Lecture 16 Flashcards
What is a virus?
An infectious agent that must grow/reproduce inside a host cell
- > considered obligate intracellular parasites
- > composed of nucleic acid genetic material and protein coat
What is a bacteriophage?
A virus that infects bacterial cells
Describe the shape of Phage Lambda
- A double stranded DNA genome
- Long flexible tail and icosahedral head
What is a temperate phage?
It can choose between lytic or lysogenic pathway
What does the lifecycle decision of the temperate phage depend on?
The state of the host bacterium
Define lytic
The lifecycle resulting in bacterial cell lysis upon release of progeny phage
Define lysogenic
The lifecycle resulting in stable carriage of the phage (prophage) within the host cell (lysogen)
What is a virulent phage?
A phage that’s only able to undergo replication via the lytic cycle
What is a lysogen?
A host cell harbouring a prophage during lysogeny
What is a prophage?
Latent form of a temperate phage that remains within the lysogen (i.e is integrated into the host chromosome)
Describe the genome in the phage head
Linear with staggered ends which give a ssDNA region of complementarity allowing DNA to stick together
Following insertion what happens to the lambda genome?
The genome circularises using cohesive end sites (cos sites)
Give the overview for the Lytic cycle
(Phage assembly and release)
- Head and tail proteins synthesised
- DNA packed into heads
- Tails added
- Host is lysed releasing the new phage
Give the overview for the Lysogenic cycle
(phage integration and maintenance)
- Lambda integrates
- Prophage is stably maintained
- Prophage passed to daughter cells
Which of the lifecycles is considered default?
The lytic pathway
How does integration occur (lysogeny) ?
- Occurs between two sites
1. attP (phage)
2. attB (bacteria) - Site specific recombination requiring the enzyme integrase, (rather than using general homology it uses these sites)
Is the expression pathway shared up until the decision point?
Yes, the same patterns for very early and early gene expression are shared and following the decision -> gene expression patterns differ
Describe the Very Early Gene expression events
Following injection, the host RNA polymerase transcribes from 2 promotors
- PL = produces N
- PR = produces Cro
What is Cro?
- Cro encodes the Cro protein
- Expressed early in infection and is a major player in establishing lytic growth
- Is a dominant protein
What does N encode?
- An anti-terminator protein that enable transcription past 2 terminators resulting in expression
- > enables RNA pol to ignore transcription termination signals
- Binds to nut site (N utilisation sites)
What is the effect of Cro?
Is a DNA-binding protein that REPRESSES transcription
->promotes the lytic pathway by switching transcription off at certain times
What is the effect of CII?
Is a DNA-binding protein that ACTIVATES transcription
->promotes the lysogenic cycle
What is the effect of CI?
A DNA binding protein that can ACTIVATE OR REPRESS transcription
- > CI activates it’s own expression
- > represses genes required for lytic cycle
- > maintains lysogeny
How is the decision of lytic/lysogenic pathway made?
Through relative concentrations of CII and Cro
In the ‘decision’, what is the deciding protein?
CII
What would happen for the lytic pathway to be chosen?
A healthy cell produces lots of proteases which degrade CII, once this occurs the LYTIC pathway takes place
Describe late lytic events
- Cro = represses expression of CI, all other genes, then itself and the replication genes
- Q = an anti-terminator that enables expression of late lytic genes (head, tail, lysis genes)
What would happen for the lysogenic pathway to be chosen?
Starved cells produce less proteases therefore CII remains intact and stable
Describe late lysogenic events
CII is an activator protein that turns on:
- Intergrase resulting in lambda integration into host
- CI repressor which activates its own expression and binds to OL and OR to repress all other phage genes
- >maintains prophage state
Describe maintenance of lysogeny
- Maintained as a prophage by CI
- CI is a repressor of all phage genes but is an activator of itself so keeps the phage genome ‘silent’ in bacterial chromosome until induction
- > keeps fitness cots low for E.Coli cell
- CII activates integrase and CI, which cause integration of phage and maintenance of lysogeny (respectively)
What is PRE?
- Promotor for repressor establishment
- Establishes expression of CI and ensures the phage is silent
