Hayward B E, Zavanelli M, Furano A V
Laboratory of Molecular and Cellular Biology, National Institute of Diabetes, Maryland 20892-0830, USA.
Genetics. 1997 Jun;146(2):641-54. doi: 10.1093/genetics/146.2.641.
Mammalian L1 (long interspersed repeated DNA. LINE-1) retrotransposons consist of a 5' untranslated region (UTR) with regulatory properties, two protein encoding regions (ORF I, ORF II, which encodes a reverse transcriptase) and a 3' UTR. L1 elements have been evolving in mammals for > 100 million years and this process continues to generate novel L1 subfamilies in modern species. Here we characterized the youngest known subfamily in Rattus norvegicus, L1mlvi2, and unexpectedly found that this element has a dual ancestry. While its 3' UTR shares the same lineage as its nearest chronologically antecedent subfamilies, L13 and L14, its ORF I sequence does not. The L1mlvi2 ORF I was derived from an ancestral ORF I sequence that was the evolutionary precursor of the L13 and L14 ORF I. We suggest that an ancestral ORF I sequence was recruited into the modern L1mlvi2 subfamily by recombination that possibly could have resulted from template strand switching by the reverse transcriptase during L1 replication. This mechanism could also account for some of the structural features of rodent L1 5' UTR and ORF I sequences including one of the more dramatic features of L1 evolution in mammals, namely the repeated acquisition of novel 5' UTRs.
哺乳动物的L1（长散在重复DNA，即LINE-1）逆转座子由具有调控特性的5'非翻译区（UTR）、两个蛋白质编码区（开放阅读框I、开放阅读框II，后者编码逆转录酶）和一个3'UTR组成。L1元件在哺乳动物中已经演化了超过1亿年，这一过程在现代物种中持续产生新的L1亚家族。在这里，我们对褐家鼠中已知的最年轻亚家族L1mlvi2进行了特征描述，意外地发现该元件有双重起源。虽然它的3'UTR与其时间上最近的前序亚家族L13和L14具有相同的谱系，但其开放阅读框I序列却并非如此。L1mlvi2的开放阅读框I源自一个祖先开放阅读框I序列，该序列是L13和L14开放阅读框I的进化前体。我们认为，一个祖先开放阅读框I序列通过重组被纳入现代L1mlvi2亚家族，这种重组可能是由L1复制过程中逆转录酶的模板链切换导致的。这种机制也可以解释啮齿动物L1 5'UTR和开放阅读框I序列的一些结构特征，包括哺乳动物L1进化中一个更显著的特征，即新的5'UTR的反复获得。
