Adey N B, Schichman S A, Graham D K, Peterson S N, Edgell M H, Hutchison C A
Department of Microbiology and Immunology, University of North Carolina at Chapel Hill 27599-7290.
Mol Biol Evol. 1994 Sep;11(5):778-89. doi: 10.1093/oxfordjournals.molbev.a040158.
All mammalian genomes contain approximately 100,000 copies of the transposable element LINES-1 (L1). Phylogenetic analysis indicates that the L1 progenitor predates the mammalian radiation; since that time, the open reading frames encoded in L1 have evolved under selection. The least conserved regions within L1 are the 5'-terminal transcriptional regulatory sequences. In rodents, four types of L1 elements (A, F, and V from mouse and R from rat) have been defined according to the type of apparently nonhomologous promoter sequence present at the 5' end. In this study, we investigate the relationships between these four types of promoters. DNA sequence was determined from approximately 1.5-kb regions from the 5' ends of seven F- and three V-type L1 elements. These sequences were aligned with 29 previously reported L1 elements. Phylogenetic analysis was then performed on the homologous regions of the alignment. The results indicate that in mouse all of the A-, F-, and V-type elements belong to a single dominant lineage but were inserted into the genome during different time periods; V-type elements are the oldest, while A-type elements are the most recently inserted. V-type elements also appear ancestral to the R-type elements found in rat and therefore were replicatively competent prior to the divergence of rat and mouse. Analysis of sequence identity indicates that the different 5' promoters did not derive from a common ancestor. Therefore, the dominant L1 lineage appears to have acquired novel promoter sequences from non-L1 sources. Transposable elements from a wide range of species show similar structural rearrangements, suggesting that acquisition of new sequences may be a common theme in their evolution.
所有哺乳动物的基因组都含有大约10万个转座元件L1（长散在核元件-1）拷贝。系统发育分析表明，L1的祖先是在哺乳动物辐射之前就已存在；从那时起，L1中编码的开放阅读框就在选择作用下不断进化。L1中最不保守的区域是5'端转录调控序列。在啮齿动物中，根据5'端存在的明显非同源启动子序列的类型，定义了四种类型的L1元件（小鼠的A、F和V型以及大鼠的R型）。在本研究中，我们探究了这四种类型启动子之间的关系。从7个F型和3个V型L1元件的5'端大约1.5 kb区域测定了DNA序列。这些序列与之前报道的29个L1元件进行了比对。然后对该比对的同源区域进行了系统发育分析。结果表明，在小鼠中，所有的A、F和V型元件都属于一个单一的优势谱系，但在不同时期插入到基因组中；V型元件最古老，而A 型元件是最近插入的。V型元件似乎也是大鼠中发现的R型元件的祖先，因此在大鼠和小鼠分化之前就具有复制能力。序列同一性分析表明，不同的5'启动子并非来自共同祖先。因此，优势L1谱系似乎是从非L1来源获得了新的启动子序列。来自广泛物种的转座元件显示出相似的结构重排，这表明获取新序列可能是它们进化中的一个共同主题。
