The Role of the Cia5 Gene in the Carbon Concentrating Mechanism in Chlamydomonas Reinhardtii

碳浓缩机制(CCM)对地球的光合作用生产力和碳循环能力起着重要作用。在衣藻和大多数其他真核和原核藻中，CCM对于大气中二氧化碳水平的光合作用是必不可少的。通过这一为期三年的项目获得的关于莱茵梭子藻CCM的知识，加上来自其他实验室的信息，应该能够更全面地了解藻细胞如何能够在二氧化碳有限的水环境中生存和繁衍。本研究的主要目的是阐明莱茵衣藻CCM的组成和调控机制。该项目的重点是对Cia5基因的完整特征及其作为细胞对环境CO2和HCO3浓度变化的反应的主调节器的作用。根据现有证据的权重，似乎Cia5基因编码一种转录因子，该转录因子是一个或多个允许或逆转形成功能性CCM的信号转导级联的一部分。本文概述了对CIA5基因进行全面研究的实验，以及对CIA5蛋白、其亚细胞定位、翻译后修饰(S)、DNA结合特性以及它在控制CCM组装中所需的其他功能的研究。提出了一个模型，在该模型中，CIA5蛋白既可以作为转录的诱导者，也可以作为转录抑制因子，在细胞从高二氧化碳水平向低二氧化碳水平转变的过程中，这些基因的表达显著增加。概述了将解决CIA5是作为基因表达的诱导者还是抑制者的不确定性的实验。从长远来看，对CCM的理解增加有可能在农业中实际应用。例如，某些作物已被证明在二氧化碳水平升高的情况下产量更高。如果CCM的适当部分能够被转移到这些作物上，它们增加内部二氧化碳浓度的结果可能会导致作物产量的大幅提高。

The carbon concentrating mechanism (CCM) plays a major role in the earth'sphotosynthetic productivity and its capacity to cycle carbon. The CCM isessential for photosynthesis at atmospheric levels of CO2 in Chlamydomonasand most other eukaryotic and prokaryotic algae. Knowledge gainedregarding the CCM in C. reinhardtii through this three-year project,coupled with information from other laboratories, should allow a morecomplete understanding of how algal cells are able to survive and flourishin their CO2-limited, aqueous environment. The primary goal of thisresearch project is to elucidate components and mechanisms of regulation ofthe CCM in Chlamydomonas reinhardtii. The focus of the project is thecomplete characterization of the Cia5 gene and its role as a masterregulator of the cell's response to changes in environmental CO2 and HCO3concentrations. Based on the weight of present evidence, it appears theCia5 gene encodes a transcription factor that is part of one or more signaltransduction cascades that allow, or reverse, formation of a functionalCCM. Experiments to fully characterize the Cia5 gene are outlined alongwith studies of the CIA5 protein, its subcellular location,post-translational modification(s), DNA binding specificities and otherfeatures required for its function in controlling assembly of the CCM. Amodel is proposed in which the CIA5 protein could act either as an induceror repressor of transcription of genes whose expression is dramaticallyincreased during a shift of cells from high to low CO2 levels. Experimentsare outlined that will resolve the uncertainty of whether CIA5 acts as aninducer or repressor of gene expression. In the long term, an increasedunderstanding of the CCM has the potential for practical application inagriculture. For example, certain crop plants have been demonstrated toproduce higher yields under elevated levels of CO2. If appropriateportions of the CCM can be transferred to those crops, their resultantability to increase internal CO2 concentrations could lead to substantiallyimproved crop yields.