study material for PG Botany students

1. LYTIC AND LYSOGENIC REGULATORY
CASCADE
SRUTHI,C.C
ASSISANT PROFESSOR OF BOTANY
SVR NSS COLLEGE,VAZHOOR

2. Lytic and Lysogenic regulatory cascade

3. bacteriophage (or phage) – A bacterial virus.
lytic infection – Infection of a bacterium by a phage that ends in the destruction
of the bacterium with release of progeny phage.
virulent phage – A bacteriophage that can only follow the lytic cycle.
prophage – A phage genome covalently integrated as a linear part of the
bacterial chromosome.
lysogeny – The ability of a phage to survive in a bacterium as a stable prophage
component of the bacterial genome.
temperate phage – A bacteriophage that can follow the lytic or lysogenic pathway.

5.  In lysogenic bacterium the genes of the prophage that encodes products involved in
the lytic pathway must not be expressed, ie genes specifying enzymes involved in the
replication of phage DNA , structural proteins required for phage morphogenesis,
and the lysozyme that catalyses cell lysis must be turned off to maintain a stable
lysogenic state.
 The lytic pathway genes of the prophage are kept turned off in lysogenic cells by a
simple repressor- operator-promotor-circuit.
 The mechanism that involve whether lambda enters the lytic pathway or lysogenic
pathway involves a elegant switch that sends the infecting phage chromosome down
one of the 2 pathways .
1)The autogenously maintained repressor circuit of lysogeny.
2) Lytic regulatory cascade
 the interplay between these two regulated by
The λ repressor occupies key regulatory site lysogeny occurs
If another λ protein Cro occupy these site lytic development occurs.

6. Repression of Lambda Lytic pathway genes during lysogeny
• C1 gene encodes a repressor, binds to operator region as
dimer
• Operator regions OL and OR overlap with promoter
sequence a which RNA polymerase binds to initiate
transcription of lytic genes.
• λ operator contains 3 repressor binding sites OR1,OR2,OR3
and OL1,OL2,OL3.
• Repressor has highest affinity to OL1 and OR1, , exhibits
cooperative binding to OR2 and OL2., but does not extend
to OL3 and OR3.
• synthesis of repressor is autogenously regulated , λ C1
gene which encodes the repressor is transcribed from
promoter PRM located between C1 gene and OR3, PRM
overlaps with OR3.
• Transcription initiated at PRM occurs in opposite direction
from transcription initiated at PR.
• Repressor bound at OR2 act as positive regulator for
transcription from PRM.
• In the absence of repressor at OR2 RNA pol. Cannot bind
at PRM to initiate transcription of C1 gene., vice versa
• Repressor bind to OR3 and blocks transcription from PRM
if repent in higher concentration.

7. The lambda Lytic regulatory cascade
 The lambda genes can be placed in 3 groups
1) immediate early genes- Cro and N ; both encoding proteins required for lytic development
2) delayed early genes- 12 genes; specify products required for DNA replication, recombination and
additional regulation
3) Late genes- 23 genes encode proteins involved in head and tail morphogenesis and in lysis of the host cell.
 One of the immediate early gene products, N protein required to express the delayed early genes.
 One of delayed early gene Q protein must be present to express the λ late genes.

8. • In the absence of repressor the C1 gene
product ; the RNA polymerase initiates
transcription at promoters PL and PR.
• The first gene transcribed starting from
PR is Cro, which binds to the same DNA
sites OL and OR but Cro has higher
affinity for OR3 , and keeps the
synthesis of repressor turned off by
blocking initiation of transcription at
PRM
• Later when more Cro protein has
accumulated in the cell it also binds to
OR1 and OR2 ; OL1 and OL2 which
supress transcription from PR and PL
decreasing he rate of synthesis of early

9.  Synthesis of delayed early gene products requires the product of
second immediate early gene N, which is first transcribed from PL.
 The gene N product functions to prevent the termintion of
transcription at two sites one (tR1) located adjacent to Cro & the
other(tR1) just downstream from gene N itself
 N protein binds to the nut sites and with the aid of E coli proteins
Nus and ribosomal protein S10 modifies the specificity of RNA
polymerase as it passes the nut site; continues into delayed early
genes.
 One of the delayed early genes Q encode another transcription
terminator that is required for late gene expression .
 Q protein recognises DNA sequence qut and prevents the arrest of
termination initiated at the constitutive promoter PR at termination
site tR3 allowing transcription to continue through the late genes.
 In the absence of λ repressor dimers Cro and N protein are
synthesised . Cro binds to OR3 & prevents the synthesis of
repressor . With Cro and N protein but no repressor lytic

11. The Lambda switch – Lytic development or lysogeny
 The genetic switch that sends lambda down either the lysogenic or the lytic pathway involves two key
regulatory genes. C II and CIII and one promoter PRE . PRE is located on the opposite side of the Cro gene
from PRM . The transcript initiated from PRE contains the non coding or antisense sequence of the Cro gene
and the normal mRNA coding sequence of the C1 gene .
 both CII and CIII are delayed early genes , so their expression is dependent on N protein.
 C II protein is required for the initiation of transcription at PRE., however CII protein is unstable , rapidly
degraded by host cell protease . CIII stabilises CII.
 Under poor growth conditions CIII protects CII from degradation, favouring lysogeny.
 under optimal growth conditions CIII is less active , thus CII is less stable favouring lytic development.

12.  The transcript initiated at PRE favours establishment of the lysogenic pathway in two ways
1) Lambda repressor is produced by translation of the C1 coding sequence.
2) The 5’ portion of this transcript contains the antisense sequence of the Cro gene .
 Thus the Cro antisense and sense RNA hybridize with each other and blocks translation of Cro
mRNA.
 Once repressor has been synthesised by use of the CII-C III PRE circuit , it binds at OR1 and OR2,
inhibits the expression of lytic function genes by blocking the initiation of transcription at PR.
• In each infected cell the genetic switch
for either lysogeny or lytic
development is set by the race between
Cro protein and λ repressor for the
occupancy of OL and OR.
• If Cro occupies lytic phase occurs
• If repressor occupies lysogeny occurs
• CII and CIII play important role in
deciding the outcome of this race b/w
Cro and repressor

13. Lysogeny Requires Several Events
FIGURE 33: The lysogenic pathway leads to repressor
synthesis

14. The Cro Repressor Is Needed for Lytic Infection
• When Cro binds to other
operators at ORor OL, it
prevents RNA polymerase
from expressing
immediate early genes,
which (indirectly) blocks
repressor establishment.
FIGURE 34: The lytic pathway
leads to expression of cro and
late genes

15. What Determines the Balance between
Lysogeny and the Lytic Cycle?
FIGURE 35: Repressor
determines lysogeny,
and Cro determines the
lytic cycle