Fig. 1. Co-transcriptional pre-mRNA processing. A schematic representation of transcription and pre-mRNA processing events at Pol II transcription units (TUs). Pol II (black ball) initiates transcription at the promoter (arrow) and proceeds along the TU during elongation phase, terminating and releasing from the DNA template following passage through the polyadenylation signals. Several polyadenylation factors, such as CPSF and CstF, bind directly to Pol II and are shown all along the TU as a blue ball adjacent to the black one. Capping enzymes (red oval) bind to Pol II as it enters the elongation phase and then fall off the TU. The 5' cap added by the capping enzymes is symbolized by the baseball cap. Because splicing is co-transcriptional, we have hypothetically placed splicing factors recognizing the 5' and 3' splice sites (orange and yellow balls, respectively) and the assembly of the spliceosome (green oval) within the body of the TU. Additional polyadenylation factors are recruited to downstream regions, as shown by the additional dark blue ball. At termination, Pol II is released from the template and recycled, and the fragment of cleaved nascent RNA remaining will be degraded. The mRNP is released from the template and undergoes nuclear transport.

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