Lecture Notes: Class 1 Viruses

Overview

• Class 1 viruses are double-stranded DNA viruses.
• They replicate their genomes and transcribe RNA similarly to prokaryotic cells.
• Use host-derived enzymes but ensure viral mRNA is prioritized over the host's.
• These viruses are found in bacteria, archaea, and eukarya.
• Most class 1 viruses are bacteriophages.

Key Characteristics

• Bacteriophages: Extensively used in molecular biology.
  • Example: T7 bacteriophage.
• Other examples: Herpes virus, pox viruses (e.g., mpox, smallpox).
  • Large viruses with replication enzymes packaged.
  • Pox viruses replicate in the cytoplasm, unlike other eukaryotic DS DNA viruses which move to the nucleus.

Bacteriophage T7

• Infection: Infects E. coli and related gut bacteria.
• Structure: Icosahedral head, helical tail.
• Mechanism:
  • Injects double-stranded DNA into host cell; protein coat remains outside.
  • DNA entry is orientation-specific: the 'left end' enters first.

Replication Process

• Initial genes are transcribed by host's RNA polymerase.
• Early proteins include:
  • Inhibitor of host's restriction system (turns off bacterial immune response).
  • Viral tRNA polymerase gene (produces virus's own RNA polymerase).
  • Enzyme to inactivate host's RNA polymerase (shifts host machinery to virus replication).
• Middle to late proteins include lysozyme and others responsible for bacteriophage release.

Summary

• Class 1 viruses leverage host systems for replication and transcription but have mechanisms to ensure their processes dominate.
• Bacteriophage T7 serves as a model for understanding virus-host interactions and control over host machinery.