Riemenschneider Anja, Riedel Kerstin, Hoefgen Rainer, Papenbrock Jutta, Hesse Holger
Institut für Botanik, Universität Hannover, D-30419 Hannover, Germany.
Plant Physiol. 2005 Mar;137(3):892-900. doi: 10.1104/pp.104.057125. Epub 2005 Feb 22.
Plant cysteine (Cys) synthesis can occur in three cellular compartments: the chloroplast, cytoplasm, and mitochondrion. Cys formation is catalyzed by the enzyme O-acetylserine(thiol)lyase (OASTL) using O-acetylserine (OAS) and sulfide as substrates. To unravel the function of different isoforms of OASTL in cellular metabolism, a transgenic approach was used to down-regulate specifically the plastidial and cytosolic isoforms in potato (Solanum tuberosum). This approach resulted in decreased RNA, protein, and enzymatic activity levels. Intriguingly, H(2)S-releasing capacity was also reduced in these lines. Unexpectedly, the thiol levels in the transgenic lines were, regardless of the selected OASTL isoform, significantly elevated. Furthermore, levels of metabolites such as serine, OAS, methionine, threonine, isoleucine, and lysine also increased in the investigated transgenic lines. This indicates that higher Cys levels might influence methionine synthesis and subsequently pathway-related amino acids. The increase of serine and OAS points to suboptimal Cys synthesis in transgenic plants. Taking these findings together, it can be assumed that excess OASTL activity regulates not only Cys de novo synthesis but also its homeostasis. A model for the regulation of Cys levels in plants is proposed.
植物半胱氨酸（Cys）的合成可发生在三个细胞区室中：叶绿体、细胞质和线粒体。半胱氨酸的形成由O - 乙酰丝氨酸（硫醇）裂解酶（OASTL）催化，以O - 乙酰丝氨酸（OAS）和硫化物作为底物。为了阐明OASTL不同同工型在细胞代谢中的功能，采用转基因方法特异性下调马铃薯（Solanum tuberosum）中的质体和胞质同工型。该方法导致RNA、蛋白质和酶活性水平降低。有趣的是，这些株系中释放H₂S的能力也降低了。出乎意料的是，无论选择哪种OASTL同工型，转基因株系中的硫醇水平都显著升高。此外，在所研究的转基因株系中，丝氨酸、OAS、蛋氨酸、苏氨酸、异亮氨酸和赖氨酸等代谢物的水平也有所增加。这表明较高的半胱氨酸水平可能会影响蛋氨酸的合成以及随后与该途径相关的氨基酸。丝氨酸和OAS的增加表明转基因植物中的半胱氨酸合成未达到最佳状态。综合这些发现，可以推测过量的OASTL活性不仅调节半胱氨酸的从头合成，还调节其稳态。本文提出了一个植物中半胱氨酸水平调控的模型。
