RNA Processing I: Splicing:
Explain the process of RNA splicing, where introns are removed and exons are joined together to form mature mRNA.
Highlight the significance of splicing in generating protein diversity and regulating gene expression.
Mention key players in the splicing process, such...
Chapter 7 Operon and Chapter 8: Major Shift in Transcription
General Transcription Factors:
RNA Processing II: Capping and Polyadenylation
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Name: AL-Raisalmill M. Alawi SUMMARY : RNA PROCESSING II: CAPPING AND POLYADENYLATION CHAPTER 15
RNA Processing II: Capping and Polyadenylation
15.1 Capping 15.2 Polyadenylation Cleav
● By 1974, mRNA from a variety of eukaryotic species and ● The process of adding poly(A) to RNA is called ●
viruses were found to be methylated polyadenylation
● A significant amount of this methylation was clustered at the ● A long chain of AMP residues is called poly (A)
5’-end of mRNA ● Heterogeneous nuclear mRNA is a precursor to mRNA
● This methylation cluster formed a structure we call a cap ●
Poly(A)
Cap Structure ● Most eukaryotic mRNAs and their precursors have a chain of
● Early study used viral mRNA as they are easier to purify and AMP residues about 250 nt long at their 3’-ends
investigate ● Poly(A) is added posttranscriptionally by poly(A) polymerase
● The B-phosphate of a nucleoside triphosphate remains only
in the first nucleotide in an RNA Functions of Poly(A)
● Poly(A) enhances both the lifetime and translatability of
○ Cap is at the 5’-terminus of RNA
mRNA
○ The cap is made of 7- methylguanosine, m7G
● Relative importance of these two effects seems to vary from Initiat
○ Linkage is a triphosphate
one system to another ●
○ Charge on the cap area is near -5
● In rabbit reticulocyte extracts, poly(A) seems to enhance
translatability by helping to recruit mRNA to polysomes ●
Reovirus Cap Structure
The m7G contributes a positive charge
Basic Mechanism of Polyadenylation ●
Triphosphate linkage contributes 3 negative charges ● Transcription of eukaryotic genes extends beyond the
Phosphodiester bond contributes 1 negative charge polyadenylation site
● The transcript is: ●
Terminal phosphate contributes 2 negative charges ○ Cleaved
○ Polyadenylated at 3’-end created by cleavage
Cap Synthesis
● First step Polyadenylation Signals Elong
○ RNA triphosphatase removes terminal phosphate ● An efficient mammalian polyadenylation signal consists of: ●
from pre-mRNA ○ AAUAAA motif about 20 nt upstream of a specif
● Then, guanylyl transferase adds capping GMP from polyadenylation (PAB
GTP ○ site in a pre-mRNA ●
● Next, 2 methyl transferases methylate N7 of capping ○ Followed 23 or 24 bp later by GU-rich motif
guanosine and 2’-O-methyl group of penultimate nucleotide ○ Followed immediately by a U-rich motif
● This occurs early in transcription, before chain is 30 nt long. ● Variations on this theme occur in nature
○ Results in variation in efficiency of polyadenylation ●
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