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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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