regulation of lytic cycle in lambda phage

1. Regulation of lytic cycle in
λ phage
Manjunatha S
II MSc(Botany)
University of Mysore,
Mysuru

2. Table of contents
1.Introduction .
2.Life cycle of lambda phage.
3.Circularization of DNA.
4.Types of genes.
5.Gene map of λ phage.
6.Regulation of lytic cycle.
7.cro repressor.
8.Conclusion.
9.References.

3. Introduction
Viruses that infect are generally called bacteriophages, often
abbreviated to phage or simply Φ.
Usually a phage infection kills the bacterium. The process by which a
phage infects a bacterium, reproduces itself, and then kills its host is
called lytic infection.
Lambda phage usually infects E.coli
Discovered by Esther Lederberg in 1950

4. Life cycle of
λ phage

6. • Early genes: Genes that are transcribed before the replication of phage
DNA.
a) Immediate early genes: They are transcribed immediately upon
infection by the host RNA polymerase.
eg: N and cro
b) Delayed early genes: They cannot be transcribed until regulator
protein(s) coded by the immediate early genes have been synthesized.
eg: cII, cIII, Q, xis, int etc.
• Late genes: A gene transcribed when phage DNA is being replicated. It
codes for components of the phage particle.

7. Lytic development
is controlled by a
Cascade

8. Genetic map of λ phage

10. Immediate early and delayed early genes are
needed for both Lytic cycle and lysogeny
Lytic development follows if the late genes are expressed.
Lysogeny ensues if synthesis of a gene regulator called the lambda
repressor is established by turning on its gene, the cI gene.
Lambda has two immediate early genes,
• N gene codes for an antitermination factor required for turning
on the delayed early genes
• cro gene has dual functions
i) codes for a repressor that prevents expression of cI gene
ii) it turns off expression of the immediate early genes(PL and PR
regions of N and cI)

12. • pN is an antitermination factor that allows RNA
polymerase to continue transcription past the ends of thr
twi immediate early genes.
• pQ is the product of delayed early gene and is an
antiterminator that allows RNA polymerase to transcribe
the late genes.
• Lambda DNA circularizes after infection; as a result, the
late genes form a single transcription unit.

13. The cro
repressor is
needed for Lytic
cycle
• Prevents the synthesis of
lambda repressor via the
maintenance circuit, i.e.,
prevents transcription via
PSM
• Inhibits the expression of
immediate early genes
from both PL and PR

14. What to choose….?
 lytic process occurs much more frequently.
 the role of the cII protein is critical.
 if the cII protein is poorly expressed or has low activity, very low
levels of cI protein will be present.
 The Cro protein will inhibit the activity of PL and PR and the
synthesis of the cII protein.
 The cII protein is susceptible to proteolytic degradation by host
enzymes.
 The cIII protein also plays a role in the choice between lysis and
lysogeny .

15. Conclusion
• Phage infection is a cascade event.
• The delayed early stage when both cro and repressor
are being expresed is common to lysogeny and lytic
cycle.
• Cro and pQ plays a prominent role in lytic cycle
initiation and synthesis of phage particles.

16. References
• E. Krebs, S. Goldstein, T. Kilpatrick, 2013, Lewin’s Essential
Genes, 3rd edition, Jones & Bartlett learning, an ascending
Learning Company, Burlington.
• Alan J. Cann, 2005, Principles of Molecular Virology, 4th
edition, Elsevier Academic press, New York.
• N.J Dimmock, A.J Easton, K N Leppard, 2007, Introduction
to Modern Virology, 6th edition, Blackwell publishing,
Carlton, Australia.

17. Thank you