Abstract：
Cysteine biosynthesis is essential for translation and represents the entry point of reduced sulfur into plant metabolism. The two consecutively acting enzymes serine acetyltransferase (SAT) and O-acetylserine-thiol-lyase catalyse cysteine production and form the cysteine synthase complex, in which SAT is activated. Here we show that tobacco (Nicotiana tabacum) expressing active SAT in plastids (referred to as PSA lines) shows substantial cysteine accumulation in plastids. Remarkably, enhanced cysteine production in plastids entirely abolished granal stack formation, impaired photosynthesis capacity, and decreased the number of chloroplasts in mesophyll cells of the PSA lines. A transgenic tobacco line expressing active SAT in the cytosol accumulated comparable amounts of thiols but displayed no phenotype. To dissect the consequences of cysteine synthase complex formation from enhanced SAT activity in tobacco plastids, we expressed an enzymatically inactive SAT that can still form the cysteine synthase complex in tobacco plastids (PSI lines). The PSI lines were indistinguishable from the PSA lines, although the PSI lines displayed no increase in plastid-localized SAT activity. Neither PSA lines nor PSI lines suffered from an oxidized redox environment in plastids that could have been causative for the disturbed photosynthesis. From these findings, we infer that the association of the plastid cysteine synthase complex itself triggers a signaling cascade controlling sulfur assimilation and photosynthetic capacity in leaves.
摘要：
半胱氨酸生物合成对于翻译至关重要，并且代表了减少硫进入植物代谢的切入点。两种连续作用的酶丝氨酸-乙酰转移酶（SAT）和O-乙酰丝氨酸-硫醇-裂解酶催化半胱氨酸的产生并形成半胱氨酸合酶复合物（CSC），其中SAT被激活。在这里，我们表明活性SAT在烟草质体（PSA）中的表达会导致大量的半胱氨酸积累。值得注意的是,增强质体中半胱氨酸的产生完全消除了颗粒堆的形成，光合作用能力受损，并减少了PSA系的叶肉细胞中叶绿体的数量。在胞质溶胶（CSA）中表达活性SAT的转基因烟草品系积累了相当量的硫醇，但未表现出表型。为了剖析烟草质体中SAT活性增强对CSC形成的影响，我们表达了一种无酶活性的SAT，它仍然可以在烟草质体（PSI）中形成CSC。PSI线与PSA线没有区别，尽管PSI系显示质体定位的SAT活性没有增加。PSA品系和PSI品系都没有受到质体中氧化的氧化还原环境的影响，这可能是光合作用受到干扰的原因。从这些发现中,我们推断质体CSC本身的结合会触发信号级联，控制叶片中的硫同化和光合能力。
