Lytic Phage Life Cycle Flashcards
What are the 7 steps of the lytic life cycle?
- adsorption
- infection
- early gene expression
- viral genome replication
- late gene expression
- assembly of new phages
- lysis
What is adsorption?
The phage binding to its host cell
What happens during the adsorption step of the lytic life cycle?
The phage receptor protein recognizes and binds to a particular molecule on the surface of the host cell
What host cell surface molecule will be recognized by the phage?
Depends on the phage. Can be LPS sugars, transport proteins, pili
What happens during the infection step of the lytic life cycle?
The phage genome crosses the membrane and enters the host cell while the protein coat stays outside
How does T4 use its tail to get its DNA inside a cell?
Once bound to the host, the tail fibres pull up and retract, and the phage “sits down” on the outer membrane. The tail pins tightly bind the membrane, and T4 lysozyme in the phage neck chews a hole through the PG. The tail sheath then retracts and it injects its DNA through the hole it made into the cell
How do phages without tails get their genome into a cell?
Hijack membrane proteins
What happens during the early gene expression step of the lytic life cycle?
Genes with promoters that look like E coli promotors will trick E coli RNA poly into transcribing them and will transcribe and translate proteins involved in initiating the replication process
What are the 4 things expressed during early gene expression?
- Proteins that repress E coli gene function
- Enzyme that cuts up the E coli genome
- Any enzymes needed for replication
- Transcription factors for the late genes
What happens during the replication step of the lytic life cycle?
The virus uses host proteins and a few of its own to replicate its own DNA about 100 times
What happens during the late gene expression step of the lytic life cycle?
Genes with viral promotors are expressed to make the coat proteins and lysis enzymes
What needs to have been expressed for late genes to be expressed?
Transcription factors and viral RNA polymerase that were expressed by the early genes
What are the 4 things expressed by the late genes?
- Coat proteins
- Lysis proteins
- Early gene repressors
- Packaging proteins
Why do the late genes express early gene repressors?
Shifts all energy and resources in the cell towards the late genes
What happens during the morphogenesis step of the lytic life cycle?
The new phage particles get assembled
How does T4 build its capsid?
Builds a scaffold and assembles the capsid on there. Motor proteins then use host ATP to shove the DNA inside. Once inside, the scaffold and motor are removed
How does T4 build its tail?
It spontaneously assembles through protein interactions
What happens during the lysis step of the lytic life cycle?
Phage enzymes become activated and disrupt both the cell membrane and cell wall to cause the host cell to lyse and release the new phage particles
What are the 2 lysis enzymes used by T4? What do they do?
Holin: pokes holes in the membrane
Lysozyme: destroys all the PG
What promotors do the early genes use?
Host promotors
What promotors do the late genes use?
Viral promotors
How is phage gene expression controlled by a regulatory cascade?
Early genes are expressed first, which activate expression of the late genes. The late genes then repress activation of the early genes to put all energy into expressing the late genes
What is the burst size?
Average number of phage released per infected bacterial cell
What determines the burst size of a phage?
Very specific to the host-virus interaction
