Paths to Pyrimidines 16 April

In presenting pyrimi-dine metabolism, most modern biochemistry texts contain a figure describing metabolism in bacterial cells and the alternative in animal cells. For plant biochemists, this discrepancy is disconcerting. Admittedly, py-rimidine metabolism has been largely overlooked in plants in favor of more glamorous endeavors. However, the importance of pyrimidines to almost all aspects of cellular biochemistry has led us to investigate pyrimidine metabolism in plants with the goal of elucidating these pathways. Plants are true eukary-otes, so their genes are not organized in polycistrons, but rather are individually encoded and scattered throughout the genome like animal genes. The transcriptional machinery in plants differs from bacterial systems but is similar to the transcriptional machinery found in animal cells. Most of the work on pyrimidine metabolism in plants has been done in a few species, notably tobacco (Nicotiana tabacum) and pea (Pisum sativum); however , within the past decade, a small weed known as field pennycrest or Arabidopsis (Arabidopsis thaliana) has become widely accepted as a " model " plant. The Arabidopsis genome project has completely sequenced about 40% of the Arabidopsis genome, so pyrimi-dine metabolism is being best characterized in Arabidopsis. Most of what we will present in this article, unless otherwise noted, is work based upon this plant. Pyrimidine biosynthesis In animals, the first three enzymes of pyrimidine biosynthesis, Carbamoylphos-phate synthase (CPSase), Aspar-tate transcarbamoylase (ATCase) and Dihydroorotase (DHOase) have been fused during evolution to form a single polypeptide called CAD 1. In contrast, the genomic organization of pyrimidine biosynthesis in plants is more like that of prokaryotes. Plants have individually encoded CPSases, ATCases and DHOases. Carbamoylphosphate synthase Carbamoylphosphate Synthase (CPSase) is encoded within the nucleus, but the mature protein is targeted to and functions within the chlo-roplast. This enzyme responsible for the production of carbamoylphosphate for both pyrimidine and arginine bio-synthesis. Ornithine stimulates incorporation of NaHCO 3 into UMP and arginine. However, in the presence of exogeneously added uridine, incorporation of NaHCO 3 into UMP was reduced while incorporation into argin-ine was unaffected 2. Two cDNAs have recently been cloned which encode the small and large sub-unit (carA and carB) of this enzyme 3, 4. Like other enzymes of the de novo pathway, CPSase is more similar to a prokaryotic enzyme than the eukaryotic homologues. The small subunit shows 46% identity to the E. coli enzyme and 51% identity to the Synechocystis enzyme. The large subunit shares 66% identity to …