Biolchem 415: Lec. 35
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What is synthesized by RNA polymerase I?
Ribosomal precursor RNAs
What is generated by cleavage of precursor RNA?
What catalyzes the synthesis of precursors to tRNA?
RNA polymerase III
What are synthesized by RNA polymerase II?
pre-mRNA (messenger RNA precursors)
What happens to pre-mRNA? What are the steps?
- 1. 5' cap
- 2. 3' poly-adenylate tail
- 3. removal of introns (intervening RNA)
What uses does the 5' cap have?
- 1. required for further processing
- 2. protects mRNA from degradation
- 3. enhances translation
What is the 5' cap composed of?
5'-5' triphosphate linkage. Terminal G methylated at nitrogen 7
What is formation of the 3' end of mRNAs coupled to?
Transcription termination by RNA polymerase II
How far downstream does the endonucleolytic cleavage in RNA pol II happen?
What do poly(A) tails do for the mRNA?
Increase stability and promote translation
What enzyme puts on the poly(A) tails?
Poly(A) polymerase: RNA polymerase, template independent, adds up to 250 A's to mRNA
Where does processing of mRNAs happen?
What does the poly(A) binding protein (PABP) do?
Coats the poly(A) tail and promotes translation
What is deadenylation and what is it for?
Removing of poly(A) tail enzymatically to begin process of degrading mRNAs
How was splicing discovered?
The mRNA was shorter than the DNA
To remove introns from precursor mRNAs
What are exons?
Joined by spicing to form a mature mRNA
How are hemoglobin mRNAs processed?
Capping, poly-adenylation, and splicing
What catalyzes splicing?
Small nuclear ribonucleoprotein complexes (snRNPs)
What forms the spliceosome and where?
U1 nRNP binds at 5' splice site. U2 binds at the branch site. Binding of U4-UU6 tri-snRNP completes spliceosome formation. Assemble on the pre-mRNA.
What is the splicing reaction?
2 sequential transesterification reactions (no energy needed to break/form phosphodiester bonds, but ATP needed to assemble and rearrange spliceosome)
What are the two steps to the splicing mechanism?
- 1. 5' splice site is attacked by 2' OH group of the branch site adenosine residue
- 2. 3' splice site is attacked by the newly formed 3' OH of the upstream exon
Splicing defects may cause what percentage of diseases?
How is thalassemia caused?
A to G mutation in the first intron of the hemoglobin beta chain
What is alternative splicing?
pre-mRNA can be spliced in different patterns, generating proteins with different functions
What is an example of alternative splicing?
- Calcitonin mRNA in thyroid cells (regulates calcium absorption)
- CGRP mRNA in neuronal cells (controls vasodilation and pain)
How are transcription and mRNA processing coupled?
Coordinated by the carboxyl-terminal domain (CTD) of RNA polymerase II
What are functions of the phosphorylated CTD?
- 1. Recruit enzymes to synthesize 5' cap
- 2. Recruit components of the spliceosome complex
- 3. Recruit endonuclease that cleaves the pre-mRNA to expose the site for poly(A) tail addition
Give an example of how mRNAs can be edited to change the function of the encoded protein?
Apobec enzyme: RNA editing activity, CAA to UAA: generates translational stop codon
What is a ribozyme?
A RNA that can function as a catalyst and self-splice
What are the steps to ribozymes?
- 1. Splicing catalyzed snRNA--U6 with U2 snRNA
- 2. Ribonuclease P cleaves 5' leader of pre-tRNA and contains catalytic RNA as component
- 3. Ribosomal rRNA catalyzes protein synthesis
- 4. Self splicing introns: intron removed fro preursor to yield 26S rRNA (in the absence of protein--requires guanoside)
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