Q.50 5′ capping of mRNA transcripts in eukaryotes involves the following events:
P. Addition of GMP on the 5′ end
Q. Removal of γ-phosphate of the triphosphate on first base at the 5′ end
R. 5′-5′ linkage between GMP and the first base at 5′ end
S. Addition of methyl group to N7 position of guanine
Which one of the following is the correct sequence of events?
(A) P, Q, R, S
(B) P, R, Q, S
(C) Q, P, R, S
(D) Q, P, S, R
Understanding 5′ Capping in Eukaryotic mRNA Processing
The 5′ capping process modifies the 5′ end of nascent pre-mRNA transcripts shortly after transcription initiation by RNA polymerase II. This cap structure protects mRNA from degradation, aids nuclear export, and facilitates translation initiation. The correct sequence of events determines the answer to this multiple-choice question on mRNA capping steps.
Correct Answer and Sequence
The correct option is (C) Q, P, R, S. Eukaryotic mRNA capping begins with RNA triphosphatase removing the γ-phosphate from the 5′ triphosphate end (pppN) of the first nucleotide, yielding a diphosphate end (ppN) [Q]. Guanylyltransferase then adds GMP from GTP to this ppN, forming a 5′-5′ triphosphate linkage (GpppN) [P, R]. Finally, guanine-7-methyltransferase methylates the N7 position of the added guanine (m7GpppN) [S].
Detailed Explanation of Events
Step 1: γ-Phosphate Removal (Q)
RNA triphosphatase hydrolyzes the γ-phosphate from the 5′ triphosphate (pppN), creating a 5′ diphosphate RNA (ppN). This step is essential as it prepares the end for GMP addition.
Step 2: GMP Addition and Linkage Formation (P, R)
Guanylyltransferase transfers GMP to the 5′ diphosphate, cleaving pyrophosphate from GTP and establishing the unusual 5′-5′ triphosphate bridge (GpppN). This backward linkage distinguishes eukaryotic caps.
Step 3: N7 Methylation (S)
Methyltransferase adds a methyl group to the N7 nitrogen of the guanine, forming the mature cap-0 structure (m7GpppN), which enhances mRNA stability and eIF4E binding.
Analysis of All Options
| Option | Sequence | Why Incorrect/Correct |
|---|---|---|
| (A) P, Q, R, S | GMP addition first, then γ-phosphate removal | GMP cannot add to triphosphate end; γ-phosphate must be removed prior. |
| (B) P, R, Q, S | GMP addition and linkage before γ-phosphate removal | Reverses first two steps; diphosphate end required first. |
| (C) Q, P, R, S | γ-phosphate removal, GMP addition, 5′-5′ linkage, N7 methylation | Matches enzymatic order: triphosphatase, guanylyltransferase, methyltransferase. |
| (D) Q, P, S, R | Methylation before 5′-5′ linkage formation | Linkage forms GpppN first; methylation follows on structured cap. |
This sequence ensures efficient co-transcriptional capping, typically within 20-30 nucleotides of transcription start.
