The role of plastid-encoded RNA polymerase-associated proteins (PAPs) in chloroplast biogenesis of plants

叶绿体是植物进行光合作用的特殊细胞器。它们代表了包括人类在内的陆地生物圈的能量和代谢支持的结构和功能基础。没有叶绿体，我们今天所知的高级生命就不可能存在。尽管这些细胞器在地球上的生命中扮演着重要的角色，但对它们生物发生的基本分子调控步骤的理解仍然难以捉摸。叶绿体通常来自未分化的前体，即原质体，或来自停滞的发育阶段，即病质体。这些分化过程是严格依赖于光的，从黑暗生长的幼苗暴露在光下变得明显。在几个小时内，黄色的、无叶绿素的子叶变成绿色，并发育出光合作用活跃的叶绿体。这需要对数千个编码叶绿体结构和功能成分的基因进行主动和快速的调控。这些基因大多在细胞核中编码，其基因产物需要经过细胞质翻译后导入质体。然而，一小部分基因被编码在质体自身的基因组——质体中，并且需要细胞器自身来表达。因此，为了叶绿体的成功生物发生，细胞核和质体基因组的紧密协调是必不可少的。在这种协调中起重要作用的是质体编码RNA聚合酶（PEP）。这种酶与另一种核编码RNA聚合酶（NEP）一起转录质体的基因。PEP由四个核心亚基组成，这些亚基与代表质体内共生祖先残余的原核RNA聚合酶高度相似。所有四个亚基都在质体中编码，并仅由NEP酶表达。生化和分子遗传学实验随后表明，PEP核心复合体在叶绿体生物发生的初始阶段通过物理添加12个核编码亚基进一步扩增。这些额外的亚基被命名为pep相关蛋白（pap），它们非常重要，因为缺乏它们中的任何一个都会阻碍叶绿体的发育，最终导致相应突变体的白化表型。因此，pap显然代表了叶绿体生物发生的关键成分。在这个项目中，我们旨在揭示它们在叶绿体生物发生中的具体作用。我们强调研究它们在光敏色素介导的光信号网络中的参与，它们的表达调控以及它们在PEP复合体中的时间依赖性组装。将特别关注它们作为从质体向细胞核的逆行信号的潜在功能，这是一个令人着迷的新想法，这一尚未被理解的信号通路可能使质体控制其自身的生物发生。

Chloroplasts are the specific cell organelles of plants which perform the process of photosynthesis. They represent the structural and functional base for the energetic and metabolic support of the terrestrial biosphere including mankind. Without chloroplasts life of higher order as we know it today would be impossible. Despite this eminent role of these organelles for life on Earth understanding of essential molecular regulatory steps of their biogenesis is still elusive. Chloroplasts typically emerge from undifferentiated precursors, the proplastids, or from arrested developmental stages, the etioplasts. These differentiation processes are strictly light-dependent as becoming evident from a dark-grown seedling exposed to light. Within a few hours the yellow, chlorophyll-free cotyledons become green and develop photosynthetically active chloroplasts. This requires active and rapid regulation of thousands of genes that encode structural and functional components of chloroplasts. Most of these genes are encoded in the cell nucleus and their gene products need to be imported into the plastids after cytosolic translation. However, a small set of genes is encoded in the plastid-own genome, the plastome, and need to be expressed by the organelles themselves. Therefore, for a successful biogenesis of chloroplasts a tight coordination of the nuclear and plastid genomes is mandatory. An important key role in this coordination plays the plastid encoded RNA polymerase (PEP). This enzyme transcribes the genes of the plastome together with another nuclear encoded RNA polymerase (NEP). The PEP is comprised of four core subunits that are highly similar to those of prokaryotic RNA polymerases representing a remnant of the endosymbiotic ancestry of plastids. All four subunits are encoded in the plastome and are expressed exclusively by the NEP enzyme. Biochemical and molecular-genetic experiments have subsequently shown, that the PEP core complex is further expanded by the physical addition of 12 nuclear-encoded subunits during the initial steps of chloroplast biogenesis. These additional subunits were named PEP-associated proteins (PAPs) and they are of eminent importance since lack of any of them blocks chloroplast development ending finally in an albino phenotype of the corresponding mutant. PAPs, thus, represent apparently key components of chloroplast biogenesis. In this project we aim to unravel their specific role during chloroplast biogenesis. We emphasize to study their involvement in the phytochrome-mediated light signalling network, their expression regulation as well a their time-dependent assembly into the PEP complex. Special focus will be given to their potential function as retrograde signals from plastids towards the cell nucleus, a fascinating new idea for this still not understood signalling route that potentially allows plastids to control their own biogenesis.