1. Microbial Genetics (Micr340)
Lecture 11
Phage l and Lysogeny

2. Lysogeny: another life-style for some phages
Lysogenic phage – able to maintain a stable relationship with the host cell in which they neither multiply nor are lost from the cell.
In lysogenic state, the phage DNA is either integrated into host chromosome or replicates as a plasmid
The phage DNA in the lysogenic state is callled a prophage and the bacterium harboring a prophage is a lysogen for that phage

3. Phage l

4. Phage l: lytic development
Fairly large phage
Three major stages of gene expression during development
First, genes N and cro.
Genes involved with replication and recombination
Third, genes encoding head and tail proteins and enzymes involved in cell lysis

5. Transcriptional antitermination
Transcription begins at the promoter but then soon terminates unless certain conditions are met.
Phage l uses antitermination of regulation in at least two stages in its development
The N protein regulates the synthesis of its recombination and replication functions
The Q protein regulates the synthesis of its head and tail proteins

6. Transcriptional antitermination

7. Transcriptional antitermination

8. Antitermination sites

9. Phage l DNA replication
Phage l DNA is linear within its head
It cyclizes after entering the cell
Its cohesive ends, cos sites, are important for cyclization

10. Lambda () Phage cos sites
(not drawn to scale)
cos sites (complementary overhang sites) or cohesive ends, consist of complementary 12 base pair overhangs, one on the 3’ end of each strand.
Note, wild type  phage genome is about 48.5 kb long

11. Phage l DNA replication

12. Phage l cloning vectors
Cosmids
cos site
Only head of DNA can be packaged if flanked by cos sites
Packaging extracts
Lysogenic vectors, retains phage l’s capability to integrate into host chromosome as a single copy DNA

13. Phage l: lysogeny The cII gene product Phage l integration
Maintenance of lysogeny
Regulation of repressor synthesis
Immunity to superinfection
The induction of l

14. Formation of lysogens

15. Integration of Phage l DNA

16. Regulation of repressor synthesis

17. Induction of l

18. Cro’s role in l induction

19. Competition between lytic and lysogenic cycles

20. Competition between lytic and lysogenic cycles

21. Specialized transduction
In specialized transduction, only bacterial genes close to the attachment site of the prophage can be transduced. In contrast, generalized transduction, essentially any gene of the donor bacterium can be transferred.
In specialized transduction, phage carries both bacterial genes and phage genes. In contrast, generalized transduction phage carries only bacterial genes

22. Specialized transduction

23. Specialized transduction