## 5' Capping of mRNA **Key Point:** The 7-methylguanosine (m^7^G) cap is added co-transcriptionally to the 5' end of eukaryotic mRNA by guanylyltransferase (also called capping enzyme), which is a component of RNA polymerase II. ### Mechanism of Capping 1. **Phosphatase reaction**: RNA triphosphatase removes one phosphate from the 5'-triphosphate of nascent RNA, leaving a diphosphate. 2. **Guanylylation**: Guanylyltransferase transfers GMP from GTP to form a 5'-5' triphosphate linkage (Gppp-RNA). 3. **Methylation**: Guanine-7-methyltransferase adds a methyl group to the N-7 position of the terminal guanine, forming m^7^Gppp-RNA. 4. **2'-O-methylation**: The first and second nucleotides of the mRNA are also methylated at the 2'-OH position of the ribose. ### Functions of the 5' Cap - **Protection**: Protects mRNA from degradation by 5' exonucleases. - **Translation**: Recognized by cap-binding proteins (eIF4E) during translation initiation. - **Splicing**: Enhances spliceosome assembly and efficiency. - **Nuclear export**: Required for mRNA export from the nucleus. - **Innate immunity**: Distinguishes cellular mRNA from viral RNA. **High-Yield:** The cap is added **while transcription is still ongoing** (co-transcriptionally), not after transcription is complete. **Mnemonic:** **CAP** = **C**o-transcriptional **A**ddition of **P**urine (7-methylguanosine).
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