Belancio Victoria P, Whelton Megan, Deininger Prescott
Tulane Cancer Center, SL66, and Department of Epidemiology, Tulane University Health Sciences Center, 1430 Tulane Ave., New Orleans, LA 70112, United States.
Gene. 2007 Apr 1;390(1-2):98-107. doi: 10.1016/j.gene.2006.07.029. Epub 2006 Aug 18.
LINE-1 (L1) is the only active, autonomous, non-LTR, human retroelement. There are about 5x10(5) L1 copies in the human genome, the majority of which are truncated at their 5' ends. Both truncated and full-length L1 insertions contain a polyadenylation (polyA) signal at their 3' ends. A typical polyA site consists of the three main cis-acting elements: a conserved hexamer, cleavage site, and a GU-rich downstream region. A newly inserted L1 copy contains the conserved AATAAA hexamer at the end of its sequence. However, the GU-rich downstream region has to be provided by the neighboring genomic sequences and therefore it would vary for every L1 copy. Using northern blot analysis of transiently transfected L1 expression vectors we demonstrate that L1 element contain sequence that allow efficient polyadenylation at the L1 3' end upon retrotransposition into a new genomic location independent of the base composition downstream of the insertion site. The strategy of polyadenylation at the 3' end of L1 parallels the approach the element employs at its 5'UTR by having an unusual internal polymerase II promoter, making new insertions less dependent on the properties of the flanking sequences at the new locus.
LINE-1（L1）是唯一活跃的、自主的、非长末端重复序列（non-LTR）的人类反转录元件。人类基因组中约有5×10⁵个L1拷贝，其中大多数在其5'端被截断。截断的和全长的L1插入序列在其3'端都含有一个聚腺苷酸化（polyA）信号。一个典型的polyA位点由三个主要的顺式作用元件组成：一个保守的六聚体、切割位点和一个富含GU的下游区域。一个新插入的L1拷贝在其序列末端含有保守的AATAAA六聚体。然而，富含GU的下游区域必须由相邻的基因组序列提供，因此每个L1拷贝都会有所不同。通过对瞬时转染的L1表达载体进行Northern印迹分析，我们证明L1元件包含一些序列，这些序列允许在反转录转座到新的基因组位置时，在L1的3'端进行有效的聚腺苷酸化，而与插入位点下游的碱基组成无关。L1 3'端的聚腺苷酸化策略与该元件在其5'非翻译区（5'UTR）采用的方法类似，即具有一个不寻常的内部聚合酶II启动子，使得新的插入对新位点侧翼序列的特性依赖性较小。
