Virology Lecture Notes - RNA Viruses and RNA Synthesis

Introduction

This lecture introduces the molecular mechanisms of RNA viruses, particularly focusing on their capacity for RNA synthesis from RNA. The history of RNA viruses will be explored, along with notable experiments and discoveries that have shaped our understanding of RNA synthesis.

Historical Context of RNA Viruses

  1. Stanley and Tobacco Mosaic Virus (TMV)

    • In 1935, Wendell Stanley at the Rockefeller Institute in Princeton crystallized the tobacco mosaic virus (TMV), the first discovered virus.

    • He noted that TMV crystals contained 5% RNA and erroneously suggested it was a contaminant; he theorized that proteins were the genetic material.

    • Stanley received the Nobel Prize for his work, although this viewpoint on RNA as mere contaminant has been widely criticized.

  2. Key Discoveries in Genetic Material

    • In 1944, it was established through work by Avery, MacLeod, and McCarty that DNA served as the genetic material in bacteria.

    • The Hershey-Chase experiment of 1952 confirmed that the DNA of bacteriophages was the hereditary material.

    • The double helical structure of DNA was elucidated in 1953, a seminal year.

    • By 1956, further experiments confirmed that RNA served as genetic material in TMV.

    • By 1959, RNA had been identified across various animal viruses.

Baltimore Classification of Viruses

  • The lecture refers to the Baltimore classification system which categorizes viruses based on their genome type and method of replication, focusing on:

    1. Double-stranded RNA viruses (Reoviruses and Rotaviruses)

    2. Negative-strand RNA viruses (Influenza virus and VSV)

    3. Positive-strand RNA viruses (Poliovirus)

    4. Retroviruses, which undergo a DNA intermediate.

RNA Virus RNA Synthesis: Experiments and Mechanisms

Polio Virus RNA Replication

  1. Infection of Cells in Culture

    • Infected cells are harvested, and extracts are created for analysis, with the addition of radiolabeled triphosphates for tracking RNA synthesis.

    • A graph illustrates the timeline of infection showing infectivity and RNA polymerase activity, culminating around 4-5 hours post-infection, indicating rapid viral replication.

RNA Polymerase Activity

  • Negative-strand Viruses

    • The RNA-dependent RNA polymerase (RDRP) is located within the virus particle.

    • When viral RNA enters the host cell, it requires this polymerase to synthesize mRNA as it cannot be translated directly.

  • Positive-strand Viruses

    • Lack an RDRP in the particle since their RNA can be directly translated into proteins post-entry into the host cell.

    • Examples: Poliovirus, Coronaviruses, Flaviviruses.

    • These viruses do not generally have protein coats on their RNA, except for retroviruses, which include encapsidated proteins.

Structural Aspects of Essential Molecules

  • Nucleocapsid Structure

    • The RNA of viruses is often bound by proteins, with structural variations like helical and cylindrical formations.

    • Use of structural models depicting RNA-protein interactions illustrate the intricate design necessary for function.

RNA as a Structured Molecule

  • RNA can form complex three-dimensional structures, such as:

    • Stem loops

    • Pseudoknots

  • Such secondary structures are significant in binding proteins and facilitating functions necessary for the viral lifecycle.

Universal Rules of RNA Synthesis

  1. Complete Coverage: The entire genomic RNA must be copied end-to-end for RNA synthesis to be successful.

  2. Translatable mRNAs: The synthesis must produce mRNAs that can be successfully translated by the host cellular machinery.

RNA Synthesis Process

  • Starts at specific template sites, with potential for denovo initiation or primer-dependent initiation.

  • Polymerase Mechanism:

    • Two-metal mechanism involves magnesium ions, facilitated by specific amino acids (e.g., aspartates), mediating nucleophilic attack and releasing pyrophosphate, forming phosphodiester bonds.

Types of RNA Polymerases

  1. RNA-dependent RNA polymerase (RDRP)

  2. RNA-dependent DNA polymerase (Reverse transcriptase)

  3. DNA-dependent DNA polymerase

  4. DNA-dependent RNA polymerase

Insights into Viral RNA Replication Strategies

Poliovirus as Model Example

  • Genome Characteristics: A long RNA that is translated into a polyprotein, subsequently cleaved into functional proteins.

  • Replication Mechanism: Characteristic reliance on cellular membranes for RNA synthesis, indicating tailored strategies for effective replication processes.

Flavivirus and Alphavirus Variations

  • Both have replication cycles that include the generation of subgenomic RNA, a characteristically important layer in their structure and transmission efficiency.

Coronaviruses' Unique Mechanism of Subgenomic Synthesis

  • Include leader sequences, representing a diversity-driving mechanism through recombination, increasing their potential to adapt rapidly in new environments.

Negative-Stranded RNA Viruses

  • Switching from mRNA to Genome Synthesis: Governed by the emergence of specific proteins (like N protein) which enable polymerase switching from mRNA to genome RNA synthesis.

Summary of Mistakes and Variability in Replication

  • RNA viruses, typically without error correction mechanisms, experience higher mutation rates compared to their DNA counterparts, leading to greater variability and adaptability.

Error Correction in Coronaviruses

  • Coronaviruses possess an exonuclease (Exo) enabling them to maintain lower mutation rates compared to other RNA viruses, which is hypothesized to contribute to their large genomic size.

Implications of Recombination in Viruses

  • RNA viruses, such as poliovirus and coronaviruses, utilize recombination as a tool for mutation. Modifying vaccines aims at stabilizing these recombinant channels to prevent potential losses in vaccine efficacy and fitness.

Conclusion

The lecture ends with an introduction to the upcoming discussions on DNA viruses and their mechanisms of mRNA synthesis, transitioning from the intricacies of RNA viruses to explore additional viral replication paradigms.