Danilevskaya O N, Tan C, Wong J, Alibhai M, Pardue M L
Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
Proc Natl Acad Sci U S A. 1998 Mar 31;95(7):3770-5. doi: 10.1073/pnas.95.7.3770.
HeT-A was the first transposable element shown to have a bona fide role in chromosome structure, maintenance of telomeres in Drosophila melanogaster. HeT-A has hallmarks of non-long-terminal-repeat (non-LTR) retrotransposable elements but also has several unique features. We have now isolated HeT-A elements from Drosophila yakuba, showing that the retrotransposon mechanism of telomere maintenance predates the separation of D. melanogaster and D. yakuba (5-15 million years ago). HeT-A elements from the two species show significant sequence divergence, yet unusual features seen in HeT-Amel are conserved in HeT-Ayak. In both species, HeT-A elements are found in head-to-tail tandem arrays in telomeric heterochromatin. In both species, nearly half of the HeT-A sequence is noncoding and shows a distinctive imperfect repeat pattern of A-rich segments. Neither element encodes reverse transcriptase. The HeT-Amel promoter appears to be intermediate between the promoters of non-LTR and of LTR retrotransposons. The HeT-Ayak promoter shows similar features. HeT-Amel has a frameshift within the coding region. HeT-Ayak does not require a frameshift but shows conservation of the polypeptide sequence of the frameshifted product of D. melanogaster.
HeT-A是首个被证明在染色体结构中发挥真正作用的转座元件，它参与了黑腹果蝇端粒的维持。HeT-A具有非长末端重复（non-LTR）反转录转座元件的特征，但也有一些独特之处。我们现在已经从雅库布果蝇中分离出HeT-A元件，这表明端粒维持的反转录转座机制早于黑腹果蝇和雅库布果蝇的分化（500万至1500万年前）。来自这两个物种的HeT-A元件显示出显著的序列差异，但在HeT-Amel中发现的不寻常特征在HeT-Ayak中得以保留。在这两个物种中，HeT-A元件都以头对头的串联阵列形式存在于端粒异染色质中。在这两个物种中，HeT-A序列的近一半是非编码的，并且呈现出富含A片段的独特不完全重复模式。这两种元件都不编码逆转录酶。HeT-Amel启动子似乎介于非LTR和LTR反转录转座子的启动子之间。HeT-Ayak启动子也表现出类似的特征。HeT-Amel在编码区内有一个移码突变。HeT-Ayak不需要移码突变，但显示出黑腹果蝇移码产物多肽序列的保守性。
