Mazan S, Michot B, Bachellerie J P
Centre de Biochimie et Génétique Cellulaires du CNRS, Université Paul Sabatier, Toulouse, France.
Eur J Biochem. 1989 May 15;181(3):599-605. doi: 10.1111/j.1432-1033.1989.tb14766.x.
We have characterized three mouse U3-RNA-processed pseudogenes. Together with previous analyses of mouse functional genes and of rat genes and pseudogenes, these data provide some insights into the processes of U3 retrogene formation during the evolution of rodents. All the mouse retrogenes correspond to a full-length U3B-coding sequence with a 3'-poly(A) tail and are precisely flanked by a pair of direct repeats, in agreement with formation through an RNA intermediate followed by insertion at staggered nicks in the genome. All the rodents U3 retrogenes identified so far are produced from a U3B-RNA form, with two of them (one for each rodent) formed from a 3'-elongated U3-RNA precursor, pointing to the particular susceptibility of RNA precursors forms to serve as templates for retrogene formation. Rodent full-length U3 retrogenes are not inserted randomly in the genome but are systematically found in a context of simple sequences, prone to slipped-strand mispairings and likely to favour the appearance of single-stranded DNA. Moreover their flanking repeats share not only the same size (15 bp) but also common sequence features (which extend to vicinal upstream nucleotides) suggesting that common mechanisms, specific to this class of retrogenes, have been involved in their formation. For the first time, a model accounting for the generation of full-length cDNA copies from nonpolyadenylated RNA templates is proposed. Rodent U3 retrogenes appear to be of rather recent origin (posterior to the mouse/rat divergence) and some of them could have undergone subsequent genetic exchanges with the functional genes.
我们已对三个小鼠U3-RNA加工的假基因进行了特征描述。连同先前对小鼠功能基因以及大鼠基因和假基因的分析，这些数据为啮齿动物进化过程中U3反转录基因形成的过程提供了一些见解。所有小鼠反转录基因都对应一个带有3'-聚腺苷酸尾巴的全长U3B编码序列，并且精确地两侧都有一对同向重复序列，这与通过RNA中间体形成并随后插入基因组中交错切口的情况相符。到目前为止，所鉴定出的所有啮齿动物U3反转录基因均由U3B-RNA形式产生，其中两个（每种啮齿动物一个）由3'-延长的U3-RNA前体形成，这表明RNA前体形式特别容易作为反转录基因形成的模板。啮齿动物的全长U3反转录基因并非随机插入基因组，而是系统地存在于简单序列的环境中，易于发生滑链错配，并且可能有利于单链DNA的出现。此外，它们两侧的重复序列不仅具有相同的大小（15个碱基对），而且具有共同的序列特征（延伸到相邻的上游核苷酸），这表明这类反转录基因特有的共同机制参与了它们的形成。首次提出了一个解释从非聚腺苷酸化RNA模板产生全长cDNA拷贝的模型。啮齿动物U3反转录基因似乎起源较近（在小鼠/大鼠分化之后），其中一些可能随后与功能基因进行了基因交换。
