Transduction

Transduction

• Definition: Gene transfer from a donor to a recipient by way of a bacteriophage

• Bacteriophage (phage): A virus that infects bcteria

Phages as DNA carriers

Bacteriophages are natural vectors that transduce DNA from one bacterial cell to another.

A bacteriophage cannot “live” or reproduce without getting inside a bacterial cell

Bacteriophage - structure

Phage Composition and Structure

• Composition– Nucleic acid– Protein

• Structure (T4)– Size (80 X 100 nm)

– Head or capsid– Tail

Core

Tail Fibers

Base Plate

Head/Capsid

Contractile Sheath

Infection of Host Cells by Phages

• Irreversible attachment–Base plate

• Adsorption–Tail fibers– Receptor is LPS for T4

• Nucleic acid injection

• Sheath Contraction

• 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 transduction1. 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

• Release of phage

• Phage replication and degradation of host DNA• Assembly of phages particles

• Infection of recipient• Homologous recombination

• Infection of Donor

Potentially any donor gene can be transferred

Generalized Transduction

Specialized TransductionLysogenic 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