Fig. 6. A model for transcript production. (A) The CTD has the potential to associate with sites involved in capping (Fong and Bentley, 2001), transcript degradation (Andrulis et al., 2002; Libri et al., 2002; Lykke-Andersen et al., 2002), translational proofreading (Iborra et al., 2001; Wilkinson and Shyu, 2002), proteolysis (Iborra et al., 2001), splicing and polyadenylation (labelled A) (Maniatis and Reed, 2002). (B) Transcription began as the template bound to the polymerizing complex and was reeled in as the transcript was extruded; the CTD is now hyper-phosphorylated, and a cap has been added. (C) The transcript continues to be extruded through a splicing site as the ribosome/NMD machinery begins proofreading the now-spliced message (and so does not read introns that may contain many termination codons). (D) Once introns are removed (lariat), the transcript is cleaved, poly-adenylated, and exported to the cytoplasm; but if errors are detected, the faulty transcript and peptide produced during proofreading are degraded by nucleases and proteasomes.