Ch. 10 Processing of Viral Pre-mRNA Flashcards
Why is RNA processing needed?
It is needed for recognition by the protein synthesizing machinery and translation by cellular ribosomes.
What are two of the important RNA modifications?
5’ capping and 3’ polyadenylation.
How can translation be regulated
Small RNA’s can bind to mRNA in cytoplasm leading to RNA silencing/interference
Virtually all cellular and viral mRNA possess the same cap structure. What is it?
m7GpppN, where N is any nucleotide. (7 methyl guanosine triphosphate)
What are the three mechanisms in which mRNA can be capped?
De novo synthesis by cellular enzymes; synthesis by viral enzymes; “cap stealing” by acquisition from cellular pre-mRNA’s.
What stabilizes the 5’ cap?
Methylization of 2’ OH group on ribose or terminal guanosine
Cellular and viral polymerases methylate differently. Cellular pol methylates ____ cap addition and viral pol methylates ____ 5’ cap addition
After, before
Formation of cap structure by cellular pol occurs when the nucleotide is _____ nucleotides long.
20-30
How does the cell signal RNA Pol II to bind?
Phosphorylation of C-term domain of large subunit
What proteins do the capping enzymes have?
5’ Triphosphatase; Guanylyltransferase
When is it necessary for viral enzyme synthesis of a 5’ cap?
When mRNA is synthesized in the cytoplasm (cellular machinery in nucleus)
What are viral capping enzymes associated with? What was one of the first viruses where this was discovered?
RNA Pol; Vaccinia virus
How does cap snatching work? Which viruses use this method?
Cap snatching works by the cap and 10 to 13 nucleotides. A G protein peptide bonded to the (-) RNA is incorporated into the cap primer, and then the strand is elongated. Bunyavirus, orthomyxovirus
What is the purpose of the 3’ Poly (A) tail? What do mRNA lacking a tail do to prevent nucleolytic attack?
Serves to stabilize mRNA and increase translation efficiency.
They form a stem loop structure
Cellular poly adenylation: What is the nucleotide sequence recognized by Cpsf (cleavage and polyadenylation specificity protein)?
5’-AAUAAA-3’
Cellular poly adenylation: What occurs after Cpsf cleaves the end of mRNA?
Poly (A) pol attaches and adds 10-15 A residues. Other proteins stabilize it and it adds a total of 200 A residues
Viral poly adenylation: Capping protein-RNA Pol II complex stay together until reaching termination signal. What happens next?
Viral Poly (A) Pol adds Poly (A)
What is pre-spliced mRNA called? Do they have a 5’ Cap and Poly A tail? What happens to the sequences immediately adjacent to these features?
heterogenous nuclear RNA;
YES;
They are conserved in mature mRNA.
What are the nucleotide sequences found at the ends of almost all introns?
GU and AG.
What type of reaction eliminates introns? What is the product of the first? the second?
two trans-esterification reactions; One 5’ exon and one intron-3’ exon complex; two attached exons and one intron
What is a spliceosome comprised of?
Many proteins and 5 small nuclear (snRNA’s) U1,U2,U4, U5, and U6
What is Constitutive splcing?
Splicing where all exons are conserved.
What is Alternative splicing?
splicing of different combinations of exons.
How is viral mRNA edited?
By the insertion of nucleotides not specified in the template or the alteration of a base
