Bacteriophage Lambda Flashcards
What are the 3 phage classifications:
- Filamentous phage (simple) - a piece of nucleic acid wrapped in protein sub-units -> less limitation
- Isocahedral head with/without tail
- Nucleic acid can be single-stranded, double-stranded/ linear
In which state is replication reduced in a bacteriophage?
Lysogeny
What is the difference between a bacteriophage which is virulent and one that is temperate?
Virulent - only lytic growth
Temperate- have choice of either
Prophage
Phage DNA in lysogen
Immunity
Bacteria can’t be infected by other phages because single integrated prophage protected by repressor -> replication can’t occur since PL and PR switched off
Induction
Prophage is freed from lysogeny by excision
Lysogeny favours
- low levels of nutrients
- high multiplicity of infection (more virus than bacteria)
- > if clear plaque more virus
Bacterial growth occurs
once lysogeny is established
Early infection
Entry of DNA to replication
- relies on transcription apparatus of host cell -> only a few genes expressed and promotors are same as host
- accumulate phage genome
Late infection
replication to release of prophage
- synthesis of protein components -> head/ tail
- assembly proteins
When structural components assemble into heads/ tail
Host cells lysed to release new viral particles
Lytic cycle is under ____ control
positive
Expression starts when ____ protein is coded by ____ genes
Regulator, early
Does the bacteriophage continue to express the initial set of genes?
Depends on control circuit:
1. If transcription initiation: early genes can be switched off when middle genes transcribed since ind. transcription units + mRNA
2. If transcription termination:
early genes continued to be expressed since transcription units must be adjacent for control at termination. -> single mRNA for both sets of genes + phage functions mainly controlled by 2 sets of early genes.
Late genes
Expressed when phage DNA replicated -> embed additional regulator gene in previous set of genes e.g. anti-termination factor
A sigma event
redirects specificity of host RNA polymerase
An antitermination event
Allows host RNA polymerase to read through termination sites between adjacent early genes + join adjacent sequences ( early gene seq. 3’ + new at 5’)
Example of an antitermination event in lambda
pN (coded by gene N) allows RNAp to read through terminators located at ends of immediate early genes -> pQ later acts with pN to transcribe late genes
How are phage promoters recognised?
Switching. Sigma factor from host is replaced with another factor that redirects its specificity in initiation so a new phage RNAp is synthesised which has different host promoters than host RNAp.
Lambda Characteristics
Isocahedral head (DNA linear) with tail but is circular in bacteria.
- double- stranded
- 12bp cos ends
- 46 genes, 48,514 bp
- cascade system so not all genes expressed at same time -> works b/c circular
Lambda has 2 immediate early genes:
N and cro - transcribed by host RNAp
Delayed early genes:
3 of products are regulators including cii and ciii
Lytic cycle requires which genes?
early gene cro and delayed early gene Q
Late genes form a
single transcription unit
Lambda repressor protein is coded by
cI -> transcribed from PRE
Why are cii and ciii regulatory?
ci is not immediately recognised by RNAp so cii needed for conformational change so RNAp can recognise weak promotor + cause anti-sense transcription of cro (repressor lysogeny)
ciii binds to cii which prevents degradation by HFL-A proteases present in bacterial cells
Functions of lambda repressor
Acts at OL and OR to block transcription of immediate early genes -> turns off PL and PR + turns on PRM
-> immediate early genes trigger a regulatory cascade which prevents lytic cycle
PI turns on….
additional expression of integrase to integrate lambda to bacterial DNA
anti-PQ
anti-sense hybridisation to decrease expression of Q transcript -> decrease lysis
How does integration occur ?
Lambda and bacteria integrate with complimentary attachment sites: attP + attB (POP + BOB) where O is the same sequence
- > intersome for site-specific recombination is coded by integrase + IHF
- > once integrated -> lambda POB + BOP joined
Excision uses
xis ( recognises att sites) and integrase
