Besides splicing, eukaryotic cells perform several other kinds of processing on their RNAs. Messenger RNAs are subject to two kinds of processing, known as capping and polyadenylation. In capping, a special blocking nucleotide (a cap) is added to the 59-end of a pre-mRNA. In polyadenylation, a string of AMPs (poly[A]) is added to the 39-end of the pre-mRNA. These steps are essential for the proper function of mRNAs and will be topics in this chapter. | Molecular Biology Fourth Edition Chapter 15 Messenger RNA Processing II: Capping and Polyadenylation Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Capping By 1974, mRNA from a variety of eukaryotic species and viruses were found to be methylated A significant amount of this methylation was clustered at the 5’-end of mRNA This methylation cluster formed a structure we call a cap 15- Cap Structure Early study used viral mRNA as they are easier to purify and investigate The b-phosphate of a nucleoside triphosphate remains only in the first nucleotide in an RNA Cap is at the 5’-terminus of RNA The cap is made of 7-methylguanosine, m7G Linkage is a triphosphate Charge on the cap area is near -5 15- Reovirus Cap Structure The m7G contributes a positive charge Triphosphate linkage contributes 3 negative charges Phosphodiester bond contributes 1 negative charge Terminal phosphate . | Molecular Biology Fourth Edition Chapter 15 Messenger RNA Processing II: Capping and Polyadenylation Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Capping By 1974, mRNA from a variety of eukaryotic species and viruses were found to be methylated A significant amount of this methylation was clustered at the 5’-end of mRNA This methylation cluster formed a structure we call a cap 15- Cap Structure Early study used viral mRNA as they are easier to purify and investigate The b-phosphate of a nucleoside triphosphate remains only in the first nucleotide in an RNA Cap is at the 5’-terminus of RNA The cap is made of 7-methylguanosine, m7G Linkage is a triphosphate Charge on the cap area is near -5 15- Reovirus Cap Structure The m7G contributes a positive charge Triphosphate linkage contributes 3 negative charges Phosphodiester bond contributes 1 negative charge Terminal phosphate contributes 2 negative charges 15- Cap Synthesis First step RNA triphosphatase removes terminal phosphate from pre-mRNA Then, guanylyl transferase adds capping GMP from GTP Next, 2 methyl transferases methylate N7 of capping guanosine and 2’-O-methyl group of penultimate nucleotide This occurs early in transcription, before chain is 30 nt long 15- Functions of Caps Caps serve at least four functions: Protect mRNAs from degradation Enhance translatability of mRNAs Transport of mRNAs out of nucleus Efficiency of splicing mRNAs 15- Polyadenylation The process of adding poly(A) to RNA is called polyadenylation A long chain of AMP residues is called poly (A) Heterogeneous nuclear mRNA is a precursor to mRNA 15- Poly(A) Most eukaryotic mRNAs and their precursors have a chain of AMP residues about 250 nt long at their 3’-ends Poly(A) is added posttranscriptionally by poly(A) polymerase 15- Functions of Poly(A) Poly(A) enhances both the lifetime and translatability of mRNA
