RNA Processing

1
Lecture 19

• RNA Processing

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General features of RNA processing
(AAUAAA)
Transcription
(heterogeneous nuclear RNA)
Capping, methylation, poly A addition Splicing,
transport
Processing
3
Pre-mRNA is capped shortly after the
initiation of transcription.
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Methylation of the first two bases may also occur
5
Polyadenylation of 3? ends
Transcription and RNA capping
RNA Polymerase II
Poly (A) signal
3 cleavage at specific site 10-30 nt downstream
of poly (A) addition site
Cleavage and polyadenylation
Poly (A) polymerase adds 100-200 adenylyl
residues to 3 end cleaved RNA
6
RNA Editing
LDL receptor binding
Lipoprotein assembly
CAA
ApoB-100 (found in LDL)
RNA editing by deamination of C
UAA
Stop
ApoB-48 (found in chylomicrons)
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Introns Must be removed before translation
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Types of introns

• Group I
• A few rRNAs

Self-Splicing introns

• Group II
• Mitochondrial and chloroplast mRNAs

• tRNA
• Some tRNAs
• Requires ATP

• Others mRNA
• Largest class
• Nuclear mRNAs
• Requires spliceosome and ATP

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Splicing occurs via a transesterification reaction
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Group I Intron Self-Splicing Group II
Introns
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Ribozymes have enzymatic properties

• Saturable
• Binding of guanosine cofactor
• Specificity
• Catalyzes reactions at only certain bases
• Internal guide sequence
• Can catalyze multiple reactions
• Usually does not happen because it is a self
  splicing mechanism

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Splicesomes
mRNA processing is mediated by snRNPs(small
nuclear ribonucleoproteins) or snurps
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Splicesome assembly
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Splicesome action

• The catalytic center of the spliceosome is formed
  by U2 and U6 snRNAs which are base paired.

• Base pairing of U2 snRNA with the branch point is
  thought to displace and activate the adenosine
  whose 2 OH attacks the 5 splice site

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Alternative processing can result in different
polypeptides