Lecture 13 Flashcards
What is a virus?
An infectious agent that must grow or reproduce inside a host cell
Considered obligate intracellular parasites
What are viruses composed of?
Nuclei acid, genetic material and protein coat (some also contain lipids in their coats)
What is a bacteriophage?
Viruses that infect bacterial cells
Features of phage lambda
- infects E.coli
- siphoviridae
- double stranded DNA genome (dsDNA)
- long flexible tail, icosahedral head
Two phage lambda pathways
- lytic pathway
- lysogenic pathway
Lambda and its two pathways
Yap
- phage finds an appropriate cell and uses its tail fibres to recognise receptors on cell surface
- Pham lambda is a TEMPERATE phage which means it can choose between two pathways
- lytic cell entrains a phage taking over the cell, making more of itself and causing cell lysis
- lysogenic it forms a stable relationship with e.coli and inject its DNA into the chromosome - forming a PROPHAGE - everything it reproduces the page is inherited vertically - phage can predict conditions and go into prophage induction (cell lysis) or lysogenic growth
Lytic lifecycle definition
The phage lifecycle that results in lysis of the bacterial cell upon release of progeny phage
Lysogenic life cycle
The phage lifecycle that results in stable carriage of the phage (prophage) within the host cell (lysogenic)
Virulent phage definition
A phage that is only able to undergo replication via the lytic cycle
Temperate phage definition
A phage that can replicate via either the lytic or lysogenic cyclones (e.g phage lambda)
Lysogenic definition
A host cell that is harbouring a prophage during lysogeny
Prophage definition
The latent form of a temperate phage that remains within the lysogen (e.g integrated into a host chromosome)
Phage lambda lifecycle
Step one in the formation of a bacteriophage
DNA injection and circularisation
- genome is linear in phage head
- following injection, the lambda genome circularises (staggered ends that are complementary )
- Aided by cohesive end sites (cos sites)
Phage lambda genome in head vs in host
- linear in phage
- circularised in host
Phage genome
Lytic cycle: process of the phage assemble and release
- Head and tail proteins are synthesised
- DNA is packaged into heads
- Tails are added
- The is lysed and releasing new phage
Lysogenic cycle: process of phage integration and maintainence
- Lambda integrates
- Prophage is stably maintained
- Prophage passed to daughter cells
In lysogeny how does integration of lambda DNA into host chromosome occur ?
- integration occurs between two sites
= attP (attachment on phage)
= attB (attachment on bacteria) - there is a site-specific recombination requiring enzyme int (integrate)
- lambda is then linearised in host chromosome
3 main questions to decide if lytic or lysogenic pathway
The cascade of gene expression upon infection
Very early gene expression events
- following injection, host RNA polymerase transcribes from two promoters
- PL (produces N). (Anti terminator)
- PR (produces Cro) (cro encodes the Cro protein. Expressed early in infection and the major player in establishing lytic growth)
N encodes an anti-terminator protein that enables transcription past 2 terminators resulting in early gene expression
Protein products for both possible pathways are produced early - but hasn’t committed yet
Principle of anti-termination
What are the three DNA-binding proteins of lambda
- Cro
- Cl
- CII
What is Cro
A DNA-binding protein that represses transcription
- Cro promotes the lytic cycle
What is CII
A DNA-bidning protein that can activate or repress transcription
- CI activates its own expression
- represses genes requires for lytic cycle
- maintains lysogeny
What does CII do and Cro do
CII promotes lysogenic pathway
Cro enables lytic pathway
What protein is the deciding protein in lytic vs lysogenic - lots of protease
CII
- host protease degrade CII
- healthy cells produce high levels of protease
- in actively growing cells, CII gets degraded
- Cro protein wins the battle and the lytic cycle proceeds
Status of CII and Cro in the late lytic cycle
CII is degraded
Cro predominates
- Cro represses expression of CI, all early genes then itself and the replication genes
- Q is an anti-terminator that enables expression of late lytic genes (head,tail and lysis genes)
What is the deciding protein - lytic vs lysogenic - less protease
- starved cells produce less protease
- CII remains intact and stable
- CII wins the battle over Cro and activates lysogenic pathway
- CII wins the battle over Cro and activates lysogenic pathway
Late lysogenic - status of CII
Not degraded
CII is an activator protein that turns on:
1. Int resulting in integration of lambda into host chromosome
2. CI repressor, which activates own expression and binds to OL and OR to repress all other phage genes
Process of lysogeny maintenance
- maintained as a prophage by CI
- CI is a repressor of all genes but an activator itself
- keeps phage genome ‘silent’ in bacterial chromosome until induction
So
CII activates int and Cl, which cause integration of phage and maintenance of lysogeny, respectively
Summary of the decision.
Maybe make into flash card
