Regulation of gene expression

In bacteriophage lamdaRASHIKA RAJASUNDAR

Lysis or LysogenyLysis: Infection by phage produces many progeny

and breaks open (lyses) the host bacteriumLysogeny: After infection, the phage DNA

integrates into the host genome and resides there passivelyNo progenyNo lysis of the hostCan subsequently lyse

Bacteriophage lambda can do either.

Growth and induction of the Lambda lysogen

Genes responsible for phage expressionN gene – anti terminatorCro gene – lytic switch ONC I gene – lysogenic switch ONC II gene – activation of C I geneC III gene – protects C II geneQ gene – anti terminatorint gene – integrase enzyme

The genetic map of the lambda phage

Recombination Control region Replication LysisVirus head&tail

originoR Pint oL PL PRM PR PRE PR‘

tR3 tL1 tR1 tR2 t6S

attint

xisred

gam

cIII N cI cro cII O P Q S R A…J

promoteroperatorterminator

Late control

cos

Immediate early transcription.

6S RNA

oR Pint oL PL PRM PR PRE PR‘

tR3 tL1 tR1 tR2 t6S

attint

xisred

gam

cIII N cI cro cII O P Q S R A…J

N Cro

Anti-termination by N protein leads to early gene expression.

Pint PL PRM PR PRE PR‘tR3

tL1 tR1 tR2 t6S

attint

xisred

gam

cIII N cI cro cII O P Q S R A…J

N N N

N protein Cro6S RNA

CIII

Recombination proteins

CII

Replication proteins

Q protein

Cro turns off cI, Q protein action leads to late gene expression.

oR Pint oL PL PRM PR PRE PR‘

tR3 tL1 tR1 tR2 t6S

attint

xisred

gam

cIII N cI cro cII O P Q S R A…J

Cro Cro Q

Lytic functionsReplication proteinsViral head & tail proteins

Lysogeny cascade

oR Pint oL PL PRM PR PRE PR‘

tR3 tL1 tR1 tR2 t6S

attint

xisred

gam

cIII N cI cro cII O P Q S R A…J

CIII CII

CI Repressor

PRE = promoter for repressionestablishmentInt

tint

CII

In simple words

Gene expressed first takes up the role.......

Role of Lambda repressor and cro

Summary Both lysis and lysogeny:

synthesize N, Croantitermination by N : synthesize cIII, cII, Q

Lysis:Low [Cro] : binds OR3, shuts off PRM (cI)High [Cro] : shuts off PR and PL

antitermination by Q + activation of PR’ by Cro

Lysogeny:cII stimulate expression from PRE (cI repressor)

and PINT (integrase)cIII stabilizes cIIcI repressor shuts off PR, PL, PR’ (no lytic

functions), stimulates PRM