DE-NOVO PURINE SYNTHESIS IN ARABIDOPSIS-THALIANA .2. THE PUR7 GENE ENCODING ORIBOSYL-4-(N-SUCCINOCARBOXAMIDE)-5-AMINOIMIDAZOLE SYNTHETASE IS EXPRESSED IN RAPIDLY DIVIDING TISSUES

The small genome size and excellent genetics of Arabidopsis, as well a s the ease with which it is transformed, make it a superb candidate fo r 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 oribosyl-4-(N-suc cinocarboxamide)-5-aminoimidazole synthetase. This enzyme, encoded by the PUR7 gene, catalyzes aspartate addition at the alpha-amino group t o the growing purine backbone. The expression of the PUR7 as directed by the 5' region, containing the promoter, mRNA leader, and leader int ron, was examined in Arabidopsis using a transgenic reporter system. O ur analysis demonstrates that the highest level of purine biosynthesis occurs in mitotically active tissues of the plant. Furthermore, purin e biosynthesis appears to be under developmental and hormonal regulati on. 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 f or a variety of metabolic processes. Our findings begin to identify so me of the regulatory mechanisms that affect the production of purine n ucleotides in Arabidopsis and may give important insights into nitroge n metabolism in general.