Exon circularization in mammalian nuclear extracts

Correct ligation of exons in pre-mRNA splicing requires splice site juxtaposition (splice site pairing), usually involving a 5' splice site and a downstream 3' splice site. Splicing of a 5' splice site to an upstream 3' splice site, however, is predicted to result in a circular RNA. This mode of splice site pairing across the exon has been hypothesized to account for rare RNAs containing scrambled exons (Nigro JM et al., 1991, Cell 64:607-613; Cocquerelle C et al., 1992, EMBO J11:1095-1098). Additionally, this mode of splice site pairing has been postulated to explain the formation of SRY circular transcripts in mouse testis (Capel Bet al., 1993, Cell 73:1019- 1030). Here we show that splice site pairing across the exon can result in exon circularization in vitro. These results indicate that spliceosome- mediated exon circularization indeed can account for the formation of scrambled exons and circular RNAs. Exon circularization efficiency decreased dramatically as the length of the exon was increased from 95 nt to 274 nt. Circularization of this longer exon was restored, however, when intronic complementary sequences were included in the RNA substrate. These complementary sequences could form a stem that served to bring the splice sites into proximity and thereby promote splice site pairing. Therefore, the splicing of this structured RNA recapitulated SRY-like exon circularization in vitro.