L13: Transcriptional Regulation in Prokaryotes II (bacteriophage λ) Flashcards
explain bacteriophage lambda (λ)
it infects E. coli and undergoes either:
1. lytic growth
2. lysogenic growth
what is lytic growth
- reproduction of viruses
- uses the host cell to product more viruses and then it bursts out of the cell
what is lysogenic growth
- virus reproduction
- phage genome integrates itself into the E. coli genome through site-specific recombination
- this cell then becomes a prophage
what is a prophage?
a dormant phage in lysogenic phase
explain the regulatory region of bacteriophage lambda (λ)
- three promoters:
1. P-RM
2. P-L
3. P-R
regulatory region of bacteriophage lambda (λ) - P-RM
- repressor maintenance
- active during lysogenic growth
- its a weak promoter that requires an activator to induce gene expression
- it transcribes the cl (l = 1) gene than encodes lambda (λ) repressor
P-RM - what are the two domains of the lambda (λ) repressor
- amino-terminal DNA binding domain (helix-turn-helix motif)
- carboxyl-terminal dimerization domain
P-RM - what can lambda (λ) repressor act as
- a repressor
- an activator
lambda (λ) repressor - acting as a repressor
by binding DNA and physically excluding RNA Pol
lambda (λ) repressor - acting as an activator
by binding DNA and recruiting RNA Pol using an activating region on the amino terminus
regulatory region of bacteriophage lambda (λ) - P-L and P-R
- both are active during lytic growth
- Cro is a dedicated repressor transcribed from P-R and it binds as a dimer using a helix-turn-helix motif
what are the operators
- multiple operators can be bound by the lambda repressor and Cro
- operator O-R1
- operator O-R2
- operator O-R3
explain operator O-R1
- in promoter P-R
- has the greatest affinity for lambda repressor
explain operator O-R2
- overlaps with P-RM and P-R
- the cooperative binding at O-R1 helps recruits the lambda repressor to O-R2
explain operator O-R3
- in promoter P-RM
- has the greatest affinity for Cro
what is the gene layout in lysogenic stage
- P-L promoter
- cl gene
- P-RM promoter (all of O-R3 and some of O-R2) ←
- P-R promoter (some of O-R2 and all of O-R1)
- cro gene
what is the gene layout in lytic stage
- P-L promoter ←
- cl gene
- P-RM promoter (all of O-R3 and some of O-R2)
- P-R promoter (some of O-R2 and all of O-R1) →
- cro gene
what happens to the gene layout during lytic growth
- the Crop repressor dimer binds O-R3 at P-RM
- RNA Pol then binds to P-R and P-L to activate lytic genes
lytic growth - what does binding O-R3 at P-RM do?
it prevents the RNA pol from transcribing the lambda repressor
lytic growth - what happens after the binding of RNA Pol
- lytic genes are activated
- no basal activation bc P-R and P-L do not require activators since they are strong promoters
what happens to the gene layout during lysogenic growth
- the lambda repressor dimer binds O-R1 at P-R
- cooperative binding of lambda repressor spreads to weaker O-R2
- activating region of lambda repressor at O-R2 brings RNA Pol to P-M
lysogenic growth - what does binding O-R1 at P-R do?
prevents RNA Pol from binding and activating cro expression
lysogenic growth - what does the recruitment of RNA Pol to P-RM do?
results in expression of more lambda repressors (positive autoregulation)
how does the cell choose between lysogenic and lytic development?
- its dictated by growth conditions of E. coli
- growth conditions affect the stability of the regulator cll (II = 2)
- the activity of Cll dictates lysogenic vs lytic development
- cII activity is dictated by HflB
choice between lysogenic and lytic development? - why is cII activity influenced by HflB?
HflB is a protease that degrades cII from bacterial host cells
choice between lysogenic and lytic development? - poor conditions for growth
- few host cells to infect
- HflB activity is low → cII levels are high → cI levels are high
- lysogenic development is favored
choice between lysogenic and lytic development? - good conditions for growth
- numerous host cells to infect
- HflB activity is high → cII levels are low → cI levels are low (no repressor)
- lytic development is favored
choice between lysogenic and lytic development? - what is the cll?
- the protein is a transcriptional activator
- cll expression is controlled by P-R
- cll protein binds upstream of P-RE
- therefore, cl is transcribed by both P-RM and P-RE
choice between lysogenic and lytic development? - what role does cI play?
- cI codes for the lambda repressor
- cI expression is established by “P-RE and maintained by P-RM (via positive autoregulation)
- this cII expression promotes lysogeny by inducing cI
gene layout when lytic development is favored
- P-L promoter
- cl gene
- P-RM promoter
- P-R promoter →
- cro gene
- P-RE promoter
- cll binding site
- cll
gene layout when lytic development is favored - what happens?
- cro is transcribed from promoter P-R (→)
- P-RE is not activated
- cII is degraded (unstable)
- no cI transcription from P-RE
- results in no repression of lytic genes
- if cII does become stables, cell will move to a more intermediate state
gene layout when intermediate development is favored
- cI gene
- P-RM promoter
- P-R promoter
- cro gene
- P-RE promoter ←
- cII binding site
- cII gene
gene layout when intermediate development is favored - what happens?
- if cII becomes stable
- cII binds near P-RE and activates cI
- cI represses Cro and activates the cI gene
- if enough cI is made, it will cause lysogeny
gene layout when lysogenic development is favored
- cI gene
- P-RM promoter ←
- P-R
- cro gene
- P-RE promoter
- cII binding site
- cII gene
gene layout when lysogenic development is favored - what happens?
- cI is now active and abundant
- cI binds to O operator and represses P-R (stopping cro and lytic genes)
- cl activates P-RM
- lysogeny is established
what is the SOS response?
- if the survival of the host cell is at risk, induction to lytic growth will occur
- E. coli senses DNA damage and activates RecA
SOS response - what does RecA do?
- facilitates homologous recombination
- triggers degradation of repressor LexA
- resulting in de-repression of ENA-repair enzymes
SOS response - what is the lambda repressor’s role in this?
- the lambda repressor has evolved to resemble LexA
- cleavage of the lambda repressor during SOS response promotes lytic growth
