Lecture 24 Euk RNA processing

Lecture 24 Euk RNA processing - Eukaryotic RNA Processing...

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Eukaryotic RNA Processing Dr. Matera 20 Mar 2009 The 3D structure of human U1 snRNP as  determined by cryo-electron microscopy at 2.5  nm resolution. (H. Stark, P. Dube, R.  Lührmann, B. Kastner) U1 snRNP Cryo-EM structure                              3D model
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Expanded view of Central Dogma in Euks In eukaryotes, the presence of a nuclear membrane imposes a spatial and temporal separation between transcription and translation. Eukaryotes Prokaryotes
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In 1977, two groups showed that, in addition to being modified at both ends, pre-mRNAs are processed via the removal of internal segments, termed “ introns .” The expressed sequences are called “ exons .” The presence of sequences within genes that are not represented in their mRNA products was first demonstrated by R-loop mapping. Phil Sharp and Rich Roberts shared the Nobel Prize for this discovery in 1993. Gene, interrupted 
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(longer) export polyA signal Fig. 8.14
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RNA polymerase II carries pre-messenger RNA processing proteins, including the capping enzymes, on its C-terminal “tail” (a.k.a. CTD). A similar arrangement ensures efficient transfer of certain key splicing and polyadenylation factors to the nascent pre-mRNA. The C-terminal domain of RNA pol II Facilitates co-transcriptional RNA processing events
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Capping occurs  co-transcriptionally , shortly  after initiation of RNA synthesis by polymerase II  pppNpN m 7 G pppN m pN m A methylguanosine is added to the 5' end of the nascent RNA and, in some cases, the first two encoded nucleotides are methylated.
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Unusual 5’ to 5’ linkage A closer look at the 5’ cap  Capping protects the RNA from degradation, and is required for efficient export from nucleus and translation in cytoplasm Fig. 8.13
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Eukaryotic mRNAs are also modified at their 3' ends via polyadenylation , which is mechanistically linked to transcription termination . cleavage of the primary transcript occurs ~10-30 nts downstream from the AAUAAA consensus sequence polyadenylation is catalyzed by poly(A) polymerase ~200 adenosine residues are added in a non-templated reaction protects mRNA from degradation m 7 GpppNpN AAUAAA cleavage 3' m 7 GpppNpN AAUAAA A A A A A A polyadenylation 5'
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Insulin gene promoter +1 poly-A signal 100 bp +1411 1 exon 2
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Lecture 24 Euk RNA processing - Eukaryotic RNA Processing...

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