Phage Composition and Structure• Composition – Nucleic acid Head/Capsid – Protein Contractile Core Sheath • Structure (T4) Tail Fibers – Size (80 X 100 nm) Base Plate – Head or capsid – Tail
Infection of Host Cells by Phages• Adsorption –Tail fibers – Receptor is LPS for T4• Irreversible attachment – Base plate• Sheath Contraction• Nucleic acid injection• DNA uptake
Types of Bacteriophage• Lytic or virulent – Phage that multiply within the host cell, lyse the cell and release progeny phage (e.g. T4)• Lysogenic or temperate phage: Phage that can either multiply via the lytic cycle or enter a quiescent state in the bacterial cell. – Expression of most phage genes repressed – Prophage – Phage DNA in the quiescent state – Lysogen – Bacteria harboring a prophage
Transduction• Definition: Gene transfer from a donor to a recipient by way of a bacteriophage• Lederberg & Zinder – 1951
Transduction• Types of transduction 1. Generalized - Transduction in which potentially any donor bacterial gene can be transferred. 2. Specialized- Transduction in which only certain donor genes can be transferred
Generalized Transduction• Infection of Donor• Phage replication and degradation of host DNA• Assembly of phages particles• Release of phage• Infection of recipient• Homologous recombination Potentially any donor gene can be transferred
Specialized Transduction Lysogenic Phage• Excision of the prophage• Replication and release of phage• Infection of the recipient• Lysogenization of the recipient – Homologous recombination also possible
Specialized Transduction• In specialized or restricted transduction, the transducing particle carries only specific portions of the bacterial genome.• Specialized transduction is made possible by an error in the lysogenic life cycle.
• When a prophage is induced to leave the host chromosome, excision is sometimes carried out improperly.• The resulting phage genome contains portions of the bacterial chromosome (about 5 to 10% of the bacterial DNA) next to the integration site.• A transducing phage genome usually is defective and lacks some part of its attachment site. The transducing particle will inject bacterial genes into another bacterium, even though the defective phage cannot reproduce.
• The best-studied example of specialized transduction is the lambda phage. The lambda genome inserts into the host chromosome at specific locations known as attachment or att sites.• The phage att sites and bacterial att sites are similar and can complex with each other.
• Normal out looping• Phage excised out• Rare abnormal outlooping• dgal• Defective in gal,• defective in integration site• Helper phage – hybrid attachment site