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Traitement pré-ARNm : Modification des Extrémités pré-ARNm
Dans les cellules eucaryotes, les transcrits fabriqués par l'ARN polymérase sont modifiés et traités avant de quitter le noyau. L’ARN non traité est a…
Dans les cellules eucaryotes, l’ARNm nouvellement transcrit est appelé ARNm précurseur, ou pré-ARNm. Chaque pré-ARNm reçoit deux modifications importantes. L’une à l’extrémité cinq premiers de la molécule, appelée le chapeau, et l’autre à l’extrémité trois premiers de la molécule, appelée la queue.
La coiffe à cinq premiers points est composée d’un seul 7-méthylguanosine, un nucléotide de guanine modifié, qui est attaché au premier nucléotide du pré-ARNm par une liaison triphosphate
.Un ensemble spécifique de nucléotides vers l’extrémité trois premières du transcrit du pré-ARNm, généralement A, A, U, A, A, A, est appelé signal de polyadénylation. Il recrute la machinerie de traitement des trois extrémités et dirige une endoribonucléase pour couper le transcrit au site de clivage des trois extrémités.
Une autre enzyme, la polyadénylate polymérase, ou PAP, ajoute ensuite une longue chaîne de nucléotides d’adénine, jusqu’à 200, à l’extrémité trois premiers du transcrit.
Le capuchon à cinq nombres premiers et la queue poly-A à trois premiers protègent les extrémités du transcrit de la dégradation par les exonucléases. La queue à trois premiers signaux signale également aux molécules de transport que le transcrit de l’ARNm est prêt à quitter le noyau. À l’extérieur du noyau, le capuchon à cinq premiers aide le ribosome à se fixer au transcrit afin qu’il puisse commencer la traduction.
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Q1: What is the 5' cap and why is it added to pre-mRNA?
The 5' cap is a 7-methylguanosine, a modified guanine nucleotide attached to the first nucleotide of pre-mRNA via a triphosphate linkage. It protects the transcript from degradation by exonucleases and helps the ribosome attach during translation. The cap also helps cells distinguish mRNA from other RNA types.
Q2: How does the poly-A tail protect mRNA and signal its readiness for translation?
Polyadenylate polymerase adds approximately 200 adenine nucleotides to the 3' end of pre-mRNA, forming the poly-A tail. This tail protects mRNA from degradation by exonucleases in the cytoplasm and signals to transport molecules that the transcript is ready to exit the nucleus for translation.
Q3: What is a polyadenylation signal and how does it direct 3' end processing?
A polyadenylation signal is a specific sequence of nucleotides, typically A, A, U, A, A, A, located toward the 3' end of pre-mRNA. It recruits the 3' end processing machinery and directs an endoribonuclease to cut the transcript at the 3' cleavage site, allowing polyadenylate polymerase to add the poly-A tail.
Q4: What is the difference between exons and introns in pre-mRNA?
Exons are RNA sequences that remain in the mature transcript and are expressed as proteins. Introns are portions removed during splicing by the spliceosome. Interestingly, a single RNA segment can function as an exon in one cell type and an intron in another, enabling alternative rna splicing to produce multiple proteins from one gene.
Q5: When does the 5' cap get added during transcription?
The 5' cap is added early during transcription, once approximately 20-40 ribonucleotides have been joined together by RNA polymerase. A group of enzymes replaces the 5' phosphate with modified 7-methyl guanosine, which helps the cell recognize mRNA and prepares it for subsequent translation.
Q6: How do the 5' cap and 3' poly-A tail work together to protect mRNA?
Both the 5' cap and 3' poly-A tail protect mRNA ends from degradation by exonucleases. The 5' cap marks the transcript as mRNA and facilitates ribosome binding, while the 3' tail signals nuclear export and protects the transcript in the cytoplasm, ensuring the mRNA remains stable for translation.
Q7: What happens to pre-mRNA after the 5' cap and poly-A tail are added?
After both modifications are complete, the mature mRNA exits the nucleus for translation. The 5' cap and 3' poly-A tail signal to transport molecules that the transcript is ready to leave the nucleus. Outside the nucleus, the 5' cap helps the ribosome attach to initiate protein synthesis.