Figure 3.
Genesis of the SVA master locus on CH10. The AluSc on CH9 and the AluSp on CH10, hereafter 5′ Alu and 3′ Alu, respectively, are shown present in the human–chimp common ancestor. The original target-site duplications (TSDs) for the 5′ Alu and the 3′ Alu are indicated by blue and red arrowheads, respectively. (A) After the human–chimp split, the initial SVAF1 was formed due to an alternative splicing event. A SVA mRNA was transcribed from either the founder SVAF1 insertion or some other unknown SVAF1 locus in the human genome, derived from the original SVAF1 founder insertion. (B) The SVA mRNA is retrotransposed directly downstream from the 5′ Alu on CH9 as displayed by the SVA insertion flanked by TSDs (green arrowheads). (C) Transcriptional initiation occurred upstream of the 5′ Alu, generating a composite mRNA containing in order (1) 185 nt of 5′ transduced sequence from CH9, (2) the 5′ Alu, and (3) the SVAF1. (D) The composite mRNA from CH9 retrotransposed to CH10 directly upstream of the 3′ Alu. (E) The CH10 master element present in the human genome reference is shown. Directly downstream from the 3′ Alu TSD, unique sequence blocks (blue boxes), labeled X and Y, are present in the daughter insertions derived from this locus. The polyA signals utilized in the 3′ transductions are indicated by AATAAA relative to the unique sequence blocks.