Moran J V, Zimmerly S, Eskes R, Kennell J C, Lambowitz A M, Butow R A, Perlman P S
Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas 75235-9038, USA.
Mol Cell Biol. 1995 May;15(5):2828-38. doi: 10.1128/MCB.15.5.2828.
Group II introns aI1 and aI2 of the yeast mitochondrial COXI gene are mobile elements that encode an intron-specific reverse transcriptase (RT) activity. We show here that the introns of Saccharomyces cerevisiae ID41-6/161 insert site specifically into intronless alleles. The mobility is accompanied by efficient, but highly asymmetric, coconversion of nearby flanking exon sequences. Analysis of mutants shows that the aI2 protein is required for the mobility of both aI1 and aI2. Efficient mobility is dependent on both the RT activity of the aI2-encoded protein and a separate function, a putative DNA endonuclease, that is associated with the Zn2+ finger-like region of the intron reading frame. Surprisingly, there appear to be two mobility modes: the major one involves cDNAs reverse transcribed from unspliced precursor RNA; the minor one, observed in two mutants lacking detectable RT activity, appears to involve DNA level recombination. A cis-dominant splicing-defective mutant of aI2 continues to synthesize cDNAs containing the introns but is completely defective in both mobility modes, indicating that the splicing or the structure of the intron is required. Our results demonstrate that the yeast group II intron aI2 is a retroelement that uses novel mobility mechanisms.
酵母线粒体COXI基因的II类内含子aI1和aI2是可移动元件，它们编码一种内含子特异性逆转录酶（RT）活性。我们在此表明，酿酒酵母ID41 - 6/161的内含子可特异性插入无内含子的等位基因中。这种移动伴随着附近侧翼外显子序列的高效但高度不对称的共转化。对突变体的分析表明，aI2蛋白是aI1和aI2移动所必需的。高效移动依赖于aI2编码蛋白的RT活性以及一种与内含子阅读框的锌指样区域相关的独立功能，一种假定的DNA内切酶。令人惊讶的是，似乎存在两种移动模式：主要模式涉及从未剪接的前体RNA逆转录而来的cDNA；次要模式在两个缺乏可检测RT活性的突变体中观察到，似乎涉及DNA水平的重组。aI2的一个顺式显性剪接缺陷突变体继续合成含有内含子的cDNA，但在两种移动模式中均完全缺陷，这表明内含子的剪接或结构是必需的。我们的结果表明，酵母II类内含子aI2是一种利用新型移动机制的逆转录元件。
