Figure 4.

A schematic representing the role of U1 telescripting, suppression of premature transcription termination, by 3′ cleavage and polyadenylation (CPA) from polyadenylation signals (PASs) scattered throughout genes’ nascent transcripts. Shown are the common components known to carry out 3′ cleavage and polyadenylation processing at the canonical terminal PAS in the last exon; it is unknown which of these play a role in premature 3′-end cleavage and polyadenylation (PCPA) in introns. The precise location of U1 small nuclear ribonucleo protein (snRNP) binding relative to PASs is unknown but is likely to be generally within 1 kb. Transcription length depends on the balance between available U1 and CPA factors, and on the density of potential U1 base-pairing sites, which include 5′SS, cryptic 5′SS, and other sequences that U1 can bind, as well as PASs.