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
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.
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)
11.
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.