Ryan S C, Dugaiczyk A
Department of Biochemistry, University of California, Riverside 92521.
Proc Natl Acad Sci U S A. 1989 Dec;86(23):9360-4. doi: 10.1073/pnas.86.23.9360.
We discovered the presence of an Alu and an Xba repetitive DNA element within introns 4 and 7, respectively, of the human alpha-fetoprotein (AFP) gene; these elements are absent from the same gene in the gorilla. The Alu element is flanked by 12-base-pair direct repeats, AGGATGTTGTGG ... (Alu) ... AGGATGTTGTGG, which presumably arose by way of duplication of the intronic target site AGGATGTTGTGG at the time of the Alu insertion. In the gorilla, only a single copy of the unoccupied target site is present, which is identical to the terminal repeat flanking the human Alu element. There are two copies of an Xba repeat in the human AFP gene, apparently the only two in the genome. Xba1 and Xba2, located within introns 8 and 7, respectively, differ from each other at 3 of 303 positions. Xba1 is referred to as the old (ancestral) repeat because it lacks direct repeats. The new (derived) Xba2 is flanked by direct repeats, TTTCTTTTT ... (Xba) ... TTTCTTCTT, and is thought to have arisen as a result of transposition of Xba1. The ancestral Xba1 and a single copy of the Xba2 target site are present at orthologous positions in the gorilla, but the new Xba2 is absent. We conclude that the Alu and Xba DNA repeats emerged in the human genome at a time postdating the human-gorilla divergence and became established as genetic novelties in the human lineage. We submit that the chronology of divergence of primate lines of evolution can be correlated with the timing of insertion of new DNA repeats into the genomes of those primates.
我们分别在人类甲胎蛋白（AFP）基因的第4和第7内含子中发现了一个Alu重复DNA元件和一个Xba重复DNA元件；在大猩猩的同一基因中不存在这些元件。Alu元件两侧是1112个碱基对的直接重复序列，AGGATGTTGTGG……（Alu）……AGGATGTTGTGG，推测这是在Alu插入时内含子靶位点AGGATGTTGTGG重复产生的。在大猩猩中，只存在一个未被占据的靶位点拷贝，它与人类Alu元件侧翼的末端重复序列相同。人类AFP基因中有两个Xba重复序列拷贝，显然是基因组中仅有的两个。分别位于第8和第7内含子中的Xba1和Xba2，在303个位置中的3个位置上彼此不同。Xba1被称为旧的（祖先的）重复序列，因为它没有直接重复序列。新的（衍生的）Xba2两侧是直接重复序列，TTTCTTTTT……（Xba）……TTTCTTCTT，被认为是Xba1转座的结果。祖先的Xba1和Xba2靶位点的一个拷贝存在于大猩猩的直系同源位置，但新的Xba2不存在。我们得出结论，Alu和Xba DNA重复序列在人类与大猩猩分化之后的某个时间出现在人类基因组中，并作为遗传新奇物在人类谱系中得以确立。我们认为，灵长类进化系的分化时间顺序可以与新DNA重复序列插入这些灵长类基因组的时间相关联。
