Lec22 - Lecture 22 Eukaryotic RNA Processing 1 Processing...

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Lecture 22 Eukaryotic RNA Processing 1 Processing of mRNA A. 5’ Capping B. C. Splicing Lodish 6th edition Chapter 8 L.5th edition Chapter 12
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In contrast to the situation in bacteria (where transcription and translation are coupled) in eukaryotes the presence of a nucleus allows: 1. Uncoupling of transcription and translation 2. RNA “processing” before translation 3. Control of gene expression at additional steps between transcription and translation (RNA processing and RNA transport to cytoplasm). Virtually all the primary transcripts or “ pre-RNAs” newly synthesized by RNA polymerases I , II and III are “processed” . All RNA processing occurs in the nucleus prior to export of the mature RNAs to the cytoplasm. Processing involves many different kinds of RNA modifications and result in RNAs that differ from the original DNA template (and from the primary transcripts) in length and sequence.
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ATG TGA Intron 1 Intron 2 Intron 3 Intron 4 Intron 5 Exons 1 2 3 4 5 6 Gene (DNA) Pre-mRNAs Poly(A) site Transcription termination sites CAP 5’ 3’ 5’ 3’ 5’ 3’ AAAA. .A 100-250 5’ 3’ AAAA. .A AUG UGA AUG UGA mRNA Transcription “capping” 3’-end cleavage (by endonuclease) Polyadenylation (by Poly A polymerase + ATP) Splicing (excision of pre-mRNA introns) Overview of pre-mRNA processing in eukaryotes Poly(A) signal AAUAAA Poly (A) Poly (A) CAP hnRNAs (heterogeneous nuclear RNAs) 1 2 3 4 5 6 CAP site
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RNA-processing factors with the phosphorylated CTD of RNA polymerase II Due to the remarkable length of the CTD many factors can interact at the same time with Pol II The CTD contains repeats of the heptamer consensus sequence YSPTSPS (26 repeats in yeast, 52 in mammals) (yeast) (Mammalian Pol II CTD twice as long!!) 2 5 S2 Phosphorylation by CDK9 subunit of P-TEFb -> promoter clearance & Elongation -> 3’- end processing of mRNA S5 Phosphorylation by CDK7 subunit of TFIIH (also by CDK8 subunit of Mediator) -> Recruitment of Capping enzyme & capping stimulated -> Recuitment of P-TEFb and subsequent S2 phosphorylation Componetns of the basal transcription machinery contain cyclin-CDK kinase pairs that phosphorylate the CTD:
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Lec22 - Lecture 22 Eukaryotic RNA Processing 1 Processing...

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