Figure 1.

Regular linear splicing (a) and two models for the formation of exonic circRNAs (b, c). (a) Upon folding, the branch point adenosine (bpA) attacks the 5′- splice site, delivering the 5′-exon with free 3′-OH group and the lariat intermediate with the intron still linked to the 3′-exon. Nucleophilic attack of the 3′-OH group of the 5′-exon onto the 3′-splice site leads to ligation of the two exons, and to release of the intron as lariat. (b) Direct backsplicing. Two unspliced introns interact by complementary base pairing, thereby juxtaposing the branch point of the 5′-intron and the 3′-intron–exon junction (3′-splice donor) for nucleophilic attack and cleavage. Then, the 3′-splice donor attacks the 5′-intron–exon junction (5′-splice acceptor) joining the two introns and releasing the circularized exon. (c) Exon skipping. Through skipping of an exon, an exon containing lariat is created following the normal mechanism of splicing. Backsplicing then occurs as described above, but within the lariat. As a result, the intron lariat is released and a circular RNA is produced.