5′-Transducing SVA elements on chromosome 19 and their source elements. (A) Full-length elements and their 5′-transduced sequences. Source elements (SEs) of SVAs H19_132, H19_153, and H19_76 could be identified and were aligned with their respective descendant SVA copy. Cryptic splice sites located within oppositely oriented Alu sequences (Kreahling and Graveley 2004) of the 5′-transducing pre-mRNA of source element H10_3 led to alternative splicing, which resulted in a 360-bp deletion within the 5′ transduction of H19_153. Consecutive and nested SVA-mediated 5′ transduction events generated SVA H19_76. The primary 32-bp 5′ transduction in source element H10_10 originated from the SVA-free source locus (SL) on chromosome 6q21. Transcriptional control of this source element by an external promoter is indicated by EST BQ431925 covering the junction between H10_10 SVA element and the 5′-flanking genomic sequence. Subsequent transduction of a GTC trinucleotide and of the H10_10 5′ TSD during retrotransposition resulted in the formation of the 5′-transducing H19_76 element. 5′- and 3′-transduced sequences are shown as yellow boxes. Numbers indicate lengths of VNTR regions and 5′ transductions in nucleotides. Blue ovals, TSDs of source elements; red ovals, TSDs flanking chromosome 19 SVAs; GA, low complexity GA-rich sequence; TA, (TTAAA)n repeats; MER93B, endogenous retrovirus repetitive sequence; FAM and FLAM_C, Alu monomer from primates (Jurka et al. 2005). (B) Members of the MAST2 5′ transduction group. Identical junctions between the 5′-ends of the truncated Alu-like region and the shared 5′-transduced MAST2 sequences indicate that these SVAs originated from a common source element. MER, DNA transposon MER115; black bar, query sequence 1.
