What are the two results of getting infected with phage lambda?
1) lytic growth cycle: cell infected and phages grow, killing the cell upon release
2) lysogenic growth cycle: the phage DNA enters a prophage state
Structure of the virion
- head is a naked icosahedral capsid (T=7)
- 63 nm diameter
- flexible, tube shaped tail
- terminal fibers
GEnome of lambda
- genome codes of approx. 60 proteins
- transcribed by cellular RNA polymerase
- genes are grouped by function
- there is a sequential expression of genes (immediate early, early, and late)
- transcription is controled by
- CI, cro--repressors
- CII-- activators
Phages that can either grow lytically or exist as a repressed prophage are called __.
What is the genome of phage lambda?
- it looks like T7
- head and tail configuration
- long tube-shaped tail
- flexible with fibers that recognize bacterial cells that attach to and infect
What do phage lambdas encode for?
a large number of proteins, a lot of them structural
There are a couple of genes that are crucial
Transcription is controlled by repressors and activators
Genes are arranged in a pretty ordered fashion and all that are responsible for a certain function are near each other
Explain infection of lamda.
- 1) binding to cell surface
- 2) injection of DNA
- 3) circularization of DNA, sealed by ligase
- 4) transcription by host RNA polymerase that begins at PL or PR, which express the N and cro genes, which allow the lytic cycle to proceed
Why would a virus evolve to have two types of replication cycles?
- depends on how active hte bacteria is
- The temperate phage enters, circularizes, and goes into the lytic or lysogenic cycle
What determines the cycle?
- How metabolically active the infected cell is
- How quickly the cell is growing
- How many nutrients are around
- If the virus infects a quickly growing cell with a lot of metabolism, the virus will take advantage of it and be lytic
- If the virus infects a slow growing bacteria, the virus becomes lysogenic until the resources are once again available
What is early gene expression?
host RNA pol transcribes competing genes, some of which favor lytic and some of which favor lysogenic cycles
- DNA is injected through the tail and into the cell using mannose permease
- DNA circularizes using cos sequences
- Host RNA polymerase begins transcription at promoters PL and PR
What is the fork in the road?
WHat happens is yo have genes that are turned on by cro (which allows the virus to enter the lytic cycle) or active cI, which allows lysogeny to occur
The way viruses sense this is by using the promoter, cII, which tells cI to turn on and go quiet
It is not very stable. The more active the cell is, the more quickly the cell degrades cII. Levels of cII may be low in a growing bacterium, which allows the lytic cycle to be carried out.
CII promotes cI production and is degraded by active cells
When you have cI, what has to be?
- there has to be another regulator
- What cI actually is a repressor that blocks expression of the lytic program by regulating the PL, PR, and PRM promoters. It also recruits RNA polymerases to the cI gene to continue increasing the level of cI made.
This occurs in stationary phase cells
How does one enter the lytic cycle?
To turn off cI is to stress the bacterial cell with, for example, host DNA damage, UV light, heat shock, etc.
This makes it turn on different genes and activate the stress genes and activates RecA, which cleaves cI, unblocking transcription of PL and PR and allows the lytic cycle to be activated
How does the repressor bind to CI
- as a dimer
- it blocks transcription from PR
- activates transcription from PRM, producing more CI transcript
- Phage DNA then integrates
High concentration of CI...
- results in PRM repression
- All operators are bound
- CI dimerizes and octamerizes to form a DNA loop
- used for repressor maintenance
- located near PR but initiates transcription in the opposite direction, directing transcription of cI mRNA
only effective if CI tetramers are formed. They then come together to form octamers, which causes a DNA loop that brings Or3 and OI3 together, repressing PRM
binds to the same operators as CI. It first binds to OR3 and blocks PRM, preventing CI expression and allowing lysis to occur