03/09b RNA Viruses II Flashcards
What is the closest analogue to (-) strand RNA in eukaryotic cells?
Pre-mRNA that must be processed before being translated
What are the most important (-) strand RNA viruses?
Influenza viruses (A, B, and C)
How does (-) strand RNA virus replication differ from (+) strand RNA virus replication?
1) (-) strand RNA viruses must supply replication proteins to initiate their life cycle, as host proteins can’t translate from (-) strand RNA; (+) strand RNA viruses can use their genomes as a translation template
2) Certain (-) strand RNA viruses include a nuclear step in their replicative cycle; (+) strand RNA viruses never enter the nucleus
What are the similarities between (-) and (+) strand RNA virus genome replication?
Both viruses have an RNA-dependent RNA polymerase that has a very high error rate
Both viruses have adapted to high error rates by minimizing their genomes and enabling genetic plasticity
How does the influenza virus gain access to the host cell?
Fusion of the viral envelope with the cell membrane
What happens to the influenza genome after it enters the cell?
Gets uncoated in the cytoplasm to release the genome and the necessary replication enzymes
Genome and enzymes enter the host cell nucleus
What three types of RNA are generated by the influenza virus?
(+) strand mRNA - copy of the original (-) strand, must be done quickly to start making viral proteins; does NOT have a cap or a polyA tail
(+) strand template RNA - identical to mRNA, but doesn’t require translation termini because it only functions as a template
(-) strand viral RNA - to be packaged into new virions
Where does the influenza virus get a 5` cap for its viral mRNA?
Steals it from the host cell via cap-snatching
How does cap-snatching work?
Viral polymerase complex (enters with the viral genome) recruits viral genomic mRNA segments as well as nascent host mRNAs in the nucleus
Viral protein complex has endonuclease activity - degrades host mRNA from the 3` end
Left with an mRNA ‘stump’ with a cap - serves as a primer for viral (+) strand synthesis
How does the influenza virus generate a polyA tail for its (+) strand mRNA?
All viral genome segments contain a polyU sequence, which causes the viral polymerase to ‘stutter’ and add a polyA tail at the 3` end
How does generation of the viral template RNA different from synthesis of (+) strand mRNA?
Occurs without cap-snatching (no 5` cap)
Occurs without termination and polyadenylation (no polyA tail)
Mechanisms are poorly understood
Why does influenza have a segmented genome?
We dunno!
Possible advantages to a segmented genome - complex orchestration of gene expression (differential regulation of certain gene products), genetic variability (emergence of new strains), maximizing coding capacity
Why must mRNA and viral RNA be made at different times?
If they were made simultaneously they would have to compete for the same template
When during the virus life cycle does mRNA synthesis predominate? When does vRNA synthesis predominate? Why?
mRNA synthesis predominates early in the infectious cycle, and decreases later - virus wants to make viral proteins early, when they are needed
vRNA synthesis predominates later in the infectious cycle, so that it can be packaged into new virions
What is NS1?
Non-structural protein 1
Viral protein that is critical for temporal regulation of the influenza genome
What happens in the host cell before NS1 is expressed?
Both virus and host mRNA production and translation progress and coexist
Virus initiates cap-snatching, but the host cell is not yet overcome
What happens in the host cell at low levels of NS1 expression?
NS1 enters the nucleus and inhibits splicing and polyadenylation - blocks full maturation of host mRNA and production of host defense proteins
Creates a large pool of host mRNA for cap-snatching
Shift from host mRNA production to viral mRNA production
NS1 binds vRNA and prevents its export from the nucleus
What happens in the host cell at high levels of NS1 expression?
Production of lots of viral mRNA at the expense of host mRNA
vRNA remains trapped in the nucleus
What happens in the host cell when NS1 stops functioning?
Viral mRNA synthesis ceases, vRNA synthesis increases
vRNA is released and exported from the nucleus
How does influenza virus generate the NS1 and NS2 gene products?
Splicing! Occurs in the nucleus
What is antigenic drift? Why is it important?
Genetic variability of a virus (like influenza) brought about by spontaneous mutagenesis, recombination, and reassortment Occurs because (+) and (-) strand RNA-dependent RNA polymerases have very high error rates Results in changes to endemic flu viruses each season
What is antigenic shift? Why is it important?
Dramatic change in viral genomes as a result of genetic reassortment that can occur when virus strains that are endemic to different species come into contact with each other
Can result in radically novel influenza strains with pandemic potential
What do the ‘H’ and ‘N’ refer to in influenza strains?
H = hemagglutinin
N = neuraminidase
Code for exterior parts of the viral particles that are responsible for antigenic and pathogenic properties
What were the origins of the H5N1 avian influenza strain?
Goose strain from China
Duck strain from Hong Kong
Quail strain that was linked to transmission in humans
Reassortment probably occurred in a poultry market
What three phenotypic markers determine the impact of emerging influenza strains on human health?
Host range - expansion to humans
Virulence and pathogenicity - relates to hemagglutinin and neuraminidase
Transmission - human-to-human spread is most dangerous
