Questions: RNA Polymerase II CTD and Coupling to mRNA Processing

Question 1

A mutation eliminates all Serine 5 phosphorylation sites on the RNA Pol II CTD. What is the most likely consequence for mRNA maturation?

• A. Pol II cannot assemble at the promoter because Ser5 is required for preinitiation complex formation
• B. The 5' cap is not added to nascent mRNA, because capping enzymes are recruited by Ser5 phosphorylation
• C. Splicing factors cannot be recruited, because they depend on CTD phosphorylation
• D. 3' cleavage and polyadenylation fail, because Ser5 recruits the polyadenylation machinery

Answer: (B)

Ser5 phosphorylation (catalyzed by TFIIH's kinase activity early in transcription) recruits the capping enzyme complex to the nascent transcript. Without Ser5 phosphorylation, 5' capping fails — the 7-methylguanosine cap is not added. Splicing factors and 3' end processing machinery are recruited by Ser2 phosphorylation, which occurs later during elongation. This temporal division is the CTD phosphorylation code: Ser5 early (capping), Ser2 late (splicing and polyadenylation).

Question 2

Promoter-proximal pausing — where NELF and DSIF stall RNA Pol II 30–60 nucleotides downstream of the start site — provides which regulatory advantage?

• A. It allows the cell to permanently silence genes during differentiation by locking Pol II in place
• B. It prevents premature 5' capping before Ser5 is phosphorylated
• C. It enables rapid transcriptional responses by pre-loading Pol II at genes before the signal to elongate arrives
• D. It coordinates Pol II elongation speed with the rate of ribosomal translation

Answer: (C)

A paused polymerase is loaded and ready at the gene, waiting for the signal to release — provided by P-TEFb (CDK9/Cyclin T) phosphorylating NELF (releasing it), DSIF (converting it to a positive factor), and CTD Ser2 (enabling productive elongation). This is far faster than assembling an entirely new preinitiation complex from scratch in response to a signal. Genes important for stress responses and developmental decisions are often regulated at this step — their Pol II is already waiting, enabling a response within minutes rather than hours.

Question 3

mRNA capping normally occurs before splicing and polyadenylation because Ser5 CTD phosphorylation precedes Ser2 phosphorylation during the transcription cycle.

• T. True
• F. False

Answer: True

The temporal sequence of CTD phosphorylation events directly controls the order of mRNA processing. TFIIH phosphorylates Ser5 shortly after initiation (near the promoter), recruiting capping enzymes to the nascent 5' end. As Pol II moves into productive elongation, P-TEFb kinase phosphorylates Ser2, which recruits splicing factors during elongation and 3' end processing machinery near the end of the gene. This phosphorylation code converts the linear act of transcription into a temporally ordered mRNA assembly line.

Question 4

RNA Pol II's CTD is unphosphorylated during productive elongation and primarily becomes phosphorylated after transcription terminates to prepare for the next round.

• T. True
• F. False

Answer: False

The opposite is true. The hypophosphorylated (unphosphorylated) CTD is the form that assembles into the preinitiation complex — this is the form that general transcription factors recognize at the promoter. Phosphorylation is progressive and dynamic during transcription: Ser5 phosphorylation occurs early (initiation/early elongation), Ser2 phosphorylation increases during productive elongation. After transcription terminates, phosphatases remove these marks, regenerating the hypophosphorylated CTD for the next initiation event.

Question 5

Why is the RNA Pol II CTD described as a 'coordination platform' for mRNA processing, and how does its phosphorylation code achieve temporal ordering of capping, splicing, and polyadenylation?

Model answer: The CTD carries the instructions for mRNA processing embedded in its phosphorylation state, so that processing factors are recruited automatically at the correct stage of transcription rather than requiring separate recruitment events. Ser5 phosphorylation (early, near the promoter) recruits capping enzymes, ensuring the 5' cap is added co-transcriptionally as soon as the nascent RNA emerges. Ser2 phosphorylation (during elongation) recruits splicing factors for co-transcriptional splicing and 3' end processing factors for cleavage and polyadenylation at the gene's end. Each phosphorylation mark is a signal that physically brings the right machinery to Pol II at the right moment.

The functional payoff is efficiency and quality control: coupling processing to transcription means each step happens at the right moment without the transcript being released to the cytoplasm prematurely. In vitro transcription systems (where the CTD is absent or truncated) show dramatically less efficient processing, confirming that the coupling is not incidental but mechanistically essential. The CTD is why mRNA maturation is so well-coordinated in living cells.