During RNA polymerase II-mediated mRNA transcription in eukaryotes, the heptapeptide repeat domain (CTD) of RNA Pol II large subunit must be phosphorylated at serine-5 for which step, and at serine-2 for which step?
- A Ser-5: promoter clearance/capping; Ser-2: elongation/3' processing ✓
- B Ser-5: elongation; Ser-2: initiation complex formation
- C Ser-5: poly-A signal recognition; Ser-2: 5' cap addition
- D Ser-5: splicing; Ser-2: nuclear export
Correct answer: A. Ser-5: promoter clearance/capping; Ser-2: elongation/3' processing
Explanation
The CTD of RNA Pol II functions as a molecular scaffold coupling transcription to RNA processing. Phosphorylation of Ser-5 by TFIIH (CDK7) occurs during promoter clearance and is required for recruitment of the 5' capping enzyme. Ser-2 is phosphorylated by P-TEFb (CDK9) during productive elongation and recruits 3' processing (cleavage/polyadenylation) machinery. This co-transcriptional coupling ensures mRNA is capped and polyadenylated efficiently.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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