Rapid and concerted evolution by hypervariability of Megasatellite DNA

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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.