Post-transcriptional modification
Chemical alterations to RNA after transcription produce functional molecules.
Post-transcriptional modification encompasses a range of chemical alterations applied to RNA primary transcripts after transcription, processes common to most eukaryotic cells. These modifications are crucial for producing functional RNA molecules that can exit the nucleus and perform diverse cellular roles. A primary example is the conversion of precursor messenger RNA (pre-mRNA) into mature mRNA capable of protein translation. This involves three major steps: adding a 5' cap, adding a 3' polyadenylated tail, and RNA splicing. The cap, typically 7-methylguanosine attached via a 5'5' triphosphate link, protects the RNA from ribonucleases that target phosphodiester bonds; additional methylation of adjacent ribose sugars can create cap structures of increasing complexity. In some organisms, metabolites like nicotinamide adenine dinucleotide also serve as caps, potentially linking gene expression to metabolic states. The 3' end processing involves cleavage at a specific site, often following a polyadenylation signal sequence (such as AAUAAA), and the subsequent addition of roughly 200 adenine residues by polyadenylate polymerase. This poly(A) tail, bound by poly(A)-binding proteins, shields the 3' end from degradation. RNA splicing removes non-coding introns and joins coding exons, a reaction catalyzed by the spliceosome. Many pre-mRNAs can be spliced in alternative patterns, generating diverse protein products from a single gene. Post-transcriptional modifications also apply to other RNA types, such as transfer RNA and ribosomal RNA. Notably, core histone mRNAs undergo a distinct processing pathway, as they lack introns and poly(A) tails.
- field
- Molecular biology
- known_for
- Chemical alteration of RNA after transcription to produce functional RNA molecules
- processes
- 5' capping, 3' polyadenylation, RNA splicing
Lore & Background
Post-transcriptional modification encompasses a set of biological processes common to most eukaryotic cells, chemically altering an RNA primary transcript after transcription to produce a mature, functional molecule. The three major steps that significantly modify the RNA’s chemical structure are the addition of a 5' cap, the addition of a 3' polyadenylated tail, and RNA splicing. The 5' cap is formed by adding 7-methylguanosine to the 5' end, a process requiring removal of the terminal phosphate by RNA triphosphatase, followed by guanosyl transferase catalyzing a 5'5' triphosphate link, and finally methylation of the guanine ring by a methyltransferase using S-adenosyl methionine. This cap structure protects the 5' end from ribonucleases that target 3'5' phosphodiester bonds. In plants, humans, and yeast, nicotinamide adenine dinucleotide has also been identified as a cap, possibly regulating gene expression via metabolites. At the 3' end, cleavage and polyadenylation occur, typically triggered by a polyadenylation signal sequence (AAUAAA) near the 3' end, followed by a CA cleavage site and a downstream GU-rich region. Multi-subunit proteins, including cleavage and polyadenylation specificity factor and cleavage stimulation factor, bind these sequences, and polyadenylate polymerase adds about 200 adenine units using ATP, forming a poly(A) tail that binds poly(A)-binding protein to protect against ribonuclease digestion. RNA splicing removes non-coding introns and joins coding exons, catalyzed by the spliceosome, a complex of proteins and small nuclear RNAs. Many pre-mRNAs, such as those for antibodies, undergo alternative splicing, producing diverse proteins from limited DNA. Histone mRNA processing differs, as core histone mRNAs lack poly(A) tails and introns, and thus do not undergo splicing. These modifications are vital for correct translation, as precursor mRNA often contains both exons and introns, and the cap and tail facilitate transport to ribosomes and protect against degradation.
Reader's Guide
Post-transcriptional modification encompasses a set of biological processes common to most eukaryotic cells, chemically altering an RNA primary transcript after transcription from a gene to produce a mature, functional RNA molecule. A key example is the conversion of precursor messenger RNA into mature mRNA, which involves three major steps: addition of a 5' cap, addition of a 3' polyadenylated tail, and RNA splicing. The 5' cap is formed by adding 7-methylguanosine to the 5' end, requiring removal of the terminal phosphate by RNA triphosphatase, followed by guanosyl transferase creating a diphosphate end that attacks a GTP molecule to form a unique 5'5' triphosphate link. A methyltransferase then transfers a methyl group to the guanine ring, producing a cap 0 structure; further methylation of the ribose of adjacent nucleotides yields cap 1, cap 2, and cap 3 structures. This cap protects the 5' end from ribonucleases. In plants and some other eukaryotes, nicotinamide adenine dinucleotide has also been identified as a cap, possibly regulating gene expression via metabolites. At the 3' end, cleavage and polyadenylation occur primarily when a polyadenylation signal sequence (AAUAAA) is near the 3' end, followed by a CA cleavage site and a downstream GU-rich sequence. Alternate signals like UGUA upstream can also direct cleavage. Multi-subunit proteins, including cleavage and polyadenylation specificity factor, cleavage factor I, and cleavage stimulation factor, bind these sequences. The complex cleaves the RNA, and polyadenylate polymerase adds about 200 adenine units using ATP. The poly(A) tail binds poly(A)-binding protein, protecting the 3' end from digestion by enzymes including the CCR4-Not complex. RNA splicing removes non-coding introns and connects exons, catalyzed by the spliceosome. Alternative splicing allows production of multiple protein variants from a single gene. Histone mRNA processing differs, lacking poly(A) tails and introns, instead using a stem-loop structure. These modifications are fundamental to the diversity and regulation of the eukaryotic transcriptome.
Did You Know?
- The 5' cap is added via a 5'5' triphosphate link and can be methylated to form cap 0, cap 1, cap 2, or cap 3 structures.
- Nicotinamide adenine dinucleotide has been identified as a cap in plants, human, and yeast, possibly regulating gene expression via metabolites.
- Core histone mRNAs lack poly(A) tails and introns, and their 3' processing uses a stem-loop structure and U7 snRNA.
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