9.10
Pre-mRNA 처리: pre-mRNA 끝의 수정
진핵세포에서는 RNA 폴리머라제에 의해 만들어진 전사체가 핵을 빠져나가기 전에 변형되고 처리됩니다. 처리되지 않은 RNA를 성숙한 mRNA와 구별하기 위해 전구체 mRNA 또는 프리-mRNA라고 합니다.
약 20-40개의 리보뉴클레오티드가 RNA 중합효소에 의해 결합되…
진핵 세포에서 새로 전사된 mRNA를 전구체 mRNA 또는 pre-mRNA라고 합니다. 각 pre-mRNA는 두 가지 중요한 변형을 받습니다. 하나는 캡(cap)이라고 하는 분자의 5 프라임 말단에 있고, 다른 하나는 꼬리(tail)라고 하는 분자의 3 프라임 끝에 있습니다.
five-prime cap은 변형된 구아닌 뉴클레오티드인 단일 7-메틸구아노신으로 구성되며, 이는 triphosphate linkage에 의해 pre-mRNA의 첫 번째 뉴클레오티드에 부착됩니다.
pre-mRNA 전사체의 3-프라임 말단을 향한 특정 뉴클레오티드 세트(일반적으로 A, A, U, A, A, A)를 폴리아데닐화 신호(polyadenylation signal)라고 합니다. 3-프라임 말단 처리 기계를 모집하고 엔도리보뉴클레아제(endoribonuclease)가 3-프라임 절단 부위에서 전사체를 절단하도록 지시합니다.
그런 다음 또 다른 효소인 폴리아데닐레이트 중합효소(polyadenylate polymerase, PAP)는 200개에 달하는 긴 아데닌 뉴클레오티드 줄을 전사체의 3-프라임 말단에 추가합니다.
5-프라임 캡(five-prime cap)과 3-프라임 폴리-A 꼬리(three-prime poly-A tail)는 엑소뉴클레아제(exonuclease)에 의한 분해로부터 전사체의 끝을 보호합니다. 쓰리 프라임 꼬리는 또한 mRNA 전사체가 핵을 떠날 준비가 되었다는 신호를 운반 분자에 보냅니다. 핵 외부에서, five-prime cap은 리보솜이 전사체에 부착되어 번역을 시작할 수 있도록 도와줍니다.
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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.