Figure 1.

When transcription has completed but splicing has not, the splicing machinery is retained at the site of transcription, independently of the polymerase. Image from an RNA FISH experiment on the U2OS E3 (3 exons, 2 introns) cell line. Co-transcriptional accumulation of the spliceosomal U2 snRNA (red) on the actively transcribing gene site (mRNA in green). Bar, 5 µM. The scheme depicts RNAP II transcription on an intron-containing gene. U1 snRNP might accompany the polymerase by associating with the CTD prior to transcription of the splice sites, and could thereby be in close proximity to the emerging transcript. Splicoeosomes are recruited to each intron and after splicing an exon-junction complex (EJC) is deposited on the exon-exon junctions. If the polymerase has completed transcription but splicing has not, the polymerase leaves the site of transcription while the transcript is delayed until splicing has completed. Only then can the transcript diffuse out into the nucleoplasm.