Devarenne Timothy P, Ghosh Anirban, Chappell Joe
Agronomy Department, University of Kentucky, Lexington 40546, USA.
Plant Physiol. 2002 Jul;129(3):1095-106. doi: 10.1104/pp.001438.
Squalene synthase (SS) represents a putative branch point in the isoprenoid biosynthetic pathway capable of diverting carbon flow specifically to the biosynthesis of sterols and, hence, is considered a potential regulatory point for sterol metabolism. For example, when plant cells grown in suspension culture are challenged with fungal elicitors, suppression of sterol biosynthesis has been correlated with a reduction in SS enzyme activity. The current study sought to correlate changes in SS enzyme activity with changes in the level of the corresponding protein and mRNA. Using an SS-specific antibody, the initial suppression of SS enzyme activity in elicitor-challenged cells was not reflected by changes in the absolute level of the corresponding polypeptide, implicating a post-translational control mechanism for this enzyme activity. In comparison, the absolute level of the SS mRNA did decrease approximately 5-fold in the elicitor-treated cells, which is suggestive of decreased transcription of the SS gene. Study of SS in intact plants was also initiated by measuring the level of SS enzyme activity, the level of the corresponding protein, and the expression of SS gene promoter-reporter gene constructs in transgenic plants. SS enzyme activity, polypeptide level, and gene expression were all localized predominately to the shoot apical meristem, with much lower levels observed in leaves and roots. These later results suggest that sterol biosynthesis is localized to the apical meristems and that apical meristems may be a source of sterols for other plant tissues.
鲨烯合酶(SS)是类异戊二烯生物合成途径中的一个假定分支点,能够将碳流专门转向甾醇的生物合成,因此被认为是甾醇代谢的一个潜在调控点。例如,当悬浮培养的植物细胞受到真菌激发子的刺激时,甾醇生物合成的抑制与SS酶活性的降低相关。当前的研究旨在将SS酶活性的变化与相应蛋白质和mRNA水平的变化联系起来。使用一种SS特异性抗体,在受到激发子刺激的细胞中,SS酶活性的最初抑制并未反映在相应多肽绝对水平的变化上,这意味着该酶活性存在翻译后控制机制。相比之下,在经激发子处理的细胞中,SS mRNA的绝对水平确实下降了约5倍,这表明SS基因的转录减少。通过测量SS酶活性水平、相应蛋白质水平以及转基因植物中SS基因启动子 - 报告基因构建体的表达,也开始了对完整植物中SS的研究。SS酶活性、多肽水平和基因表达主要都定位于茎尖分生组织,在叶片和根中观察到的水平要低得多。这些后期结果表明,甾醇生物合成定位于顶端分生组织,并且顶端分生组织可能是其他植物组织甾醇的来源。