Relationship between histidine and PRA biosynthesis. (A) E. coli cells can generate PRA by direct synthesis using PRPP and glutamine (L-gln) by the action of PurF. In the ΔpurF strain, PRA can be made by diverting the histidine biosynthetic intermediate ProFAR (1-(5-phosphoribosyl)-5-[(5-[phosphoribosylamino)methylideneamino] imidazole-4 carboxamide) (Koenigsknecht et al. 2012). Thus, the concentration of PRA is expected to be at its lowest when ΔhisB cells deplete the PRA precursor PRPP in a futile attempt to generate sufficient histidine. (B) Interpretation of the agar-diffusion assay with the ΔhisB strain grown on minimal medium and wherein histidine is added to the filter disk. The plate image is taken from Figure 3D.
