George J A, Burke W D, Eickbush T H
Department of Biology, University of Rochester, New York 14627, USA.
Genetics. 1996 Mar;142(3):853-63. doi: 10.1093/genetics/142.3.853.
R2 elements are non-long terminal repeat retrotransposable elements that insert into 28S rRNA genes of most insect species. The single open reading frame of R2 encodes a protein with both endonuclease activity, which cleaves the target site, and reverse transcriptase activity, which uses this cleavage to prime reverse transcription. This target-primed reverse transcription mechanism is also used by group II introns. Little is known of the mechanism by which the 5' end of R2 is integrated after reverse transcription. We have determined the 5' junction sequence of 94 R2 elements from 14 different species of Drosophila. Only 37% of the full-length elements contained precise 5' junctions; the remainder contained deletions of the 28S gene and/or insertions of additional sequences. Because the 5' junctions of truncated copies were similar to full-length elements, no sequences at the 5' end of R2 appear to be required for element integration. A model in which the R2 reverse transcriptase is capable of switching templates from the R2 RNA transcript to the upstream 28S gene can best explain the observed 5' junction sequences. This template jumping is analogous to the template switching of retroviral reverse transcriptases during formation of the double-stranded integration products.
R2元件是非长末端重复逆转座子元件，可插入大多数昆虫物种的28S rRNA基因中。R2的单一开放阅读框编码一种具有内切核酸酶活性和逆转录酶活性的蛋白质，内切核酸酶活性用于切割靶位点，逆转录酶活性则利用这种切割作用来引发逆转录。II类内含子也采用这种靶标引发的逆转录机制。对于R2逆转录后5'端的整合机制，人们知之甚少。我们确定了来自14种不同果蝇物种的94个R2元件的5'连接序列。只有37%的全长元件含有精确的5'连接；其余元件包含28S基因的缺失和/或额外序列的插入。由于截短拷贝的5'连接与全长元件相似，因此R282似乎R2 5'端的序列对于元件整合并非必需。一种模型认为，R2逆转录酶能够将模板从R2 RNA转录本切换到上游的28S基因，这可以最好地解释所观察到的5'连接序列。这种模板跳跃类似于双链整合产物形成过程中逆转录病毒逆转录酶的模板切换。
