Bacteriophage Life Cycles and Mechanisms

Bacteriophage T4 and Lambda Phage

Introduction to Bacteriophages

  • Bacteriophage (Phage): A virus that infects bacteria, abbreviated as .
  • Infection Mechanism: Bacteriophages subvert the host machinery to generate a high number of progeny.

Types of Bacteriophages

  • Virulent Phages:

    • Life Cycle: Single, operates via the lytic cycle.
    • Process:
    1. Enter the host cell.
    2. Reproduce.
    3. Released.

  • Temperate Phages:

    • Life Cycle: Two paths, lytic and lysogenic.
    • Process:
    1. Enter the cell and can be integrated into the host genome.
    2. Can exit to reproduce (induction) and then released.

Virulent Phage Life Cycle

Infection Phases:
  • Early Development:
    • DNA injected into the host cell.
  • Late Development:
    • Synthesis of phage components like heads and tails.
  • Lysis:
    • The host cell breaks, releasing new phages.

Temperate Phage Life Cycle

  • Replication: Host replicates, passing the phage genome to daughter cells.
  • Prophage Role:
    • Provides immunity by integrating into the host's genome.
  • Induction: Transition from lysogenic to lytic form through a regulated transcriptional pathway.

Lytic Development Stages

  • Early Stage:
    • Use of host factors, RNA polymerase regulators are indistinguishable from host.
  • Mid Stage:
    • Viral factors activated to express new sigma factor and replication factors.
  • Late Stage:
    • Expression of structural genes essential for phage components.

Lambda Phage Overview

  • Characteristics:
    • A temperate bacteriophage infected by E. coli.
    • Contains a 48 kb linear dsDNA genome with unique cos sites.

Lambda Phage Genome Organization

  • Directionality:
    • Genome organized with leftward and rightward transcription regions.
  • Clustering:
    • Early, delayed early, and late genes within operons for efficient co-regulation.

Antitermination in Lambda Phage

  • Definition: Involves proteins that bypass terminators, supporting gene expression at various points in development.
  • Antiterminators: Such as pN and pQ, act on specific DNA elements.

Lysogenic Cycle Dynamics

  • Integration Requirements:
    • Requires the lambda repressor coded by the cI gene to suppress transcription of N and Cro genes.
  • Operator Binding:
    • Repressor binding prevents further infection and regulates phage gene expression.
  • Maintaining Lysogeny: cI controls the transcription levels, reinforcing stability.

Immunity Mechanism in Lysogenic Phage

  • Function:
    • A lysogenic phage can confer immunity to the host against further infections by similar phages.
    • Prophage actively suppresses other phage genes to prevent their replication.

Role of cI in Regulating Phage Life Cycle

  • Structure: cI possesses DNA-binding motifs and dimerization features for operator interaction.
  • Dynamic Regulatory Role: cI helps maintain the lysogenic pathway by inhibiting transcription of lytic genes.

Choice Between Lysis and Lysogeny

  • Factors Involved:
    • Balance between cII (promotes lysogeny) and cro (promotes lytic). Stability of cII is critical for lysogeny establishment.
  • Environmental Influences: Optimal conditions skew preferences towards lysogeny.

Summary of Life Cycle Events

  • Lysogenic Pathway: cII leads to integration and establishment of prophage.
  • Lytic Pathway: N protein allows viral DNA replication and ultimately leads to host cell lysis.
  • Key Takeaway: The decision to enter either lytic or lysogenic cycles is influenced by a series of genetic interactions and environmental cues.