PHOSPHORIBOSYL DIPHOSPHATE SYNTHETASE-INDEPENDENT NAD DE-NOVO SYNTHESIS IN ESCHERICHIA-COLI - A NEW PHENOTYPE OF PHOSPHATE REGULON MUTANTS

Phosphoribosyl diphosphate-lacking (Delta prs) mutant strains of Esche richia coli require NAD, guanosine, uridine, histidine, and tryptophan for growth. NAD is required by phosphoribosyl diphosphate-lacking mut ants because of lack of one of the substrates for the quinolinate phos phoribosyltransferase reaction, an enzyme of the NAD de novo pathway. Several NAD-independent mutants of a host from which prs had been dele ted were isolated; all of them were shown to have lesions in the pstSC AB-phoU operon, in which mutations lead to derepression of the Pho reg ulon. In addition NAD-independent growth was dependent on a functional quinolinate phosphoribosyltransferase. The prs suppressor mutations l ed to the synthesis of a new phosphoryl compound that may act as a pre cursor for a new NAD biosynthetic pathway. This compound may be synthe sized by the product of an unknown phosphate starvation-inducible gene of the Pho regulon because the ability of pst or phoU mutations to su ppress the NAD requirement requires PhoB, the transcriptional activato r of the Pho regulon.