Phage Lambda Flashcards
Define lytic cycle
Virus infects a host cell
Replicates
Destroys the cell to release newly formed viral particles
Define lysogeny
Viral genome integrates into the host cell’s genome
Replicates alongside it without causing immediate lysis
In this state, the viral DNA/RNA is known as a PROPHAGE when integrated into the host genome
What state are viral genes integrated in lysogeny?
Genes are transcriptionally silent or express only a few genes = allowing the virus to persist in the host without causing immediate harm
Define prophage
A phage genome covalently integrated into a bacterial genome
Define phage immunity
The ability of a prophage to prevent another phage of the same type from infecting a cell
What does induction of a phage mean?
Under certain condition, the prophage can be induced to exit the lysogenic state and enter lytic cycle
What happens to viral gene expression in induction?
Phage DNA is released and enters the lytic cycle
Why may induction of a phage occur?
Destruction of a lysogenic repressor
Environmental stress
Changes in host cell
What can a temperate phage do?
Can choose between lytic and lysogenic pathway development
What can a virulent phage do?
Lytic phage = cannot display lysogenic cycle
What are the periods of lytic development?
Early period = before replication
Late period = after replication onset
What are phage genomes and protein particles first assembled into in lytic development?
Progeny phages
What are the two early genes?
cro = negative regulator
N = antiterminator
What is N gene required for?
Expression of delayed early genes
N = synthesizes delayed early genes
What are the 3 delayed early genes?
Regulators = cii, ciii, Q
What delayed early genes does lysogeny require?
cii and ciii
What genes does the lytic cycle require?
cro = early gene
Q = delayed early gene
What genes does the lysogenic cycle not have?
Late genes
What order does the lambda repressor bind operators?
O1-2-3
What is the outcome of lambda repressor binding operator?
Represses expression of lytic genes
Maintains lysogenic cycle
What is the role of cro?
Promotes lytic cycle
Prevents lambda repressor from binding
What promoters are the early genes transcribed by?
N = promotor L
cro = promoter R
What is the role of the anti-terminator protein? (pN)
Allows RNA polymerase to pass the terminators = tL1 tR1
What does N mRNA code for?
N protein
What is the role of Q protein?
Anti-terminator = allows RNA polymerase to transcribe the late genes for the LYTIC cycle
What happens to the lambda DNA after infection?
It circularizes = late genes form a single transcription unit
What is the late genes’ promoter called?
PR’
What gene encodes lambda repressorand what are its promoters?
cl gene
PRM = promoter right maintenance
PRE = promotor right establishment
What genes does lambda repressor block from being transcribed?
N and cro
Lambda repressor acts at operators = blocking transcription of immediate early genes
How is transcription of cl stimulated?
Lambda repressor binding at operator R = stimulates transcription of cl from PRM
What is the structure of the lambda repressor?
Monomer has two domains
NTD = DNA binding site
CTD = dimerization
What is required for lambda repressor to bind DNA?
Must be a dimer
How is the lytic cycle induced?
Cleavage of the repressor between the two domains = reduces affinity to operators
Inducing the lytic cycle
Cleavage can occur because of UV or temperature
How does lambda repressor bind DNA?
Helix-turn-helix motif = arrangement of two alpha-helices that bind DNA
What is the role of the recognition helix?
Lambda repressor recognition helix (helix 3) = recognizes operator sequence in phage genome
FIts into major groove
Describe the lambda operator sequence
17 bp sequence
Axis of symmetry through central base pair
Phage immunity occur between phages with the same what?
Immunity region
What happens in virulent mutations?
Unable to establish lysogeny because mutation in operator but repressor cannot bind
What is serial dilution used for?
To count phage
What is a plaque?
Hole in a bacterial lawn
What are the different plaque morphologies seen?
Wild-type phages = cloudy plaques
Because they contain some cells that have established lysogeny instead of being lysed
Virulent mutants = clear plaques
Because unable to establish lysogeny so the plaques only have lysed cells
What two regions overlap?
Operators overlap with promoters
How does the lambda repressors’ autoregulatory circuit work?
DNA-binding region of repressor at OR2 contacts RNAp and stabilizes its binding to PRM = promotes cl synthesis
Repressor blocks N and cro transcription = blocks entry into lytic cycle
Where is the DNA-binding region found in lambda repressor?
Helix 2
What does cooperative interaction increase?
Sensitivity of regulation
Cooperative ???
Why are cII and cIII genes needed to establish lysogeny?
These delayed early genes are need for RNAp to initiate transcription at PRE
cIII = protects cII from degradation
Transcription from PRE = synthesis of repressor and blocks cro synthesis
What is the role of cII protein?
RNAp only binds promoter in presence of cII
What is the role of cIII protein?
Protect cII from degredation
Why can transcription from PRE block cro synthesis?
…?
Where does cII act at?
PRE
What are the steps in lysogeny?
Immediate early genes N and cro are transcribed
N protein antiterminates allowing delayed early genes to be transcribed = cII and cIII
cII acts at PRE = cl is transcribed
Establishment of repressor turns off early gene expression
Repressor binds OR and OL
cl is transcribed from PRM
What are the steps in teh lytic cycle?
N and cro are transcribed
N antiterminates, allowing delayed early genes to be transcribed = cII and cIII
cro binds O3-2-1 affinity
Binding OR3 = prevents RNAp binding PRM
Cro represses immediate early genes = indirectly blocking repressor establishment
pQ activates late expression
What is the same between the lytic cycle and lysogeny?
Delayed early stage = both Cro and repressor are being expressed
What is deteremines which cycle the phage goes through?
Whether cII causes sufficient synthesis of repressor to overcome action of Cro
