Rapid and concerted evolution by hypervariability of Megasatellite DNA

Megasatellite DNA是新发现的串联重复序列，大小在3-4.7 kb之间。在人类4p16.1上发现的RS447 megasellite DNA由20-100个4746 bp单位序列的高变拷贝组成。有趣的是，RS447不是垃圾DNA，它有一个1590 bp的开放阅读框，编码泛素特异性蛋白酶17 （USP17）。在以往的基因组南方杂交研究中，RS447序列在猴、狗、牛、兔、大鼠和小鼠等哺乳动物中均有较好的保守性。在这些物种中也证实了RS447同源物的头尾串联重复结构。然而，小鼠基因组不具有串联结构。此外，重复单元的大小因物种而异。RS447同源物的分化表明，大卫星DNA进化迅速。为了了解meggasatellite DNA可能的动态进化，我们通过基因组southern blots和生物信息学研究了RS447同源物是否存在于其他生物体中。通过基因组南方杂交分析，在猫、马、羊、鸡和鲤鱼3种灵长类动物中均有RS447同源基因保守。令人惊讶的是，RS447同系物是否串联重复在不同物种之间存在差异。在生物信息学分析中，我们可以在人类、黑猩猩、猕猴、大鼠、小鼠、牛、狗、鸡、鱼、蛙、水稻、蝇、蚕等多种生物的基因组数据库中找到RS447的同源基因。作为泛素特异性蛋白酶的RS447同源物被认为是保守的，但进化迅速。RS447同源基因的拷贝数在不同物种间具有高变异性。另一方面，物种内串联重复序列的单位序列可能是一致进化的结果。

Megasatellite DNA is newly discovered tandem repetitive sequence range in size from 3-4.7 kb. RS447 megasatellite DNA found on human 4p16.1 consists of 20-100 hypervariable copies of the 4,746 bp unit sequence. Interestingly, RS447 is not junk DNA but has an open reading frame of 1590 bp encoding ubiquitin-specific protease 17 (USP17). In the previous study by genomic southern hybridization, RS447 sequence was well conserved in mammalian species : monkey, dog, cow, rabbit, rat and mouse. The head-to-tail tandem repetitive structure of the RS447 homologs was also confirmed in those species. However, the mouse genome did not have the tandem structure. In addition, the size of the repeating units differs from species to species. The divergence of RS447 homologs suggested that the megasatellite DNA had evolved rapidly.To understand the possible dynamic evolution of the megasatellite DNA, we have studied whether or not the RS447 homolog is in the other organisms by genomic southern blots and by bioinformatics. By genomic southern hybridization analysis, the RS447 homolog was conserved in three primate species, cat, horse, sheep, chicken and carp. Whether the RS447 homologs are repeated tandemly or not was surprisingly differed from species to species. In bioinformatics analysis, we could find the RS447 homologs in genome database of various organisms : e.g.human, chimp, macaque, rat, mouse, cow, dog, chicken, fish, frog, rice, fly and silkworm. The RS447 homologs as ubiquitin-specific protease are thought to be conserved but evolving rapidly. The copy numbers of RS447 homologs are hypervariable among species. On the other hand, the unit sequences of the tandem repeats within species are likely homogeneous by concerted evolution.