Fig. 7: A comprehensive model on canonical splicing, back-splicing, and interconversion between the ID and ED spliceosomes.

a A carton diagram of the transient pre-B complex assembled on an endogenous transcript. U2 snRNP is assembled on the central intron, upstream of the central exon. One U1 snRNP is assembled on the central intron, whereas another U1 snRNP is located downstream of the central exon. Free tri-snRNP with PRP28 may be recruited by SF3a120 and U2 snRNA.¹⁸ Depending on which U1 snRNP it engages, the SF3a120-linked tri-snRNP may assemble into an ID or ED pre-B complex. b Assembly of the ID pre-B complex and its conversion to the ID B complex. With the tri-snRNP engaging U1 snRNP on the central intron, the ID pre-B complex is assembled. PRP28 unwinds the U1/5′SS duplex, leading to U1 snRNP release and U6/5′SS duplex formation in the B complex. c Assembly of the ED pre-B complex and its conversion to the ED B complex. Engagement of the tri-snRNP with U1 snRNP downstream of the central exon results in assembly of the ED pre-B complex, which may be converted to the ED B complex by PRP28. But the ED pre-B complex may also be converted to the ID pre-B complex if the tri-snRNP switches over to engage the U1 snRNP on the central intron. d A schematic diagram of canonical splicing as mediated by the ID spliceosomes. e A schematic diagram of back-splicing as mediated by the ED spliceosomes. Circular exon is a final product of back-splicing by ED spliceosomes. Several scenarios are shown here.