英文PPT模板 (14)

U2 snRNA
-initially binds the branchpoint recognition sequence -forms two duplexes with U6, bringing the intron
5’-splice site close to the branchpoint
Spliceosomes are composed of Splicing Factor proteins (~40) plus small nuclear RNAs (snRNA) found in nucleoprotein (~10 proteins) complexes ... 50-60 S size ... These ribonucleoprotein complexes are called snRNP particles, or snurps snRNAs.
Processing
Prokaryotic RNA 2-5' half life; polycistronic; seldom modified; used immediately Coupled transcription and translation; many ribosomes bind same operon, and synthesize polypeptide simultaneously ... called polyribosomes or polysomes for short. Polycistronic mRNAs: coordinate expression of >1 gene: operon Multiple translational start sites per mRNA
1.1.1. Poly(A) on 3' end
Transcripts are cleaved at 3' end past a highly conserved AAUAAA sequence. Such Cleavage involves some protein cofactors
The Poly(A) polymerase (PAP) adds ~200 A residues to most mRNAs
U5 RNA
-initially complexed with U4 and U6 -binds exon sequences upstream of
the 5’-splice site and downstream of the 3’-splice site (guide sequence for holding exons
Chapter 9 RNA Processing
Very few RNA molecules are transcribed directly into the final mature RNA.
Most newly transcribed RNA molecules (primary transcripts) undergo various alterations to yield the mature product
Introns are InterVening Sequences (IVS), sequences found between Exon sequences in the genomic DNA, i.e. in the gene
2. Nuclear RNA splicing
RNA splicing occurs in the nucleus, on the initial transcripts (pre-mRNA or hnRNA) complexed with the nuclear proteins: hnRNP particles.
N-6 methylation of Adenines, at frequency of about 0.1%
1.2. Eukaryotic RNA Splicing:
Introns and Exons
Eukaryotic genes are often interrupted genes: Coding sequence is interrupted by noncoding sequences .
together?)
U6 RNA
-initially complexed with U4 and U5 -displaces U1 from 5’-splice site,
forming duplex with intron sequences -complexes with U2, bringing the
intron 5’-splice site close to the branchpoint. (catalytic activity?)
U6-U4 pairing is incompatible with U6-U2 pairing. When U6 joins the spliceosome it is paired with U4. Release of U4 allows a conformational change in U6; one part of the released sequence forms a hairpin, and the other part pairs with U2. Because an adjacent region of U2 is already paired with the branch site, this brings U6 into juxtaposition with the branch.
Then one of 3 methyl Capping reactions occurs
A) Cap 0: a methyl group is added to the G-7 posn: Guanine-7-MethylTransferase Present in all eukaryotic mRNA ... no further capping in unicelluar eukaryotes
Eukaryotic RNA:
long half life; monocistronic; always modified; transport to cytoplasm
Most post-transcription modifications of the primary transcript occur in nucleus, before transport to cytoplasm.
U4 RNA
-initially complexed with U5 and U6
-keeps U6 in an unfolded conformation
-is released after delivering U6 to the 5’-splice site

(U6 chaperone?)
B) Cap 1: a methyl group is then added to the 2'-OH of the ribose of the 1st nucleotide in the original transcript: 2'-OMethylTransferase Present in most eukaryotic mRNA ... if base is A, it too can be methylated
2.1. GU-AG rule
GU always at 5' end of intron: Donor site: AG | GUAAGU AG always at 3' end of intron: Acceptor site: (Py...Py) 12 NCAG | N
2.2. Spliceosomes
C) Cap 2: a similar methylation of the 2nd nucleotide of the original transcript Present in about 10-15% of total capped population.
Cap at 5' end
1.1.3. Internal Methylation:
Histone mRNAs are poly(A)-free.
1.1.2 Methylated G Cap at 5' end:
Guanylyl transferase catalyzes addition of GTP in 5'->5' direction, with release of PPi from the GTP, and release of Pi from the terminal triphosphate of the RNA transcript.
these modifications convert the transcript (Heterogeneous Nuclear RNA hnRNA) into mRNA
1.1 Major Eukaryotic Modifications events
G capping of 5' end of pre-mRNA Cleavage and Poly(A) addition to 3' end Splicing out (removal of) Introns Some methylation of bases
Removal of the Introns in RNA transcript modification is called RNA splicing
Exons are Expressed sequences: these sequences are those present in mature mRNA
RNA processing is the collective term used to describe the molecular events allowing the primary transcripts to become the mature RNA.
1. Basic Features of RNA
The snRNPs are U1, U2, U5, and U4/U6, containing U1, U2, U4, U5, U6 snRNAs.
Functional roles for each snRNP:
snRNP U1 :
Structure: 4 stem-loop Domains (A,B,C,D) Sm-binding site: non-paired region between Domains C and B - binding of snRNP proteins 5' non-paired end: H-bonds to Donor site of pre-mRNA ... This pairing is essential for splicing; some pairing sites more important than others
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U1 -initially bound to 5’-splice site
-is released upon recruitment of U4/U5/U6

(determines which 5’-splice site is used?)
U1 SnRNA
Mutations that abolish function of the 5’splicing site can be suppressed by compensating mutations in snRNA that restore base pairing
Poly(A)-Binding Protein (PABP) binds to the poly(A) stoichiometrically, one protein every 10-20 bases ... increases stability and enhances translation