine phosphoribosyltransferase activities and purine uptake in isolated membrane vesicles by guanosine tetraphosphate.

The ability of rel+ cell strains of Escherichia coli to take up nucleosides and bases and convert them to the corresponding ribonucleoside 5’-triphosphates is qualitatively dependent upon amino acids, much the same as the ability to accumulate RNA. Restriction of uptake in this case occurs under the same conditions that elicit guanosine 5’-diphosphate, 2’or 3’-diphosphate (ppGpp) accumulation and both phenomena are reversed by addition of chloramphenicol. The Pribose-PP-dependent transport of purine nucleosides and bases into membrane vesicles is inhibited by ppGpp, as are membrane-bound purine phosphoribosyltransferase activities. The degree of inhibition differs for different substrates; for purines, inhibition approximates the uptake restrictions observed in whole cells. Enzyme and membrane preparations obtained from rel+ and relcells were equally inhibited by PPGPP. Purified soluble 6-hydroxy purine phosphoribosyltransferase, which mediates membranous uptake of 6-hydroxy purines, is inhibited by ppGpp. Inhibition is not strictly competitive, but appears to be less severe at high P-ribose-PP levels. Adenine phosphoribosyltransferase was only mildly inhibited by ppGpp. The inhibitory effects of ppGpp on purine transport due to inhibition of purine phosphoribosyltransferase can account for purine uptake restrictions observed under different physiological conditions in whole cells.