I' Chapter 14: Messenger RNA Processing I: Splicing continued

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Molecular Genetics Bio350/516 Lecture 24, 4/20/06
I. Chapter 14 Messenger RNA Processing I
Splicing (continued) A. Review splicesome
assembly and structure B. Just when you
thought things couldnt get any more complicated
1. Splicing factors and branchpoint
bridging proteins C. The role of RNAP CTD in
splicing 1. Branchpoint A consensus
sequences D. The role of the RNAP II CTD in
splicing E. Alternative splicing F.
Self-Splicing RNAs 1. Group I introns
2. Group II introns G. tRNA splicing
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Figure 14.28
Commitment complex
ATP
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snRNP Structure
1. All snRNPs have the same 7 Sm proteins 2.
Bind to a common Sm site AAUUGUGG 3. In
addition, each snRNP has its own set of
specific proteins
U1 snRNP structure
Figure 14.29
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Splicing Factors and Bridging Proteins
1. Slu7 (recognizes 3-splice site) a. Slu7
holds the upstream exon tightly so can bind to
the correct AG 2. SC35 (SR proteins
rich in serine and arginine) a. But SR
proteins do not exist in yeast, so must be other
proteins in yeast that do the same
thing 3. Branchpoint Bridging Proteins
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Figure 14.36
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The Role of RNAP II CTD In Splicing
1. Also plays a role in capping and
polyadenylation 2. CTD recruits splicing
substrates to active spliceosomes 3. Assembles
splicing factors on exons as they are
synthesized
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Figure 14.39
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Alternative Splicing
1. 1 in 20 eukaryotic premRNAs can be spliced in
more than one way 2. In humans, 60 of
transcripts can be subject to alternative
splicing 3. Mouse immunoglobulin gene ? heavy
chain gene a) secreted form and membrane
bound form
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Figure 14.40
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Self-Splicing RNAs
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Group I Introns
1. 26S rRNA from Tetrahymena has an intron and
can self- splice 2. Questions that had to be
answered a. Was the splicing carried out by
the RNA itself, or was the RNAP somehow
involved? b. The band on the gel is the right
size for the intron, but is it really?
(sequence analysis) c. It can remove the
intron, but can it splice the exons
together? 3. Requires GTP 4. First example of a
ribozyme
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Figure 14.47
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Group II Introns
1. Mitochondrial and chloroplast genes have both
group I and group II introns 2. Hypothesis
nuclear premRNA introns descended from
bacterial group II introns a. Archaea have
group II introns as well as cyanobacteria and
purple bacteria
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snRNPs vs. Group II Introns
Ribozyme
Figure 14.23
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tRNA Splicing
1. Introns are in the same place (one nucleotide
to the 3 side of the anticodon) 2. Step 1
tRNA endonuclease cuts out the intron 3. Step 2
RNA ligase rejoins the two halves together
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Figure 14.50