生物化学原理教学（杨荣武）Chapter37 Post-transcriptional processing.pptVIP

Outline Bacterial post-transcriptional processing Processing of Primary mRNA (rare) Processing of Primary rRNA Processing of Primary tRNA Eukaryotic post-transcriptional processing Processing of Primary mRNA Processing of Primary rRNA Processing of Primary tRNA Archaeal post-transcriptional processing Processing of Primary mRNA in Eukaryotes Processing types 5end = capping 3end = tailing Internal = splicing Internal methylation Editing Mechanism Capping and Methylation Structure and types: 0, I and II type Capping reaction: co-transcriptional - Enzymes: Phosphohydrolase, guanylyl transferase and methyl transferases Functions Why have a Cap? The CAP structure promotes stability of the mRNA (prevents degradation by 5 exonucleases) The CAP structure promotes translation of the mRNA A ribosomal protein subunit interacts with the CAP and recruits mRNA to the ribosome Why are only mRNAs Capped? The CAP structure is only added to transcripts synthesized by RNAP II (i.e. mRNA transcripts) This is because the capping enzymes bind to a part of RNAP II that is unique to that enzyme (not found in RNAP I or RNAP III) Capping enzymes bind to the CTD of RNAP II They bind only to the elongating form of the CTD ( i.e. the phosphorylated CTD) As soon as the transcript emerges from RNAP II it is in contact with the capping enzymes 3-Polyadenylation A poly(A) tail is added to the 3’ end of the transcript in 2 steps… 1) cleavage: the RNA is cut 10-30 nucleotides downstream of a specific sequence in the 3’UTR 2) addition of A’s (100-200 are added) to generate a poly(A) tail Why have a Poly(A) tail? The poly(A) tail promotes stability of the mRNA (prevents degradation by 3’ exonucleases) The poly(A) tail promotes translation of the mRNA A ribosomal protein subunit that interacts with the CAP structure is stimulated in CAP binding by poly(A) binding protein Therefore the CAP and the poly(A) tail work synergistically to recruit mRNA to the ribosome Create stop codon Alternativ