De novo purine synthesis in Arabidopsis thaliana. II. The PUR7 gene encoding 5'-phosphoribosyl-4-(N-succinocarboxamide)-5-aminoimidazole synthetase is expressed in rapidly dividing tissues.

The small genome size and excellent genetics of Arabidopsis, as well as the ease with which it is transformed, make it a superb candidate for molecular genetic studies of the purine biosynthetic pathway. Herein we report the isolation, physical characterization, and dissection of the expression patterns of the single gene encoding 5'-phosphoribosyl-4-(N-succinocarboxamide)-5-aminoimidazole synthetase. This enzyme, encoded by the PUR7 gene, catalyzes aspartate addition at the alpha-amino group to the growing purine backbone. The expression of the PUR7 as directed by the 5' region, containing the promoter, mRNA leader, and leader intron, was examined in Arabidopsis using a transgenic reporter system. Our analysis demonstrates that the highest level of purine biosynthesis occurs in mitotically active tissues of the plant. Furthermore, purine biosynthesis appears to be under developmental and hormonal regulation. Inhibition of purine biosynthesis using substrate analogs results in arrested plant development and induction of purine gene expression. Purine nucleotides and their derivatives provide multiple cofactors for a variety of metabolic processes. Our findings begin to identify some of the regulatory mechanisms that affect the production of purine nucleotides in Arabidopsis and may give important insights into nitrogen metabolism in general.