RNA polymerase II transcribes protein-coding mRNA. The C-terminal domain (CTD) of its largest subunit is extensively phosphorylated during transition from transcription initiation to elongation. Phosphorylation of Ser5 and Ser2 of the heptapeptide CTD repeat serves as a landing platform for which processes?
- A Ser5-P recruits the 3' polyadenylation machinery; Ser2-P recruits the 5' capping enzyme
- B Ser5-P recruits the 5' capping enzyme; Ser2-P recruits splicing factors and polyadenylation machinery ✓
- C Both Ser5 and Ser2 phosphorylation recruit ribosome assembly factors to the nucleus
- D Ser5-P directly recruits RNA helicase to unwind the template; Ser2-P pauses elongation
Explanation
RNAP-II CTD phosphorylation couples transcription with co-transcriptional RNA processing. Ser5 phosphorylation (by TFIIH cyclin-dependent kinase CDK7) occurs at promoter-proximal regions and recruits the mRNA capping enzyme (guanylyltransferase + methyltransferase), ensuring 7-methylguanosine cap addition immediately after transcription begins. Ser2 phosphorylation (by P-TEFb/CDK9) marks productive elongation through the gene body and recruits factors for co-transcriptional splicing (SR proteins via SCAFs) and 3' end processing (polyadenylation cleavage/CPSF complex). This phospho-code coordinates all mRNA maturation events with ongoing transcription.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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