FIG. 1.

Schematic representation of the guanylic nucleotide synthesis pathway. Solid lines represent the plasma membrane. ext, extracellular medium; Gua, guanine; int, intracellular medium; PRPP, 5-phosphoribosyl-1-pyrophosphate. Genes are shown in italic type and encode the following enzymatic activities: FCY2, purine cytosine permease; GUA1, guanosine-5′-monophosphate synthetase; GUK1, guanosine-5′-monophosphate kinase; HPT1, hypoxanthine-guanine phosphoribosyltransferase; IMD2, IMD3, and IMD4, IMPDH; RNR, ribonucleotide reductase; and YNK1, nucleoside-5′-diphosphate kinase. IMD1 is not indicated here because it is not expressed (3, 12) and is thought to be a pseudogene.