A Window into the Early Steps in Plastid Evolution

这项拟议的工作解决了光合作用细胞器--叶绿体--进化的方式问题。这些研究中使用的有机体是原生动物Paulinella chroophora，它的细胞质内含有看起来像蓝藻的东西，尽管这种内部的“蓝藻”实际上是一种叫做生色团的细胞器。尽管色素体看起来不像叶绿体，但它执行许多与叶绿体相同的功能，捕捉阳光来驱动光合作用，光合作用为原生生物宿主细胞提供生长所需的能量和碳骨架。色素体包含自己的基因组(DNA)，就像叶绿体一样，但这个基因组的大小是大多数叶绿体基因组的5-10倍，但比蓝藻的基因组小得多。这些发现表明，色团是由内共生蓝藻进化成叶绿体过程中的细胞器，因此它可以揭示叶绿体进化早期事件关键机制的重要信息。我们最近发现，许多对生色团的功能至关重要的蛋白质是在生色团外和宿主原生生物细胞的细胞质中合成的，必须运输到它们发挥功能的细胞器中(例如，PsaE，一种光合作用的蛋白质)。我们研究的主要目的是识别这些蛋白质，并了解它们是如何从寄主原生生物的细胞质进入新生的叶绿体，在那里它们组装成大型功能复合体。从广义上讲，这项工作对于我们理解细胞器进化的过程以及两个生物体如何结合在一起并将它们的功能整合成一个高效的单一有机体都是至关重要的。这项工作的结果以及由此产生的概念将产生广泛的智力影响，因为它们将：(1)阐明与蛋白质进入发育中的细胞器有关的早期事件；(2)建议哪些蛋白质最适合(基于其生物物理性质)通过宿主生物体中已存在的系统进入发育中的叶绿体；(3)允许识别哪些蛋白质可能需要更广泛的剪裁才能进入细胞器(它们可能必须通过随进化时间发展的更专门的系统进入叶绿体)；(4)提供对调控事件发展的更广泛的了解，这些调控事件整合了细胞质中蛋白质的合成和叶绿体中合成的蛋白质；(5)提出了新的方法，使蛋白质可以被设计成使它们进入特定的亚细胞隔室，这可能对合成生物学的各个方面和定制细胞制造特定的产品，如生物燃料，可能是重要的；(6)培训研究生进行实验，使用分子和生化工具和技术来测试进化的概念；(7)纳入斯坦福大学正在教授的一门本科课程，该课程侧重于光合作用过程的功能、进化和调控(从光合作用到生物燃料)。

INTELLECTUAL MERITThe proposed work addresses the question of the way in which the photosynthetic organelle, the chloroplast, evolved. The organism used in these studies is the thecate protist Paulinella chromatophora, which contains what visually resembles a cyanobacterium within its cytoplasm, although this internal "cyanobacterium" is really an organelle called a chromatophore. Even though the chromatophore does not look like a chloroplast, it performs many of the same functions as chloroplasts, capturing sunlight to drive photosynthesis, which provides the protist host cell with energy and carbon backbones for growth. The chromatophore contains its own genome (DNA), like a chloroplast, but the size of this genome is 5-10 times more than what is found in most chloroplasts, but much less than what is found in cyanobacteria. These findings suggest that the chromatophore is an organelle derived from an endosymbiotic cyanobacterium that is in the process of evolving into a chloroplast, and as such it can reveal important information about mechanisms critical for the early events in chloroplast evolution. We recently discovered that a number of proteins important for the function of the chromatophore are synthesized outside of the chromatophore and in the cytoplasm of the host protist cell, and must be transported into the organelle where they function (e.g. PsaE, a protein that functions in photosynthesis). The major specific thrust of our studies is to identify those proteins and to develop an understanding of how they move from the cytoplasm of the host protist into the nascent chloroplast, where they assemble into large functional complexes. BROADER IMPACTSIn a broad sense, this work is critical to our understanding of both the processes by which cellular organelles evolved and the ways in which two organisms can come together and integrate their functions into a highly efficient single organism. The result of this work and the concepts that grow out of those results will have broad intellectual impact in that they will: (1) shed light on early events associated with the transport of proteins into developing organelles; (2) suggest which proteins are most suited (based on their biophysical properties) to enter developing chloroplasts through systems that already exist in the host organism; (3) allow for the identification of proteins that may require more extensive tailoring before they can assume the right conformation to enter organelles (they may have to enter the chloroplast through a more specialized system that develops over evolutionary time); (4) provide a broader understanding of the development of regulatory events that have integrated the synthesis of proteins in the cytoplasm of the cell with those synthesized in the chloroplast; (5) suggest new ways in which proteins can be engineered to get them into specific subcellular compartments, which may be important for aspects of synthetic biology and the tailoring cells to make specific products such as biofuels; (6) train graduate students to perform experiments that test evolutionary concepts using molecular and biochemical tools and techniques; (7) be incorporated into an undergraduate course that is being taught at Stanford University that focuses on the functionality, evolution and regulation of photosynthetic processes ("From Photosynthesis to Biofuels").