Long Interspersed Nuclear Element-1 (LINE-1, L1) is the only autonomous active transposable element in the human genome. The L1-encoded proteins ORF1p and ORF2p enable the element to jump from one locus to another via a "copy-and-paste" mechanism. ORF1p is an RNA-binding protein, and ORF2p has endonuclease and reverse transcriptase activities. The huge number of truncated L1 remnants in the human genome suggests that the host has likely evolved mechanisms to prevent full L1 replication, and thereby decrease the proliferation of active elements and reduce the mutagenic potential of L1. In turn, L1 appears to have a minimized length to increase the probability of successful full-length replication. This streamlining would be expected to lead to high information density. Here, we describe the construction and initial characterization of a library of 538 consecutive trialanine substitutions that scan along ORF1p and ORF2p to identify functionally important regions. In accordance with the streamlining hypothesis, retrotransposition was overall very sensitive to mutations in ORF1p and ORF2p; only 16% of trialanine mutants retained near-wild-type (WT) activity. All ORF1p mutants formed near-WT levels of mRNA transcripts and 75% formed near-WT levels of protein. Two ORF1p mutants presented a unique nucleolar-relocalization phenotype. Regions of ORF2p that are sensitive to mutagenesis but lack phylogenetic conservation were also identified. We provide comprehensive information on the regions most critical to retrotransposition. This resource will guide future studies of intermolecular interactions that form with RNA, proteins, and target DNA throughout the L1 life cycle.

中文翻译：

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人类逆转录转座子 LINE-1 的全面扫描诱变鉴定了功能必需的基序。


长散布核元件-1（LINE-1，L1）是人类基因组中唯一的自主活性转座元件。 L1 编码的蛋白质 ORF1p 和 ORF2p 使该元件能够通过“复制粘贴”机制从一个基因座跳转到另一个基因座。 ORF1p是一种RNA结合蛋白，ORF2p具有核酸内切酶和逆转录酶活性。人类基因组中大量截短的 L1 残余物表明，宿主可能已经进化出阻止 L1 完全复制的机制，从而减少活性元件的增殖并降低 L1 的诱变潜力。反过来，L1 似乎具有最小化的长度，以增加成功全长复制的概率。这种精简预计将带来高信息密度。在这里，我们描述了 538 个连续三丙氨酸取代文库的构建和初步表征，该文库沿着 ORF1p 和 ORF2p 扫描以识别功能上重要的区域。根据精简假说，逆转录转座总体上对 ORF1p 和 ORF2p 的突变非常敏感；只有 16% 的三丙氨酸突变体保留了近野生型 (WT) 活性。所有 ORF1p 突变体形成接近 WT 水平的 mRNA 转录本，75% 形成接近 WT 水平的蛋白质。两个 ORF1p 突变体呈现出独特的核仁重定位表型。还鉴定了对诱变敏感但缺乏系统发育保守性的 ORF2p 区域。我们提供关于逆转录转座最关键区域的全面信息。该资源将指导未来在整个 L1 生命周期中与 RNA、蛋白质和目标 DNA 形成的分子间相互作用的研究。