M07.15.011 Temperate Phages and Lysogeny

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Learning Objectives

Describe the dual life cycles of temperate phages: lytic and lysogenic.
Explain lysogeny, prophage formation, and maintenance by repressor proteins.
Understand induction and the basis of specialized transduction.
Recognize the clinical relevance of lysogenic conversion in bacterial virulence.

Definition: Phages that can choose between lytic replication and lysogeny.
Upon infection, there is a regulatory “race”: if the phage repressor is made fast enough, lytic genes are blocked, and the phage enters lysogeny.
Lysogenized cells replicate normally, passing the prophage to daughter cells as long as the repressor is functional.

Prophage: Phage DNA stably integrated into the bacterial chromosome.
State of lysogeny: The bacterial cell carries the prophage without producing virions due to repression.
Maintenance:
- The repressor protein prevents lytic replication.
- Defective phages cannot replicate unless a helper phage is present.
Induction:
- Damage to the repressor (from UV, chemicals, or temperature) triggers excision.
- Phage enters the lytic cycle, producing virions.

Example: Lambda phage inserts between gal and bio genes in E. coli.

Letter	Toxin / Factor	Organism
C	Cholera toxin	Vibrio cholerae
O	O-antigen modification	Salmonella prophages
B	Botulinum toxin	Clostridium botulinum
E	Erythrogenic toxin	Streptococcus pyogenes
D	Diphtheria toxin	Corynebacterium diphtheriae
S	Shiga toxin	Shigella

Occurs when imprecise excision of a prophage transfers adjacent bacterial genes.
Only genes next to the phage insertion site are transferred.
Lambda phage: only gal or bio genes can be transduced.
Transduced DNA can integrate into a recipient bacterium by homologous recombination, producing new genetic combinations.

Triggered by repressor damage (UV light, chemicals, temperature).
Normally, excision recreates the original circular phage genome, resulting in normal phages.
Rare errors in excision → specialized transducing phages carrying bacterial genes.

Feature	Transformation	Conjugation	Transduction
Cell-to-cell contact	No	Yes	No
Requires antecedent phage infection	No	No	Yes
Requires competency	Yes	No	No
Naked DNA involved	Yes	No	No
Recombination required	Yes	No (F⁺ × F⁻)	Yes (Hfr × F⁻)

Feature	Generalized	Specialized
Mechanism	Error in phage assembly	Error in prophage excision
Requires stable insertion of the prophage	No	Yes
Genes that may be transferred	Any bacterial gene	Only genes adjacent to the insertion site