Section of Molecular Cell and Developmental Biology, Department of Molecular Biosciences and Institute for Cell and Molecular Biology, University of Texas at Austin.
Mol Biol Evol. 2013 Dec;30(12):2666-75. doi: 10.1093/molbev/mst163. Epub 2013 Sep 18.
Reverse transcription of mRNA is thought to be an important first step in a model that explains certain evolutionary changes within genes, such as the loss of introns or RNA editing sites. In this model, reverse transcription of mRNA produces cDNA molecules that replace part of the parental gene by homologous recombination. In vivo evidence of reverse transcription of physiologically relevant mRNAs is generally lacking, however, except in genetically engineered cells. Here, we provide in vivo evidence for reverse transcription of the chloroplast psbA mRNA in two naturally occurring species of Chlamydomonas (raudensis and subcaudata) that is based on the presence of spliced cDNAs in both organisms. The psbA cDNAs, which lack the group II intron of the genomic gene, are nearly full length, and the majority of them--though not all--are in the form of RNA-cDNA hybrids. Moreover, the presence in these species of psbA cDNAs is correlated with the loss of an early group I intron from the same psbA gene. The group II intron that interrupts psbA in C. raudensis and C. subcaudata potentially encodes a protein with a reverse transcriptase domain, and the C. raudensis protein was shown to have reverse transcriptase activity in vitro. These results provide strong evidence for reverse transcription of a physiologically important mRNA (psbA) in two species of Chlamydomonas that have also lost an intron from the same gene, possibly through recombination with the cDNA.
mRNA 的反转录被认为是解释基因内某些进化变化的模型中的重要第一步，例如内含子或 RNA 编辑位点的丢失。在该模型中，mRNA 的反转录产生 cDNA 分子，通过同源重组取代部分亲本基因。然而，除了在基因工程细胞中外，通常缺乏生理相关 mRNA 反转录的体内证据。在这里，我们基于两个自然发生的衣藻物种（raudensis 和 subcaudata）中存在拼接 cDNA 提供了 psbA mRNA 反转录的体内证据。这些 psbA cDNA 缺乏基因组基因的组 II 内含子，几乎全长，并且它们中的大多数——尽管不是全部——是以 RNA-cDNA 杂交体的形式存在。此外，这些物种中存在 psbA cDNA 与同一 psbA 基因的早期组 I 内含子丢失有关。在 C. raudensis 和 C. subcaudata 中断 psbA 的组 II 内含子可能编码具有逆转录酶结构域的蛋白质，并且 C. raudensis 蛋白在体外显示出逆转录酶活性。这些结果为两个已经失去同一基因内含子的衣藻物种中生理重要的 mRNA（psbA）的反转录提供了有力证据，可能是通过与 cDNA 的重组。
