Abstract
Reductive catabolism of pyrimidine nucleotides occurs via a three-step pathway in which uracil is degraded to β-alanine, CO2 and NH3 through sequential activities of dihydropyrimidine dehydrogenase (EC 1.3.1.2, PYD1), dihydropyrimidinase (EC 3.5.2.2, PYD2) and β-ureidopropionase (EC 3.5.1.6, PYD3).A proposed function of this pathway, in addition to the maintenance of pyrimidine homeostasis, is the recycling of pyrimidine nitrogen to general nitrogen metabolism. PYD expression and catabolism of [2-14C]-uracil are markedly elevated in response to nitrogen limitation in plants, which can utilize uracil as a nitrogen source.PYD1, PYD2 and PYD3 knockout mutants were used for functional analysis of this pathway in Arabidopsis. pyd mutants exhibited no obvious phenotype under optimal growing conditions. pyd2 and pyd3 mutants were unable to catabolize [2-14C]-uracil or to grow on uracil as the sole nitrogen source. By contrast, catabolism of uracil was reduced by only 40% in pyd1 mutants, and pyd1 seedlings grew nearly as well as wild-type seedlings with a uracil nitrogen source. These results confirm PYD1 function and suggest the possible existence of another, as yet unknown, activity for uracil degradation to dihydrouracil in this plant.The localization of PYD-green fluorescent protein fusions in the plastid (PYD1), secretory system (PYD2) and cytosol (PYD3) suggests potentially complex metabolic regulation.
Highlights
Pyrimidine nucleotides are precursors in the synthesis of DNA and RNA, and pyrimidine nucleoside diphosphate sugars are activated intermediates in the synthesis of lipids, sucrose and cell wall polysaccharides (Zrenner et al, 2006)
We further demonstrate the subcellular localization of this pathway using PYDgreen fluorescent protein (GFP) fusions, and provide evidence that recycling of pyrimidine nitrogen to general nitrogen metabolism is a major function of this pathway under conditions of nitrogen limitation
Tsai & Axelrod (1965) reported that 72% and 88% of label from [1-14C]- and [2-14C]-β-alanine, respectively, were found in the lipid fraction in rape seedlings, being derived from the β-alanine aminotransferase (BAT) reaction product malic acid, with further metabolism via an acetate intermediate. These findings suggest that β-alanine derived from Ura catabolism contributes little to pantothenate synthesis in plants, the vast majority of which is metabolized via BAT
Summary
Pyrimidine nucleotides are precursors in the synthesis of DNA and RNA, and pyrimidine nucleoside diphosphate sugars are activated intermediates in the synthesis of lipids, sucrose and cell wall polysaccharides (Zrenner et al, 2006). © The Authors (2009) Journal compilation © New Phytologist (2009). New Phytologist (2009) 183: 117–132 117 www.newphytologist.org 117. In this pathway, uracil (Ura) or thymine (Thy) degradation is initiated by dihydropyrimidine dehydrogenase (DHPDH, EC 1.3.1.2, PYD1) through NAD(P)H-dependent reduction of the C-5, C-6 double bond. In Arabidopsis, each of the enzymes catalysing this three-step pathway is encoded by a single gene (Zrenner et al, 2006)
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