Q.46 Which one of the following covalent linkages exists between 7-Methyl guanosine (m7G) and mRNAs?
(A) 2′-3′ triphosphate (B) 3′-5′ triphosphate
(C) 5′-5′ triphosphate (D) 2′-5′ triphosphate
Correct Answer: (C) 5′-5′ triphosphate
The 7-methylguanosine (m7G) cap forms a unique covalent linkage with eukaryotic mRNAs at the 5′ end, essential for mRNA stability, export, and translation initiation. This cap structure protects mRNA from degradation and facilitates recognition by initiation factors like eIF4E.
Option Analysis
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(A) 2′-3′ triphosphate: Incorrect. The 2′ and 3′ hydroxyls on ribose are involved in standard phosphodiester bonds within RNA chains or RNA degradation pathways, not m7G capping.
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(B) 3′-5′ triphosphate: Incorrect. 3′-5′ phosphodiester linkages connect nucleotides in the mRNA backbone, but the m7G cap uses a reverse orientation via 5′-5′ triphosphate bridge.
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(C) 5′-5′ triphosphate: Correct. m7G is attached in an inverted manner to the first nucleotide’s 5′ end through a 5′-5′ triphosphate linkage (m7GpppN), formed by guanylyltransferase after RNA triphosphatase action.
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(D) 2′-5′ triphosphate: Incorrect. 2′-5′ linkages occur in unusual contexts like 2-5A oligoadenylates from interferon response or some viral RNAs, but not standard mRNA capping.
m7G Cap Structure
The cap forms post-transcriptionally: RNA 5′-triphosphatase removes the γ-phosphate, guanylyltransferase adds GMP via 5′-5′ triphosphate, and methyltransferase adds N7-methyl to guanosine. This yields m7G(5′)ppp(5′)N, where N is the first mRNA nucleotide, often further methylated (Cap 1/2 structures).
The 7-Methylguanosine m7G mRNA covalent linkage is a cornerstone of eukaryotic gene expression, featuring a distinctive 5′-5′ triphosphate bridge that distinguishes it from standard RNA bonds. This m7G cap structure—m7GpppN—anchors at the mRNA 5′ terminus, shielding against exonucleases while enabling nuclear export via CBC and translation via eIF4E binding.
Why 5′-5′ Triphosphate?
Unlike the 3′-5′ phosphodiester backbone, the m7G cap inverts polarity: guanosine’s 5′-triphosphate links to the mRNA’s 5′-diphosphate remnant, creating m7G(5′)ppp(5′)N. Enzymatic steps include:
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RNA triphosphatase: pppN → ppN.
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Guanylyltransferase: GTP + ppN → GpppN (retaining β-γ phosphates from GTP).
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Guanine-N7-methyltransferase: GpppN → m7GpppN.
This 5′-5′ triphosphate linkage boosts mRNA half-life and efficiency, with defects linked to diseases like cancer via dysregulated translation.
Common Misconceptions
Examinees often confuse it with:
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3′-5′ triphosphate: Backbone linkage, not cap.
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2′-3′ or 2′-5′: Rare or degradative bonds.
For biotech researchers, understanding this 7-Methylguanosine m7G mRNA covalent linkage aids synthetic mRNA design in vaccines and therapies.
