RNA Viruses

Overview of RNA Viruses

RNA viruses are a diverse group of viruses that use ribonucleic acid (RNA) as their genetic material. They can infect a wide range of hosts, including bacteria, fungi, plants, and animals. RNA viruses are classified based on the nature of their RNA genome (double-stranded or single-stranded, positive or negative sense) and whether they are enveloped or non-enveloped.

• Double-stranded RNA (dsRNA) viruses

• Single-stranded RNA (ssRNA) viruses – positive-sense (+) or negative-sense (−)

• Enveloped vs. Non-enveloped viruses

Examples of RNA virus families:

• Caliciviridae: Norwalk virus, Hepatitis E virus

• Flaviviridae: Yellow fever virus, Dengue virus, West Nile virus, Hepatitis C virus

• Paramyxoviridae: Measles virus

• Rhabdoviridae: Rabies virus

• Filoviridae: Ebola virus, Marburg virus

• Bunyaviridae: Hantavirus

• Arenaviridae: Lassa fever virus

• Reoviridae: Rotavirus, Colorado tick fever virus

Class III: Double-Stranded RNA (dsRNA) Viruses

General Features and Replication

Class III viruses possess a double-stranded RNA genome. These viruses infect a variety of hosts and are notable for their unique replication strategies, as host cells do not naturally use dsRNA and have mechanisms to destroy it.

• Host Range: Bacteria, fungi, plants, and animals (e.g., rotavirus in humans).

• Cellular Defense: Eukaryotic cells have systems to destroy dsRNA, so viral dsRNA must be protected within the cell.

• Enzyme Requirement: Host cells lack RNA-dependent RNA polymerase (RdRp), so the virus must provide this enzyme.

• Enzyme Functions: RdRp acts as both a transcriptase (to transcribe mRNA) and a replicase (to replicate the RNA genome).

Replication Cycle of dsRNA Viruses (e.g., Rotavirus)

• Upon entry, the viral nucleocapsid (NC) remains intact in the cytoplasm, protecting the dsRNA genome from host defenses.

• Replication and transcription occur inside the NC or capsid.

• mRNA is transcribed from the negative (−) strand of the dsRNA genome by the viral RdRp.

• The mRNA is translated by host ribosomes to produce viral proteins.

• Viral proteins aggregate to form new nucleocapsids, which encapsidate the RNA replicase and plus-strand RNA.

• New negative-strand RNA is synthesized within the NC, ensuring the dsRNA genome remains protected.

Key Steps in dsRNA Virus Replication

1. Virus entry into host cell.

2. Transcription of mRNA from the (−) strand by viral RdRp.

3. Translation of viral proteins from mRNA.

4. Assembly of new nucleocapsids and synthesis of new dsRNA genomes.

5. Release of new virions from the host cell.

Diagrammatic Summary

• dsRNA genome → (RdRp) → mRNA → (host ribosome) → viral proteins

• mRNA → (RdRp) → new dsRNA genome

Example: Rotavirus

• Segmented linear dsRNA genome (11 segments coding for 12 proteins).

• Causes diarrheal illness in humans, especially children.

• Replication occurs in the cytoplasm, with the genome protected inside the capsid.

Table: Key Features of dsRNA Virus Replication

Additional info:

• dsRNA viruses must avoid detection by host immune sensors such as RIG-I and MDA5, which recognize dsRNA as a sign of viral infection.

• Rotavirus is a major cause of severe diarrhea in infants and young children worldwide.