Pol II

Pol II

Productive elongation

Fig. 14.1 Phosphorylation cycle of the Pol II CTD. During the transcription cycle, hypophosphorylated Pol Ila is assembled into the pre-initiation complex (PIC). Shortly after initiation, Ser5 of the CTD is phosphorylated by CDK7, a component of the TFIIH complex. This phosphorylation facilitates both the promoter clearance to shift into an early elongation mode and the formation of the 5' cap structure in the nascent RNA chain. Subsequently, the phosphorylation on Ser2 of the CTD by P-TEFb enables Pol II to shift into a productive elongation mode. Upon the generation of a mature mRNA, the heavily phosphorylated Pol II undergoes dephosphorylation by the specific phosphatase FCP1 in order to be recycled back into the hypophosphorylated Ila form for subsequent rounds of transcription.

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