RNA Splicing in Chloroplasts: Mechanism and Function of Group I Introns

叶绿体基因组大约包含100个基因；它们的适当表达是光合作用和叶绿体发育所必需的。许多叶绿体基因表达的关键步骤是RNA剪接；将内部序列（内含子）切除，并精确定位编码序列（外显子）的过程。比较序列分析表明，大多数叶绿体内含子可归类为I类或II类，但关于叶绿体中RNA剪接机制的直接信息很少。本研究结果表明：(1)衣藻体内rrn 23S内含子的有效剪接需要至少一个核基因产物；(2)23S preRNA在体外具有自剪接能力。初步证据表明衣藻光系统II的psbA基因内含子和其他未确定的I族内含子具有自剪接能力。为了尽可能地阐明叶绿体I族内含子剪接的机制和控制，我们提出了相关的研究。体外（自剪接）和体内（遗传）方法都将被利用。了解叶绿体中RNA剪接的机制和控制至少有两个原因是重要的。首先是因为植物是这个星球上生物有用能量的主要来源。植物通过光合作用获得能量，而光合作用至少在一定程度上依赖于叶绿体编码基因的表达和调控。其次，真核生物中内含子的起源、功能和剪接有许多未解之谜；交互作用因子的作用是特别感兴趣的，因为它们可能在调节内含子相关过程中起作用。虽然叶绿体基因组与植物细胞的核基因组和线粒体基因组共同进化，但它有着不同的起源和许多独特的特征。因此，这可能为内含子参与的细胞和分子过程提供新的基础信息。

The chloroplast genome contains on the order of 100 genes; the proper expression of which are necessary for photosynthesis and chloroplast development. A critical step in the expression of a number of chloroplast genes is RNA splicing; the process where internally-located sequences (introns) are excised, and coding sequences (exons) are precisely religated. Comparative sequence analysis suggests that most chloroplast introns can be classified as Group I or Group II, however, there is very little direct information on the mechanism of RNA splicing in chloroplasts. Here, results are presented which indicate that (1) at least one nuclear gene product is required for efficient splicing of the rrn 23S intron of Chlamydomonas in vivo and, (2) 23S preRNA is capable of self-splicing in vitro. Preliminary evidence is also presented which suggests that introns of the psbA gene of Photosystem II and other unidentified Group I introns in Chlamydomonas are capable of self-splicing. Studies are proposed to elucidate, as far as possible, the mechanism and control of splicing of chloroplast Group I introns. Both in vitro (self-splicing) and in vivo (genetic) approaches will be utilized. Understanding the mechanism and control of RNA splicing in chloroplasts is important for at least two reasons. The first is due to the fact that plants are the primary source of biologically useful energy on this planet. Plants obtain energy through the process of photosynthesis, which is dependent, at least in part, on the expression and regulation of chloroplast-encoded genes. Secondly, there are many unanswered questions concerning the origin, function, and splicing of introns in eucaryotes; the role of trans-acting factors are of particular interest because of their probable role in regulating intron-related processes. Although the chloroplast genome has coevolved with the nuclear and mitochondrial genomes of plant cells, it has a distinct origin and many unique characteristics. Therefore, could be a useful source a new basic information on cellular and molecular processes that involve introns.