A7 Messenger RNA

1. A7 Messenger RNA A single-stranded RNA sequence, carrying the coding information of its protein, called as messenger RNA (mRNA).

2. Synthesis and processing • Splicing • Capping • Polyadenylation • mRNA stability • Alternative processing of mRNA

3. 1.Synthesis and processing • In eukaryotes, mRNA is produced by transcription of protein coding genes by RNA polymerase Ⅱ. • In prokaryotes, mRNA is not processed and translation of message begins even before transcription is complete. • Usually do not contain introns and splicing unnecessary.

4. 多顺反子mRNA：一个mRNA包含多个蛋白质的编码信息，即包含多个个开放可读框（ORF），这种mRNA称为多顺反子mRNA；这些蛋白质通常具有同一种相关的功能，并且成组的被一个操纵子（operon）所调控；多顺反子mRNA：一个mRNA包含多个蛋白质的编码信息，即包含多个个开放可读框（ORF），这种mRNA称为多顺反子mRNA；这些蛋白质通常具有同一种相关的功能，并且成组的被一个操纵子（operon）所调控； 原核生物mRNA通常是这种情况。真核生物里也有这种情况，较少。

5. Eukaryotic pre-mRNA is the transcript of both exons and introns， its coding information （EXON regions）is discontinuous，separated by noncoding INTRONs. • hnRNA (heterogeneous nuclear RNA): the pre-mRNAs transcribed by RNA polymerase II in nuclus show large variable lengths, so, these pre-mRNAs are called hnRNA. • 未经加工的mRNA在分子大小上差别很大，所以通常称为核不均一RNA。

6. Processing comprises of splicing, 5’ capping and 3’ tailing (polyadenylation).

7. Capping • Addition of 7-methylguanosine at the 5’ end of eukaryotic mRNAs, catalyzed by guanyl-transferase which joins GTP by 5’ →5’tri-phosphate linkage • Add -CH3 by methyl transferase at both the 7-nitrogen of the guanine ring and the 2’ hydroxyl group of the next two nucleotides

9. 5’-5’ 三磷酸连接，不同于磷酸二酯键，具有保护mRNA末端的作用。

10. The function of capping: • Protect the mRNA from being degraded from the 5’ end by exonucleases in the cytoplasm • 保护mRNA被降解掉，否则易被核酸外切酶切割 • A signal allowing the ribosome to recognize the start of a mRNA molecule • 核糖体识别翻译起始位点

11. Polyadenylation • Addition of a sequence of up to 250 adenines (poly A tail) at the 3’ ends of most eukaryotic mRNA by poly(A) polymerase

12. Polyadenylation signal: 5’ AAUAAU 3’,YA in the next 11-20 bases and GU rich sequence often present further downstream • Y=C/G

15. Function: uncertain but may serve to protect the mRNA from degradation of the coding sequence at the 3’ end by exonucleases • 保护mRNA被核酸外切酶降解 • However, some mRNAs lack poly A tail, e.g. histone genes

16. Splicing • Splicing signals: • (for majority of mRNAs) First two nucleotides are GU at the 5’ end (5’-AGGTAAGU-3’)of and the last two are AG at the 3’ end (5’-YYYYYYNCAG-3’) of an intron. • “GU---AG”法则剪切内含子

17. Branchpoint sequence (5’-CURAY-3’) located in introns 10-40 bases upstream of the 3’ signal sequence. (in vertebrate). In yeast, it is 5’-UACUAAC-3’ • 内含子的分支点序列 （RNA剪接的信号之一）

18. Splicing process: catalyzed by small nuclear ribonucleoproteins (snRNPs),U1,U2,U4,U5, and U6. • Their RNAs interact by base-pairing with the splicing signal sequences. • U1 binds to the 5’ splice site, U2 binds to branchpoint sequence • U5 and U4/U6 form a complex with U1 and U2 causing the introns to loop out and the exons to be brought together， forming Spliceosome

20. mRNA stability • Generally short-live compared to rRNAs and tRNAs。 • Half-life for mRNA: • in prokaryotic cell, a few min; • in eukaryotic cell, as much as 6h

21. Alternative splicing of mRNA • The same pre-mRNA may be alternatively processed within a single cell type, between different cell types and in the same cell type at a different stages of development. • 不同组织细胞或同种细胞不同发育阶段，由于剪接作用的差异可以产生具有不同编码的mRNA,导致翻译生成不同的蛋白质产物，称为mRNA的可变剪接。

23. RNA editing: • pre-mRNA sequence alteration. • RNA编辑是指在mRNA水平上改变遗传信息的过程。具体说来，指基因转录产生的mRNA分子中，由于核苷酸的缺失，插入或置换，基因转录物的序列不与基因编码序列互补，使翻译生成的蛋白质的氨基酸组成，不同于基因序列中的编码信息现象。

25. Key points • The difference between prokaryotic and eukaryotic mRNA. • What are capping and polyadenylation? Their functions. • alternative splicing of mRNA • RNA editing.