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 ●
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 ●
