RNA-dependent RNA synthesis Flashcards
Replication of unimolecular - strand RNA viruses
This means there is just one RNA strand packaged inside the virus particle.
1. mRNA is synthesized from the genome, as a complementary + strand is synthesized
2. More - strand RNA can also be produced from the + strand
Replication of segmented - strand RNA viruses
mRNA can be synthesized from the - strand genome. The mRNA can also be used to make more - strand RNA to be packaged in the new viruses
VSV replication cycle (5)
- (-) strand RNA virus, so it brings RNA polymerases packaged inside the virus.
- The virus is taken up into the endosome and uncoating occurs. The negative polarity viral genome is released
- Acted on by RNA dependent RNA polymerases to make mRNA
- The mRNA is latched onto by ribosomes to make viral proteins
- More - RNA can be made for packaging in the new viruses
Processing of the viral genome
RNA polymerases are designed to make subgenomic RNAs from the - strand genome rather than making the full length mRNA right away. The subgenomic RNAs are of + polarity and can directly be used to make proteins. Then, the virus must switch to make the full length RNA so the virus can reproduce
Switch from mRNA to full length RNA synthesis
After subgenomic RNAs and proteins have been made in abundance, accumulation of the N protein coats RNA. This causes the switch to replication of full length + strand RNA by causing synthesis to continue without stopping. + strand RNA can be used as a template to make the - strand RNA genome
Stop-start model of mRNA synthesis
Viral genes are interrupted by intergenic regions. Intergenic regions have sequences that won’t allow RNA polymerase to continue synthesis. When the polymerase reaches the sequence, it stutters and causes the dissociation of the new mRNA molecule
Polyadenylation of mRNAs
Polyadenylation is important for the ribosomes to use mRNA to make proteins. Multiple U residues are present on genomic RNA in intergenic regions. When new RNA is synthesized, As are added instead of a U. The multiple As serve as poly-A tails at the end of mRNA and the mRNA can be released
Influenza A virus RNA synthesis
The genome for this virus is already segmented into 8 - strand RNA segments. + polarity viral mRNAs are used to make proteins (polymerases) involved in viral replication. The proteins go back inside the cell nucleus to convert + to - RNA. The process is continued until several copies of genomic RNA are produced. The segments of RNA coding for structural proteins are shuttled in the cytoplasm where they produce enveloped proteins, which then go to the plasma membrane
Influenza A mRNA processing
8 - strand viral genome segments make 8 mRNAs. Each mRNA segment can make one or multiple proteins, which all have different functions. The mRNA produced is slightly shorter than the genomic RNA because it lacks the 3’ end
Why are mRNAs shorter than genomic RNAs?
Random cellular mRNA can be used to provide a 3’ end and act as the necessary primer to make new mRNA. Synthesis stops prematurely, leaving off a 3’ end. However, this doesn’t really matter because the virus can still make proteins. Levels of nuclear protein (NP) can accumulate and cause the switch from cap dependent to cap independent full-length synthesis of viral mRNA
Cap snatching by influenza A
The proximal piece of host mRNA with a cap (12-13 bases) is taken by influenza A virus to prime its mRNA synthesis. Cap snatching is important for synthesis of mRNA.
Proteins responsible for cap snatching
The trimeric IAV polymerase complex- PA, PB1, and PB2. These proteins cleave the host mRNA to use as a cap
Double stranded RNA viruses
These viruses have both + strand and - strand RNA. These viruses also package RNA dependent RNA polymerase. The enzyme can hold on to - strand RNA and make + strand RNA to make proteins. Or, + strand mRNA can further be used to make - strand RNA. They will intertwine to make the genome of the virus
Reovirus replication cycle
Double stranded RNA virus and may have a segmented genome. However, every segment is double stranded RNA. The virus has both an outer and inner capsid layer. It is taken into the cell by endocytosis in clathrin coated pits. It stays in the endosome until the lysosome fuses with the endosome. The virus eventually loses its top layer and is left with the core in the cytoplasm. The virus has a channel in each of its 12 faces, with an RNA segment underneath the channel. RNA dependent RNA polymerase will be activated and synthesize + RNAs that will go through the channels and produce viral proteins in the cytoplasm
