Swanstrom R, DeLorbe W J, Bishop J M, Varmus H E
Proc Natl Acad Sci U S A. 1981 Jan;78(1):124-8. doi: 10.1073/pnas.78.1.124.
We have determined the nucleotide sequence of portions of two circular avian sarcoma virus (ASV) DNA molecules cloned in a prokaryotic host--vector system. The region whose sequence was determined represents the circle junction site--i.e., the site at which the ends of the unintegrated linear DNA are fused to form circular DNA. The sequence from one cloned molecule, SRA-2, shows that the circle junction site is the center of a 330-base-pair (bp) tandem direct repeat, presumably representing the fusion of the long terminal repeat (LTR) units known to be present at the ends of the linear DNA. The circle junction site is also the center of a 15-bp imperfect inverted repeat, which thus appears at the boundaries of the LTR. The structure of ASV DNA--unique coding region flanked by a direct repeat that is, in turn, terminated with a short inverted repeat--is very similar to the structure of certain transposable elements. Several features of the sequence imply that circularization to form the SRA-2 molecule occurred without loss of information from the linear DNA precursor. Circularization of another cloned viral DNA molecule, SRA-1, probably occurred by a different mechanism. The circle junction site of the SRA-1 molecule has a 63-bp deletion, which may have arisen by a mechanism that is analogous to the integration of viral DNA into the host genome. Flanking one side of the tandem direct repeat is the binding site for tRNATrp, the previously described primer for synthesis of the first strand of viral DNA. The other side of the direct repeat is flanked by a polypurine tract, A-G-G-G-A-G-G-G-G-G-A, which may represent the position of the primer for synthesis of the second strand of viral DNA. An A+T-rich region, upstream from the RNA capping site, and the sequence A-A-T-A-A-A are present within the direct repeat sequence. These sequences may serve as a promoter site and poly(A) addition signal, respectively, as proposed for other eukaryotic transcription units.
我们已经确定了克隆于原核宿主 - 载体系统中的两个环状禽肉瘤病毒（ASV）DNA分子部分片段的核苷酸序列。所测定序列的区域代表环状连接位点，即未整合的线性DNA末端融合形成环状DNA的位点。来自一个克隆分子SRA - 2的序列表明，环状连接位点是一个330个碱基对（bp）串联直接重复序列的中心，推测它代表线性DNA末端已知存在的长末端重复序列（LTR）单元的融合。环状连接位点也是一个15 bp不完全反向重复序列的中心，因此它出现在LTR的边界处。ASV DNA的结构——独特的编码区域两侧是一个直接重复序列，而这个直接重复序列又以一个短反向重复序列终止——与某些转座元件的结构非常相似。该序列的几个特征表明，形成SRA - 2分子的环化过程中，线性DNA前体没有信息丢失。另一个克隆的病毒DNA分子SRA - 1的环化可能是通过不同的机制发生的。SRA - 1分子的环状连接位点有一个63 bp的缺失，这可能是通过一种类似于病毒DNA整合到宿主基因组的机制产生的。串联直接重复序列的一侧是色氨酸tRNA的结合位点，它是先前描述的病毒DNA第一链合成的引物。直接重复序列的另一侧两侧是一个多聚嘌呤序列，A - G - G - G - A - G - G - G - G - G - A，它可能代表病毒DNA第二链合成引物的位置。RNA加帽位点上游的一个富含A + T的区域以及序列A - A - T - A - A - A存在于直接重复序列内。如对其他真核转录单位所提出的那样，这些序列可能分别作为启动子位点和聚腺苷酸添加信号。
