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Chapter 15

BIOL 367 Chapter 15: biol 367 chapter 15

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BIOL 367
Reg Storms

CHAPTER 15 – RNA PROCESSING II: CAPPING AND POLYADENYLATION CAPPING - Cap1 is the same as the cap shown in this figure. - Cap 2 has another 2’-O-methylated nucleotide (two in a row) - Cap0 has no 2’-O-methylated nucleotides - Cap2 is found only in eukaryotic cells, cap 1 is found in both cellular and viral RNAs and cap0 is found only in certain viral RNAs Cap Synthesis a) A nucleotide phosphohydrolase (also called RNA triphosphatase) clips the gamma-phosphate off the triphosphate at the 5’ end of the growing RNA, leaving a diphosphate b) A guanylyl transferase attaches GMP from GTP to the diphosphate at the ends of the RNA, forming the 5’-5’ triphosphate linkage c) A methyltransferase transfers the methyl group from S-adenosylmethionine (AdoMet) to the 7-nitrogen of the capping guanine d) Another methyltransferase uses another molecule of AdoMet to methylate the 2’-hydroxyl of the penultimate nucleotide Functions of Caps - Caps appeaer to serve at least four functions (1)They protect mRNAs from degradation (2)They enhance the translatability of the mRNAs (3)Theyenhance the transport of mRNAs from the nucleus into the cytoplasm (4)They enhance the efficiency of splicing of mRNAs 1 Protection - The cap is joined to the rest of the mRNA through a triphosphate linkage found nowhere else in the RNA - Thecap might therefore be expected to protect the mRNA from attack by RNases that begin at the 5’ end of their substrates and that cannot cleave triphosphate linkages Translatability - Eukarotic mrNA gains access to the ribosome for translation via a cap-binding protein that recognizes the cap. If there is no cap, the cap-binding protein cannot bind and the mRNA is very poorly translated Transport of RNA - Facilitates the transport of at least some mature RNAs out of the nucleus - The cap is essential for porper splicing of a pre-mRNA POLYADENYLATION - hnRNA is a precursor to mRNA - theyshared a unique structure at their 3’ ends: a long chain of AMP residues called poly(A) Poly (A) - they cut the RNA after everynucleotide except the As, preserving only pure runsof As, next, electrophoresed the poly(A)s from nuclei and from cytoplasm to determine their sizes - both Poly(A)s have major peaks that electrophorseed more slowly than 5S rRNA at about 7S - cytoplasmic poly(A) is subject to shortening - poly(A) must be added post transcriptionally - enzyme poly(A) polymerase(PAP) that adds AMP residues one at a time to mRNA precurosrs - we know that poly(A) is added to mRNA precursors because it is found on hnRNA 2 Functions of Poly (A) + Protection of mRNA  the poly (A) RNA has a longer lifetime than the poly(A) RNA, and that poly(A) is therefore the protective agent Translatability of mRNA one of the protiens that binds to a eukaryotic mRNA during trnaslation is poly(A)-binding protein I (PAB I). binding to this protein seems to boost the efficiency with which an mRNA is translated. Excess poly(A) added to an in vitro reaction inhibited translation of a capped, polyadenylated mRNA. This finding suggested that the excesspoly(A) was competing with the poly(A) on the mRNA for an essential factor, presumably for PABI. Without this factor, the mRNA could not be translated well Basic Mechanism of Polyadenylation - RNA pol can still be elongating an RNA chain, while the polyadenylation apparatus has already located a polyadenylation signal somewhere pstream, cut the growing RNA, and polyadenylated it Overview of the Process a) Cutting. The 1 step is cleaving the transcript, which may actually still be in the rpocess of being made. The cut occurs at the end of the RNA region and that wil
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