FIGURE 3.

Consequences of Alu–Alu base pairing within SMN pre-mRNA. Diagrammatic representation of the partial SMN pre-mRNA (not to the scale) showing inclusion, skipping and circularization of exon 5. Exons are shown in colored boxes, whereas, red arrows indicated Alu elements. For simplicity, only two Alu elements per intron are shown. Base pairing between complementary Alu sequences are shown by stacked lines. Various hypothetical scenarios of splicing reactions involving different combinations of splice site pairings are shown by broken lines. Base pairing between Alu sequences within intron 5 promotes inclusion of exon 5, whereas, base pairing between intronic Alu sequences flanking exon 5 promotes exon 5 skipping. Base pairing between Alu elements within intron 5 combined with the base pairing between intronic Alu sequences upstream of exon 5 and downstream of exon 6 promotes circularization event.