13. Which one of the following statements about eukaryotic RNA processing is INCORRECT?
(1) Termination of transcription occurs co- transcriptionally at the polyadenylation site.
(2) Phosphorylation of the Ser 2 of the RNA polymerase CTD is required for the recruitment of polyadenylation factors.
(3) Polyadenylation requires both cleavage at the polyadenylation site and addition of poly A.
(4) Xrn 2 is the nuclease that degrades the cleaved RNA to release RNA polymerase II from the template.

Understanding Eukaryotic RNA Processing: Identifying the Incorrect Statement

Description: Explore the intricacies of eukaryotic RNA processing, focusing on transcription termination, polyadenylation, and the role of key enzymes. Learn which statement about these processes is incorrect and why.

Eukaryotic RNA processing involves multiple coordinated steps, including transcription termination, polyadenylation, and RNA polymerase II (RNAPII) release. Below, we analyze the given statements to identify the incorrect one:

Statement Analysis

1. “Termination of transcription occurs co-transcriptionally at the polyadenylation site.”

   • Incorrect. While polyadenylation signals (PAS) are critical for termination, transcription termination does not occur directly at the PAS. Instead, the RNA is cleaved downstream of the PAS, and termination is triggered afterward. The cleavage event (10–30 nucleotides downstream of the PAS) precedes RNAPII release, which is mediated by degradation of the downstream RNA fragment by XRN Thus, termination occurs after PAS recognition and cleavage, not precisely at the PAS.

2. “Phosphorylation of the Ser2 of the RNA polymerase CTD is required for the recruitment of polyadenylation factors.”

   • Correct. Phosphorylation of the C-terminal domain (CTD) of RNAPII at Ser2 residues is essential for recruiting polyadenylation factors like CPSF (cleavage and polyadenylation specificity factor) and CstF (cleavage stimulation factor). These factors bind to the PAS and downstream elements to initiate cleavage and polyadenylation.

3. “Polyadenylation requires both cleavage at the polyadenylation site and addition of poly A.”

   • Correct. Polyadenylation involves two steps:

     • Cleavage: The pre-mRNA is cleaved at a specific site downstream of the PAS.

     • Poly(A) tail addition: Poly(A) polymerase (PAP) adds ~200–250 adenine residues to the 3′ end of the cleaved RNA.
       Both steps are mandatory for functional mRNA maturation.

4. “Xrn2 is the nuclease that degrades the cleaved RNA to release RNA polymerase II from the template.”

   • Correct. XRN2, a 5’→3′ exonuclease, degrades the downstream RNA fragment remaining tethered to RNAPII after cleavage. This degradation dislodges RNAPII from the DNA template, terminating transcription.

Summary of the Incorrect Statement

The incorrect statement is (1). Transcription termination is not initiated at the polyadenylation site but occurs downstream after RNA cleavage and XRN2-mediated degradation. The PAS defines the cleavage site, but termination follows as a separate step.

Correct Answer: (1) Termination of transcription occurs co-transcriptionally at the polyadenylation site.

This analysis clarifies the interplay between polyadenylation signals, RNA cleavage, and transcription termination, emphasizing the precision required in eukaryotic gene expression.