leading to the synthesis of AMP and GMP begins with the transfer of an amido group from glutamine
to PRPP (Figure 1). Since PRPP is used for the both de novo and salvage synthesis of purine and pyrimidine nucleotides as well as for the synthesis of NAD, histidine and tryptophan, any stress that alters PRPP availabil ity affects multiple pathways. Given the essential nature of
PRPP, all free-living organisms contain at least one gene encoding PRPP synthetase