Yang J, Zimmerly S, Perlman P S, Lambowitz A M
Department of Molecular Genetics, The Ohio State University, Columbus 43210-1292, USA.
Nature. 1996 May 23;381(6580):332-5. doi: 10.1038/381332a0.
Some group II introns are mobile elements as well as catalytic RNAs. Introns aI1 and aI2 found in the gene COX1 in yeast mitochondria encode reverse transcriptases which promote site-specific insertion of the intron into intronless alleles ('homing'). For aI2 this predominantly occurs by reverse transcription of unspliced precursor RNA at a break in double-strand DNA made by an endonuclease encoded by the intron. The aI2 endonuclease involves both the excised intron RNA, which cleaves the DNA's sense strand by partial reverse splicing; and the intron-encoded reverse transcriptase which cleaves the anti-sense strand. Here we show that aI1 encodes an analogous endonuclease specific for a different target site compatible with the different exon-binding sequences of the intron RNA. Over half of aI1 undergoes complete reverse splicing in vitro, thus integrating linear intron RNA directly into the DNA. This unprecedented reaction has implications for both intron mobility and evolution, and potential genetic engineering applications.
一些II类内含子既是可移动元件，也是催化性RNA。在酵母线粒体基因COX1中发现的内含子aI1和aI2编码逆转录酶，这些逆转录酶促进内含子位点特异性插入无内含子的等位基因（“归巢”）。对于aI2，这主要是通过由内含子编码的内切核酸酶在双链DNA的一个断裂处对未剪接的前体RNA进行逆转录来实现的。aI2内切核酸酶涉及切除的内含子RNA，它通过部分逆转剪接切割DNA的有义链；以及切割反义链的内含子编码逆转录酶。在这里，我们表明aI1编码一种类似的内切核酸酶，该内切核酸酶对与内含子RNA不同的外显子结合序列兼容的不同靶位点具有特异性。超过一半的aI1在体外经历完全逆转剪接，从而将线性内含子RNA直接整合到DNA中。这种前所未有的反应对内含子的移动性和进化以及潜在的基因工程应用都有影响。
