Demers G W, Matunis M J, Hardison R C
Department of Molecular and Cell Biology, The Pennsylvania State University, University Park 16802.
J Mol Evol. 1989 Jul;29(1):3-19. doi: 10.1007/BF02106177.
The L1 family of long interspersed repetitive DNA in the rabbit genome (L1Oc) has been studied by determining the sequence of the five L1 repeats in the rabbit beta-like globin gene cluster and by hybridization analysis of other L1 repeats in the genome. L1Oc repeats have a common 3' end that terminates in a poly A addition signal and an A-rich tract, but individual repeats have different 5' ends, indicating a polar truncation from the 5' end during their synthesis or propagation. As a result of the polar truncations, the 5' end of L1Oc is present in about 11,000 copies per haploid genome, whereas the 3' end is present in at least 66,000 copies per haploid genome. One type of L1Oc repeat has internal direct repeats of 78 bp in the 3' untranslated region, whereas other L1Oc repeats have only one copy of this sequence. The longest repeat sequenced, L1Oc5, is 6.5 kb long, and genomic blot-hybridization data using probes from the 5' end of L1Oc5 indicate that a full length L1Oc repeat is about 7.5 kb long, extending about 1 kb 5' to the sequenced region. The L1Oc5 sequence has long open reading frames (ORFs) that correspond to ORF-1 and ORF-2 described in the mouse L1 sequence. In contrast to the overlapping reading frames seen for mouse L1, ORF-1 and ORF-2 are in the same reading frame in rabbit and human L1s, resulting in a discistronic structure. The region between the likely stop codon for ORF-1 and the proposed start codon for ORF-2 is not conserved in interspecies comparisons, which is further evidence that this short region does not encode part of a protein. ORF-1 appears to be a hybrid of sequences, of which the 3' half is unique to and conserved in mammalian L1 repeats. The 5' half of ORF-1 is not conserved between mammalian L1 repeats, but this segment of L1Oc is related significantly to type II cytoskeletal keratin.
通过测定兔β-珠蛋白基因簇中五个L1重复序列的序列以及对基因组中其他L1重复序列进行杂交分析，对兔基因组中的长散在重复DNA的L1家族（L1Oc）进行了研究。L1Oc重复序列有一个共同的3'末端，该末端终止于一个聚腺苷酸添加信号和一个富含A的区域，但各个重复序列有不同的5'末端，这表明在其合成或传播过程中从5'末端发生了极性截短。由于极性截短，L1Oc的5'末端在每个单倍体基因组中约有11,000个拷贝，而3'末端在每个单倍体基因组中至少有66,000个拷贝。一种类型的L1Oc重复序列在3'非翻译区有78 bp的内部直接重复序列，而其他L1Oc重复序列只有该序列的一个拷贝。测序得到的最长重复序列L1Oc5长6.5 kb，使用来自L1Oc5 5'末端的探针进行的基因组印迹杂交数据表明，一个全长L1Oc重复序列约为7.5 kb长，在测序区域的5'端延伸约1 kb。L1Oc5序列有与小鼠L1序列中描述的ORF-1和ORF-2相对应的长开放阅读框（ORF）。与小鼠L1中看到的重叠阅读框不同，ORF-1和ORF-2在兔和人L1中处于相同的阅读框，形成了双顺反子结构。在种间比较中，ORF-1可能的终止密码子和ORF-2提议的起始密码子之间的区域不保守，这进一步证明了这个短区域不编码蛋白质的一部分。ORF-1似乎是序列的混合体，其中3'半段是哺乳动物L1重复序列特有的且保守的。ORF-1的5'半段在哺乳动物L1重复序列之间不保守，但L1Oc的这一段与II型细胞骨架角蛋白有显著相关性。
