Figure 3.

The CTD facilitates capping and splicing by recruitment of RNA processing factors. (A) Capping enzyme (CE) is recruited to the vicinity of nascent mRNA by the CTD phosphorylated on Ser5. (B) During transcription, the CTD is phosphorylated on Ser2, while the Ser5-P is dephosphorylated and is involved in recruiting the indicated splicing factors, which defines splice sites and facilitates assembly of the spliceosome. In this and subsequent figures, green spheres above the CTD represent relevant CTD-binding proteins, while assembled functional complexes are indicated below.