RNA Viruses: Baltimore Groups III, IV, V

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Questions and Answers

What is the role of cytoplasmic DLP in rotavirus replication?

Cytoplasmic DLP catalyzes primary transcription, allowing capped mRNA to be produced and exit each vertex of the virion.

Describe what occurs during the formation of viroplasm in rotavirus replication.

Translation of rotavirus proteins leads to the formation of viroplasm, where mRNA becomes enclosed by newly formed double-layered particles (DLPs).

What results from the copying of (+) strand templates inside new DLPs?

This process results in the formation of DLPs containing double-stranded RNA (dsRNA) genomes.

How does the viral mRNA synthesized by VP1/VP3 differ from typical eukaryotic mRNA?

Viral mRNA synthesized by the VP1/VP3 complex does not have a poly(A) tail; instead, a viral nonstructural protein binds to the 3' end to facilitate translation initiation. Signup and view all the answers

What significant shift occurs a few hours after the start of the rotavirus replication cycle?

After a few hours, the virus shifts from gene expression and genome replication to the assembly phase of the replication cycle. Signup and view all the answers

What is the function of the RdRp in minus sense (-) RNA viruses during replication?

RdRp is required for the replication of the viral genome, as it synthesizes the complementary positive strand from the negative strand. Signup and view all the answers

How do Class IV (+) RNA viruses manage to express multiple proteins from a single genome?

They utilize a polyprotein strategy, where a large polyprotein is synthesized and then proteolytically processed into multiple individual proteins. Signup and view all the answers

What is the role of viral replication complexes (VRCs) in cytoplasmic RNA virus replication?

VRCs compartmentalize genome replication and protein transcription, facilitating efficient viral replication while evading host defenses. Signup and view all the answers

Why is immediate translation of viral proteins critical for (+)RNA virus replication?

Immediate translation is necessary to synthesize the viral RdRp, which is essential for replicating the viral genome and producing viral mRNA. Signup and view all the answers

What structural feature is utilized by poliovirus to initiate translation in the absence of a 5' cap?

Poliovirus uses an internal ribosome entry site (IRES) that allows ribosomes to bind internally to the viral RNA. Signup and view all the answers

How do eIF4G complexes contribute to mRNA translation?

eIF4G serves as a docking site for initiation factors and proteins, bringing together components necessary for the translation process. Signup and view all the answers

What strategy do plus (+) strand RNA viruses employ to bypass the eukaryotic mRNA cap requirement?

They utilize structures such as the IRES to bind ribosomes directly, circumventing the need for a 5' cap. Signup and view all the answers

What is the role of the IRES in the replication of (+) strand RNA viruses?

The IRES enables the direct translation of the viral genome into a polyprotein. Signup and view all the answers

Explain the significance of the VPg protein in poliovirus genome replication.

VPg acts as a protein primer that is essential for initiating the replication of the viral genome. Signup and view all the answers

How does poliovirus affect eIF4E and what is the consequence for host mRNA?

Poliovirus proteolytically degrades eIF4E, preventing cap-dependent translation of host mRNA. Signup and view all the answers

What is the primary difference between cap-dependent and IRES-dependent translation initiation?

Cap-dependent initiation requires eIF4E binding to the 5' cap, while IRES-dependent initiation directly recruits ribosomes to the mRNA's internal region. Signup and view all the answers

Describe the structural properties of the rotavirus virion.

Rotavirus has a non-enveloped, icosahedral structure with a triple capsid that contains a double-layered particle. Signup and view all the answers

Why do viruses modify host cell membranes during replication?

Viruses modify host cell membranes to create scaffolds for efficient viral replication and protection from host immune responses. Signup and view all the answers

What is the significance of the segmented genome in rotavirus?

The segmented genome allows for genetic reassortment during co-infection with different strains, enhancing genetic diversity. Signup and view all the answers

What is the function of VP1 and VP3 proteins in rotavirus DLP?

VP1 and VP3 are internal proteins that form part of the polymerase complex responsible for nucleic acid synthesis. Signup and view all the answers

How does rotavirus transcribe its dsRNA genome?

Rotavirus transcribes its dsRNA genome into capped (+)RNAs inside double-layered particles using RdRp. Signup and view all the answers

What role does NSP3 play in rotavirus mRNA processing?

NSP3 substitutes for PABP by binding to a conserved sequence at the 3’ end of viral mRNA, aiding its stability. Signup and view all the answers

Explain the phenomenon of uncoating in rotavirus infection.

Uncoating involves the release of the double-layered particle, which then facilitates the transcription and replication of the viral genome. Signup and view all the answers

Why can the genome of Baltimore Class V ss(-) RNA viruses not be used directly as mRNA?

The minus sense (-) strand must be transcribed into a complementary plus strand before it can function as mRNA. Signup and view all the answers

What occurs in the viroplasm during rotavirus replication?

Viroplasms serve as the site for viral dsRNA synthesis and become progressively fused during the infection. Signup and view all the answers

Study Notes

RNA Viruses: Baltimore Groups III, IV and V

RNA viruses replicate in the cytoplasm of host cells
(+)RNA viruses have genomes that can be directly translated into proteins by host ribosomes. They may be capped and polyadenylated.
(-)RNA viruses must carry an RNA-dependent RNA polymerase (RdRp) within their capsid to initiate replication, as their genomes cannot be directly translated.
Host ribosomes assemble on viral genomes that have entered the host cell to synthesize viral proteins.
The earliest viral proteins synthesized are those needed for genome replication and the production of RdRp.
Many RNA viruses create virus replication complexes (VRCs) within the cytoplasm to compartmentalize genome replication and transcription.
VRCs are assembled from non-structural viral proteins, viral genomes, host lipids, and host proteins.
VRCs help escape recognition from host defenses and toll-like receptors.
Within VRCs, the (+) strand genome is used as a template to synthesize full-length (-) strand copies, which remain hydrogen-bonded to the (+) strand.

Class IV (+) RNA Viruses

Class IV (+) RNA viruses express multiple proteins from a single genome using a polyprotein strategy.
Eukaryotic mRNA typically encodes only one protein, but (+) strand RNA viruses must encode at least two: a capsomer and an RdRp.
Immediate translation of the (+) strand RNA genome is crucial for viral replication because it results in the synthesis of the viral RdRp.
RdRp then synthesizes replicative forms and viral mRNA.
Class IV viruses encode a polyprotein that is proteolytically processed to release many individual proteins, including capsomers and RdRp.

Poliovirus (Class IV (+) RNA Virus)

Poliovirus lacks a 5' cap on its genome and instead utilizes an internal ribosome entry site (IRES) to initiate translation.
The IRES is a complex stem-loop structure in the 5' UTR (untranslated region) that recruits translational machinery to an internal position in the mRNA.
Poliovirus also has a VPg protein primer for genome replication.

Translation of mRNA

eIF4G (eukaryotic initiation factor 4G) serves as a docking site for initiation factors and proteins involved in RNA translation.
The cap-dependent initiation complex involves eIF4E binding to the 5' cap and forming a complex with PABP.
In poliovirus, the ITAF protein binds to the IRES and substitutes for eIF4E, allowing initiation of translation at an internal site.
Poliovirus proteolytically degrades eIF4E, preventing cap-dependent translation of host mRNA.

Baltimore Class V ss(-) RNA Viruses

Class V ss(-) RNA viruses have genomes that are not directly translatable into proteins.
These viruses package RdRp within their virion to initiate replication.
Examples include Orthobunyaviruses, Orthomyxoviridae, and Paramyxoviridae.

Mononegavirales

Mononegavirales are ss(-)RNA viruses with non-segmented genomes.
An example is Measles virus.

Baltimore Class III dsRNA Viruses

Class III dsRNA viruses have segmented, double-stranded RNA genomes.
An example is Rotavirus.
Rotaviruses are non-enveloped and have a triple capsid structure.
Uncoating leaves a double-layered particle (DLP) containing the segmented dsRNA genome.
Each RNA segment has a 5' cap but lacks a polyA tail.
Co-infection of cells with different rotavirus strains belonging to the same serogroup can lead to genetic reassortment.

Rotavirus Genome and Replication

The rotavirus genome consists of 11 segments coding for 12 proteins.
The DLP is the transcriptionally active form of the virus.
Many components of the DLP are both enzymes needed for nucleic acid synthesis and structural proteins.
VP1, a structural protein, also has enzymatic activity and is part of the VP1/VP3 flower complex.
This complex synthesizes the 5' cap structure and mRNA using dsRNA as a template.
Viral mRNA lacks a poly(A) tail. A viral nonstructural protein (NSP3) binds to a conserved sequence in the 3' end of each mRNA, substituting for PABP during translation initiation.
Rotavirus replication occurs in viroplasms, cytoplasmic sites where viral dsRNA synthesis takes place.
Viroplasms are formed by the clustering of DLPs, which catalyze primary transcription, producing capped mRNA.
New DLPs are assembled within the viroplasms, encapsulating dsRNA genomes.
The replication cycle transitions from gene expression and genome replication to assembly.

Rotavirus Replication Summary

Rotaviruses have three-layered, naked capsids surrounding a segmented dsRNA genome.
The DLP is the transcriptionally active form, containing enzymes for nucleic acid synthesis and structural proteins.
VP1 is a structural protein that also synthesizes the 5' cap of viral mRNA using dsRNA as a template.
Viral mRNA lacks a poly(A) tail, but NSP3 substitutes for PABP during translation initiation.

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Description

Explore the fascinating world of RNA viruses, focusing on Baltimore Groups III, IV, and V. This quiz delves into their replication mechanisms, the role of host ribosomes, and the formation of virus replication complexes. Test your understanding of the unique strategies employed by these viruses to evade host defenses.