The role of guanosine deaminase in ureide production in soybean nodules and in purine nucleotide catabolism of Arabidopsis

嘌呤核苷酸可以在植物体内充分分解代谢以循环养分。我们最近描述了一种来自拟南芥的酶，鸟苷脱氨酶（GSDA），参与了这一过程。GSDA催化拟南芥中鸟苷脱胺生成黄嘌呤，黄嘌呤在拟南芥中仅由GSDA合成。黄嘌呤的其他来源，如XMP的去磷酸化，似乎与此无关。然而，在大豆中，通常假设通过XMP去磷酸化产生尿素（嘌呤碱分解产物，作为从根瘤到茎部的主要氮转运化合物）。如果这是真的，大豆的GSDA将不需要用于结核中的尿素生产。(1)我们拟通过实验研究大豆根瘤中尿素生成的途径，并评估GSDA在这一过程中的作用。在拟南芥中，我们的初步实验表明GSDA可能对所有或至少大部分嘌呤核苷酸分解代谢（GMP和AMP）至关重要。(2)通过对一系列单突变体和双突变体进行代谢物分析，绘制嘌呤核苷酸分解代谢途径图，并研究GSDA的总体重要性。这将有可能导致对体内嘌呤核苷酸分解代谢的全新看法。GSDA突变体表现出明显的低发芽率和对长时间黑暗敏感等表型。(3)将对GSDA突变体在其他嘌呤核苷酸分解代谢突变体的背景下进行详细的表型分析，以评估和量化观察到的缺陷。我们推测有缺陷的糖再动员可能是造成表型改变的部分原因。GSDA是一种仅存在于植物中的新型酶。微生物具有结构相似的特异鸟嘌呤酶。(4)通过从拟南芥中获得GSDA的晶体结构，我们希望了解该酶鸟苷特异性的分子基础。

Purine nucleotides can be fully catabolized in plants to recycle nutrients. We recently described an enzyme from Arabidopsis, guanosine deaminase (GSDA), involved in this process. GSDA catalyzes the deamination of guanosine to xanthosine which in Arabidopsis is exclusively synthesized by GSDA. Other sources for xanthosine, like the dephosphorylation of XMP, seem not to be relevant. In soybean however, it is generally assumed that the generation of the ureides (purine base breakdown products that serve as primary nitrogen transport compounds from the nodules to the shoot) runs via XMP dephosphorylation. If true, GSDA of soybean would not be required for ureide production in nodules. (1) We propose to experimentally evaluate the route of ureide generation in soybean nodules and assess the role of GSDA in this process. In Arabidopsis, our preliminary experiments indicated that GSDA may be of central importance for all or at least the bulk of purine nucleotide catabolism (of GMP and also AMP). (2) By performing metabolite analyses of a range of single and double mutants, a map of of the purine nucleotide catabolic pathway will be drawn and the overall importance of GSDA investigated. This will potentially result in a completely new view on purine nucleotide catabolism in vivo.Arabidopsis mutants of GSDA show distinct phenotypes like low germination and susceptibility to prolonged darkness. (3) A detailed phenotypic analysis of GSDA mutants in the context of other mutants of purine nucleotide catabolism will be performed to evaluate and quantify the observed defects. We speculate that defective sugar remobilization may be in part responsible for the phenotypic alterations.GSDA is a novel enzyme only found in plants. Microbes have structurally similar enzymes with specificity for guanine. (4) By generating a crystal structure of GSDA from Arabidopsis we would like to understand the molecular basis for the guanosine specificity of this enzyme.