Glutamine phosphoribosylpyrophosphate amidotransferase (PRAT; EC 2.4. 2.14) is the first and rate-limiting enzyme of de novo purine biosynthesis. PRAT expression in Drosophila development was examined to determine if it is correlated with cell proliferation and/or nutritional isolation. An antiserum, raised against the 16 carboxyl-terminal amino acids of PRAT, detects two proteins corresponding to a 60 kDa proenzyme and 55 kDa mature enzyme, consistent with a 53 amino acid propeptide predicted from the gene sequence. Mature enzyme is maternally expressed, and proenzyme appears in embryos at 2-8 h, corresponding to the interval during which zygotic transcription is initiated. Upon hatching of first instar larvae, proenzyme levels are reduced and remain low relative to mature enzyme. Adult females have higher levels of both proteins relative to males, consistent with maternal expression. Maternal expression reflects a requirement for the enzyme during embryogenesis, while reduction in expression following hatching reflects a switch to an exogenous source of purines. Prat mRNA levels follow a similar overall pattern in the same developmental stages examined for the protein. Discovery of a second gene encoding PRAT with 78% amino acid identity leads to the possibility that the antiserum raised against the carboxyl-terminus detects two enzymes.