Interaction of sulfur and nitrogen metabolism: significance of different serine sources for cysteine biosynthesis

半胱氨酸是植物中所有含硫有机硫化合物的代谢前体，包括蛋氨酸、谷胱甘肽和硫代葡萄糖苷，是蛋白质生物合成所必需的。半胱氨酸是由还原硫和O-乙酰丝氨酸合成的，后者的硫受体是丝氨酸和乙酰辅酶A在丝氨酸乙酰转移酶作用下生成的。尽管参与半胱氨酸生物合成的所有酶步骤都已在分子水平上被确定，但丝氨酸提供到半胱氨酸生物合成的途径在很大程度上仍不清楚。目前也不清楚半胱氨酸生物合成中对O-乙酰丝氨酸的需求如何与碳和氮代谢相协调。本研究旨在阐明叶绿体/叶绿体与线粒体丝氨酸的合成对半胱氨酸生物合成的意义，并鉴定参与调节半胱氨酸生物合成与碳、氮代谢相互作用的植物激素和基因。本研究要检验的中心假设是：(I)光呼吸产生的丝氨酸是自养组织在光下合成半胱氨酸的主要前体；(Ii)叶绿体/叶绿体中的丝氨酸生物合成在黑暗中接管自养组织，并在异养组织中占主导地位；(Iii)光和暗期间半胱氨酸合成所需的丝氨酸供应的差异受到植物激素(特别是细胞分裂素)作用和基因表达的调节。为了验证这些假说，我们将在丝氨酸生物合成的不同途径中使用已建立的和新的突变体。此外，我们还将通过调节Rubisco的氧分压来改变光呼吸丝氨酸生物合成途径的通量，从而改变Rubisco的氧化反应。突变体和野生型植物将暴露在半胱氨酸需求扩大的条件下，并在代谢、植物激素含量和mRNA转录本的水平上进行比较。这些实验将共同揭示不同生理状态和扩大半胱氨酸需求条件下半胱氨酸生物合成的碳供应途径。

Cysteine is the metabolic precursor of all thiol containing organic sulfur compounds in plants, including methionine, glutathione, and glucosinolates; it is essential for protein biosynthesis. Cysteine is synthesized from reduced sulfur and O-acetylserine, the latter sulfur acceptor being generated from serine and acetyl CoA by the enzyme serine acetyltransferase. Although all enzymatic steps involved in cysteine biosynthesis have been identified at the molecular level, the route of serine provision to cysteine biosynthesis is still largely unknown. It is currently also not understood how the demand for O-acetylserine in cysteine biosynthesis is coordinated with carbon and nitrogen metabolism. This proposal aims at elucidating the significance of chloroplastic/plastidic versus mitochondrial serine production for the biosynthesis of cysteine and to identify phytohormones and genes involved in the regulation of the interaction cysteine biosynthesis with carbon and nitrogen metabolism in A. thaliana. The central hypotheses to be tested in this research are that (i) serine generated by photorespiration in the mitochondira is the dominant precursor of cysteine synthesis in autotrophic tissues in the light; (ii) serine biosynthesis in the chloroplasts/plastids takes over in autotrophic tissues in the dark and is dominant in heterotrophic tissues; (iii) the differences in serine supply for cysteine synthesis during light and dark are subject to regulation by phytohormone (in particular cytokinin) action and gene expression. To test these hypotheses, we will employ established and novel mutants in the various pathways for serine biosynthesis. In addition, we will vary the flux through the photorespiratory serine biosynthesis pathway by modulating the oxygen partial pressure and thereby the oxygenation reaction of RubisCO. Mutants and wild type plants will be exposed to conditions of expanded cysteine demand and comparatively profiled at the levels of metabolism, phytohormone contents, and mRNA transcripts. Jointly, these experiments will reveal the path of carbon supply to cysteine biosynthesis during different physiological states and under conditions of expanded cysteine demand.