Bacteriophage Flashcards
capsid
- the coat around the virus
- some viruses also have a lipid coat called an envelope
viral genome can be
ds or s RNA or DNA
lytic cycle
- infection
- genome insertion
- trascription and translation of viral genome
- construction of viruses
- cell lysis
stages of viral infection
- adsorption
- penetration
- gene expression
- nucleic acid replication
- synth of structural proteins
- assembly of virions
where do phages adsorb
bacterial cell surface
early and alte mRNA
- early is transcribed 1-12 minutes after transcription
- late is transcribed 8-40 minutes after infection
- early mRNA transcription ends due to dactor dependent and factor independent remination
at 12 mins after infection
-only late mRNA are transcribed, not early mRNA’s
why arent the late mRNAs transcribed earlier
- the early mRNAs code for an RNa polymerase specific for the late DNA region
- this is termed a phage-encoded RNA polymerase, specific for T7 late promoters
phage T7 RNA pol
-this is a very fast polymerase encoded for by the bacteriophage
types of penetration
- injection
- membrane fusion
Gp2 gene product
this is the late gene product of T7
-it binds to the bacterial RNApol and inhibits transcription therefore only viral genes are being made by Gp1
Gp1
- early phage gene product
- encodes a RNA pol that can bind to and initiate trsncription of the T7 late promoters
temparate phages
-can choose between the lytic or lysogenic cycle
-
bacteriophage lambda
-temperate phage
lytic growht of the lambda virus
- the N transcript is expressed
- the N product is an anti terminator that allows for the expression of the middle genes
- Q is then expressed which is an anti terminator allowing the late genes to be expressed
- C1 is the only gene not transcribed in the lytic cycle ; C1 is the pressor of the lytic cycle
lysogeny
-this process occurs when the bacterial cell is growing slowly
1. to choose this mechanism the virus needs to synthesize a high amount of C1 repressor and synthesize the Int protein, which is a DNA recombination protein for instertion into the genome
2.C2 is what modulates the response to the host growth rate, it is a substrate for a host protease
3. transcription of C1 depends on the presence of C2
C2 binds to Pre
4. C1 is transcribed from Pre and Prm
5. upon lack of nutrients, C2 is not degraded by protease and builds up which activates the positive regulator leading to C1 establishment which binds to Pl and Pr, shutting off all early and middle gene expression, preventing the lytic cycle
Induction
- this is how the lambda genome gets out of the host genome and back into the lytic cycle
- this is done via the SOS response elicited by DNA damage
- During DNA dmage, RecA is activated which causes autoproteolysis of C1, a repressor of the lytic cycle
SOS Response in bacteria
- SOS genes are not typically expressed because they are repressed by the LexA protein
- when DNA damage occurs, some of the fragmented DNA comes into contact with the RecA protein, which becomes activated
- once activated, RecA inititates the autoproteolysis of LexA, allowing the SOS genes to be transcribed
- this allows for the transcription of Xis and Int via the Pl and Pr promoters which excises the phage genome out of the host choromosome as a cirlce that is identical to the original phage DNA molecule
specialized transduction
- when the phage undergoes induction and is excised from the host chromosome, mistakes can be made which results in picking up a bacterial gene which had been flanking the phage genes
- this phage genome with the bacterial gene attached can packaged into a new phage and this phage could infect a different bacterium, giving it a new gene which may confer advantageous characteristics
-lysogenic conversion
- a bacteria picking up a new trick due to specialized transduction
- can convert nonpathogenic bacteria to pathogenic bacteria
why should you not give your patients bacterial DNa damaging antibiotics if they have an infection with a phage encoded toxin?
- the DNA damage will trigger the SOS response
- this causes the virus to be excised from the bacterial chromosome and go into the lytic cycle and produce its toxin
- these are shigga-like toxins
