Regulation of purine utilization in bacteria. VI. Characterization of hypoxanthine and guanine uptake into isolated membrane vesicles from Salmonella typhimurium

Uptake of hypoxanthine and guanine into isolated membrane vesicles of Salmonella typhimurium TR119 was stimulated by 5'-phosphoribosyl-1'-pyrophosphate (PRPP). For strain proAB47, a mutant that lacks guanine phosphoribosyltransferase, PRPP stimulated uptake of hypoxanthine into membrane vesicles. No PRPP-stimulated uptake of guanine was observed. For strain TR119, guanosine 5'-monophosphate and inosine 5'-monophosphate accumulated intravesicularly when guanine and hypoxanthine, respectively, were used with PRPP as transport substrates. For strain proAB47, IMP accumulated intravesicularly with hypoxanthine and PRPP as transport substrates. For strain TR119, hypoxanthine also accumulated when PRPP was absent. This free hypoxanthine uptake was completely inhibited by N-ethylmaleimide, but the PRPP-stimulated uptake of hypoxanthine was inhibited only 20% by N-ethylmaleimide. Hypoxanthine and guanine phosphoribosyltransferase activity paralleled uptake activity in both strains. But, when proAB47 vesicles were sonically treated to release the enzymes, a three- to sixfold activation of phosphoribosyltransferase molecules occurred. Since proAB47 vessicles lack the guanine phsophoribosyltransferase gene product and since hypoxanthine effectively competes out the phosphoribosylation of guanine by proAB47 vesicles, it was postulated that the hypoxanthine phosphoribosyltransferase gains specificity for both guanine and hypoxanthine when released from the membrane. A group translocation as the major mechanism for the uptake of guanine and hypoxanthine was proposed.