Schwartzbach Steven D
Department of Biological Sciences, University of Memphis, Memphis, TN, 38152, USA.
Adv Exp Med Biol. 2017;979:159-182. doi: 10.1007/978-3-319-54910-1_9.
Euglena can use light and CO, photosynthesis, as well as a large variety of organic molecules as the sole source of carbon and energy for growth. Light induces the enzymes, in this case an entire organelle, the chloroplast, that is required to use CO as the sole source of carbon and energy for growth. Ethanol, but not malate, inhibits the photoinduction of chloroplast enzymes and induces the synthesis of the glyoxylate cycle enzymes that comprise the unique metabolic pathway leading to two carbon, ethanol and acetate, assimilation. In resting, carbon starved cells, light mobilizes the degradation of the storage carbohydrate paramylum and transiently induces the mitochondrial proteins required for the aerobic metabolism of paramylum to provide the carbon and energy required for chloroplast development. Other mitochondrial proteins are degraded upon light exposure providing the amino acids required for the synthesis of light induced proteins. Changes in protein levels are due to increased and decreased rates of synthesis rather than changes in degradation rates. Changes in protein synthesis rates occur in the absence of a concomitant increase in the levels of mRNAs encoding these proteins indicative of photo and metabolic control at the translational rather than the transcriptional level. The fraction of mRNA encoding a light induced protein such as the light harvesting chlorophyll a/b binding protein of photosystem II, (LHCPII) associated with polysomes in the dark is similar to the fraction associated with polysomes in the light indicative of photoregulation at the level of translational elongation. Ethanol, a carbon source whose assimilation requires carbon source specific enzymes, the glyoxylate cycle enzymes, represses the synthesis of chloroplast enzymes uniquely required to use light and CO as the sole source of carbon and energy for growth. The catabolite sensitivity of chloroplast development provides a mechanism to prioritize carbon source utilization. Euglena uses all of its resources to develop the metabolic capacity to utilize carbon sources such as ethanol which are rarely in the environment and delays until the rare carbon source is no longer available forming the chloroplast which is required to utilize the ubiquitous carbon source, light and CO.
眼虫藻可以利用光和二氧化碳进行光合作用，也能将多种有机分子作为生长所需的唯一碳源和能源。光会诱导相关酶的产生，在此情况下是诱导整个细胞器——叶绿体的形成，叶绿体是利用二氧化碳作为生长所需的唯一碳源和能源时所必需的。乙醇而非苹果酸会抑制叶绿体酶的光诱导，并诱导乙醛酸循环酶的合成，乙醛酸循环酶构成了导致二碳化合物（乙醇和乙酸盐）同化的独特代谢途径。在静止、碳饥饿的细胞中，光促使储存碳水化合物副淀粉的降解，并短暂诱导副淀粉有氧代谢所需的线粒体蛋白，以提供叶绿体发育所需的碳和能量。其他线粒体蛋白在光照下会被降解，从而提供合成光诱导蛋白所需的氨基酸。蛋白质水平的变化是由于合成速率的增加和降低，而非降解速率的变化。蛋白质合成速率的变化发生在编码这些蛋白质的mRNA水平没有相应增加的情况下，这表明是在翻译水平而非转录水平上进行光和代谢调控。编码光诱导蛋白（如光系统II的捕光叶绿素a/b结合蛋白，LHCPII）的mRNA在黑暗中与多核糖体结合的比例，与在光照下与多核糖体结合的比例相似，这表明在翻译延伸水平上存在光调节。乙醇是一种其同化需要特定碳源酶（乙醛酸循环酶）的碳源，它会抑制利用光和二氧化碳作为生长所需的唯一碳源和能源时所特有的叶绿体酶的合成。叶绿体发育的分解代谢敏感性提供了一种优先利用碳源的机制。眼虫藻会利用其所有资源来发展利用乙醇等环境中罕见碳源的代谢能力，并推迟形成利用普遍存在的碳源（光和二氧化碳）所需的叶绿体，直到这种罕见碳源不再可用。
