SVA 5′ transduction events involving pre-mRNA splicing. (A) After insertion of a full-length SVA element into an intron, cryptic splice acceptor sites upstream of the element can be activated. The retrotransposed copy includes both exon and intron sequences as 5′ transduction (e.g., H1_108). Alternatively, an upstream exon can be spliced to cryptic acceptor sites within the SVA sequence itself, resulting in a 5′-truncated SVA with exon sequences as 5′ transduction. (B) Retrotransposition of a full-length SVA with a 5′-transduced segment into intronic sequences can have two alternative outcomes: Activation of intronic splice acceptors leads to the inclusion of intron sequences (stippled box) in the secondary transduction (green boxes) (H3_545). Alternatively, the 5′-transduced sequence is fused to a splice acceptor localized in the primary transduction (e.g., ZNF487 group, cf. Fig. 5B). (C) After retrotransposition of 5′-truncated SVA elements into an intron (e.g., H2_383), only activation of intronic splice sites could be observed (e.g., H7_1117). A two-exon setting was chosen for the schematic representation. The actual number of exons transduced differs between elements. Yellow boxes, primary transductions; green boxes, secondary transductions; dotted green boxes, intronic sequence included in 5′ transductions; red ovals, TSDs; blue ovals, TSD sequence of the source element present in the 5′-transducing SVA; TD, transduction.
