RNA Synthesis (Lecture 11) Flashcards
What direction does RdRp read in and synthesize in?
reads 3’-5’ and synthesizes 5’-3’
Why is it important for viral RNA to be different from eukaryotic RNA?
capsid proteins and RdRP needs to recognize viral RNA ONLY
Which viruses encode for RdRp?
ALL RNA viruses EXCEPT Retroviruses
What are the 5 rules for RNA-dependent RNA synthesis?
1) RNA synthesis starts and stops at specific sites on template || 2) RdRp can initiate via de novo or primer-dependent || 3) usually requires a template || 4) RdRp never copies cellular mRNA || 5) other viral or cellular proteins may be required to help with RNA synthesis
What is the benefit of starting and stopping RNA synthesis at specific sites on the template?
more genes for the same protein on viral genomes
What are the 2 ways to start RNA synthesis?
de novo or primer-dependent
What is the de novo way of initiating RNA synthesis?
virus will bring together 2 rNTPs to form a phosphodiester bond taking place at the 3’ end of template
What is the primer-dependent way of initiating RNA synthesis?
can use either a terminal protein that provides a free OH group (ie: tyrosine or serine) or snatch a 5’ cap fragment from host transcript and begin RNA synthesis on 3’ end
What is a special feature of RdRp?
has endonuclease activity
What are 2 ways do viruses obtain the 5’ cap?
recognizes capping sequence on ribosomal ready host mRNA and cleaves it OR steals cap during capping step of host mRNA synthesis
Why is RdRp very efficient?
its very processive = does not easily fall off its template
What are 4 ways that RdRp determines which are viral templates from host-RNA?
1) RdRp does not recognize adenosine (polyA tail on mRNA) || 2) secondary RNA structures (not very common in cell mRNA) || 3) internal RNA sequences that RdRp recognizes and loads onto || 4) membrane-bound replication complexes where reaction components and viral RNAs are centralized
What helps load RdRp onto internal sequences in RNA templates?
secondary RNA structures
What are the 2 roles of RdRp?
synthesize mRNA molecules and replicate genome; switches off between these roles?
What regulates the initiation and termination of RNA synthesis?
RdRp switching roles
What is one of the first proteins made from (+)ssRNA viruses?
RdRp
What is the first thing that happens when (+)ssRNA viruses deliver its genome in the cytoplasm?
RdRp attaches 5’ cap »_space;> host ribosomes recognize it and begin translation
How does Polio replicate its genome?
uses a terminal protein as a primer and forms membrane-bound vesicles at the site of replication
What is the method of translation that Polio takes?
full genomic translation, no sub-genomic particles; protein will be cleaved off into multiple different proteins
In what structure do (+)ssRNA viruses replicate their genomes?
in membrane-bound structures
What is the difference in genome replication between Polio and Togaviruses?
Togavirus uses a 5’cap primer to initiate RNA synthesis
What is the method of transcription that Togaviruses take?
transcription can begin at internal sequences = subgenomic mRNAs = helps form capsid proteins
How will (-)ssRNA viruses make RdRp if it isn’t (+)sense?
it can’t so it brings RdRp with them
What is the method of transcription that (-)ssRNA viruses take?
mRNAs are made through a series of start/stop sequences (sub-genomic sequences)
What dictates that the full genome will be replicated?
a viral protein (such as L protein) that will silence the start/stop sequences
What is the strategy that Rabies viruses use?
can make a bunch of mRNA transcripts due to the start/stop sequences in the genome
How does dsRNA viruses accomplish hiding of genome differently than (-)ssRNA viruses?
has 2 distinct viral particles one containing newly synthesized genome and one transcribing mRNA
What is the first biosynthetic event that dsRNA viruses must undertake?
transcribe mRNA via RdRp
In order to replicate the genome, what happens to mRNA once it leaves its viral particle?
gets encapsidated which then RdRp synthesizes second strand
What are the 3 distinguishable characteristics/features of dsRNA viruses?
dsRNA is always found in a coat, 2 processes occurs in 2 separate respective virion particles, viral genome itself never leaves the particle as a double strand
What are the 3 cellular sites of viral RNA synthesis?
nucleocapsids (in –ssRNA viruses); subviral particles of dsRNA viruses; membrane bound replication complexes
Why would a virus want to make replication complexes? What is the benefit?
protection against host cell defenses, increases replication efficiency, protect from nucleases
What are 3 reasons why RNA viruses are more diverse than DNA viruses?
RNA polymerase lacks proofreading = more mutations, reassort genomes, recombination during RNA synthesis when RNA polymerase copies one strand and then jumps on the other strand and continues synthesis, RNA editing when there’s no template
