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RNA Processing:- Eukaryotic mRNAs Are Capped at the 5 End

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  1008

2026-07-23

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RNA Processing:- Eukaryotic mRNAs Are Capped at the 5 End

Most eukaryotic mRNAs have a 5cap, a residue of 7 methyl guanosine linked to the 5-terminal residue of the mRNA through an unusual 5,5-triphosphate linkage (Fig. 1). The 5 cap helps protect mRNA from ribonucleases. The cap also binds to a specific cap binding complex of proteins and participates in binding of the mRNA to the ribosome to initiate translation . The 5 cap is formed by condensation of a molecule of GTP with the triphosphate at the 5end of the transcript. The guanine is subsequently methylated at N-7, and additional methyl groups are often added at the 2 hydroxyls of the first and second nucleotides adjacent to the cap (Fig. 1). The methyl groups are derived from S-adenosylmethionine. All these reactions occur very early in transcription, after the first 20 to 30 nucleotides of the transcript have been added. All three of the capping enzymes, and through them the 5end of the transcript itself, are associated with the RNA polymerase II CTD until the cap is synthesized. The capped 5 end is then released from the capping enzymes and bound by the cap-binding complex (Fig. 1c).

FIGURE 1 The 5cap of mRNA. (a) 7-Methylguanosine is joined to the 5end of almost all eukaryotic mRNAs in an unusual 5,5 triphosphate linkage. Methyl groups (pink) are often found at the 2position of the first and second nucleotides. RNAs in yeast cells lack the 2-methyl groups. The 2-methyl group on the second nucleotide is generally found only in RNAs from vertebrate cells. (b) Generation of the 5 cap involves four to five separate steps (adoHcy is S adenosylhomocysteine). (c) Synthesis of the cap is carried out by en zymes tethered to the CTD of Pol II. The cap remains tethered to the CTD through an association with the cap-binding complex (CBC).