L6 Regulation of Bacteriophage lambda Flashcards
Lambda life cycle
- 2 states (lytic or lysogenic), 3 potential pathways (lytic, lysogenic, induction)
Lytic cycle
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Lysogenic cycle
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Lambda genome
- clustered by function
- most genes for lytic cycle
- not all genes expressed simultaneously, there’s a cycle to it
- has multiple promoters, PL and PR are the major promoters
Lytic cycle - cascade of gene expression
- early, middle, late genes
- transition from one phase to the next uses different mechanisms
Early genes
- transcribed by host RNA pol
- transcribes N (anti-terminator) and cro protein (DNA binding transcription regulator) from PL and PR promoters
Delayed early genes
N protein allows transcripts to extent into delayed early genes including anti-terminator Q
Late genes
Q protein allows it to extent into late genes, transcription initiates at PR’ promoter
Late genes expressed from a different promoter, not PL and PR. Expressed by PR’.
PR’ is a constitutive promoter so that RNA pol can bind and initiate transcription without an activator BUT the transcripts are terminated almost immediately in the absence of Q
Once Q gene has been expressed, Q becomes associated with RNA pol and the late genes can be completed transcribed
PL
- strong constitutive promoter, no activator needed
- allows for transcription of N protein
- has tL terminator
PR
- strong constitutive promoter, no activator needed
- allows for transcription of cro protein
- has tR1 terminator
First early transcript
- will be terminated by tL and tR1
- transcript will only contain gene for N and cro protein
How does anti-termination work?
Anti terminator binds to RNA pol so that as it progresses through, it no longer recognises the termination sequence and keeps on progressing
- anti termination proteins act on specific terminators
Cro repressor protein
represses expression of C1 repressor to allow expression of lytic genes
- acts negatively at PRM repressing C1 expression
- binds preferentially to OR3
N protein
- Specific for transcripts that contain a nut sequence
- Nut sequence allows for association of factors to act on RNA pol => rho cannot terminate transcription
- For N protein, doesn’t matter where nut sequence is as long as it occurs before the termination sequence
- N protein binds to mRNA that is transcribed from nut sequence
PR’ promoter
- constitutive
- upstream of late genes
- required for synthesis of head and tail proteins and lysis
Q protein
- interacts directly with RNA pol holoenzyme
- binds to DNA and influences RNA pol as it initiates transcription
- reduces pausing at termination sites
Nut sequence
- contains BoxA and Box B
- Nut sequence allows for association of factors to act on RNA pol => rho cannot terminate transcription
C1 repressor
Lambda C1 repressor binds to PL and PR
C1 repressor critical to ongoing association in a lysogenic stage - actively preventing the lytic cycle
- maintains its own expression
- binds preferentially to OR1 and OR2 (less affinity for OR2), represses PR including cro (negative)
- activates PRM (positive)
PRM promoter
repressor maintenance promoter - transcribes the C1 repressor gene
- weak promoter, requires activator
OR and OL
- right (and left) operator
- consists of 3 operator sites
- each operator has dyad symmetry, not identical
- two different regulatory proteins can bind these operator sites: C1 repressor (repressing lytic cycle) and cro protein (promoting lytic cycle)
OR1 operator overlaps -10 and -35 region of PR promoter
OR3 operator overlaps -10 and -35 region of PRM promoter
C1 and cro
- bind to PL and PR as dimers
- binds at operators
- different affinities for O1, O2 and O3
SEE ONENOTE
Lambda repressor domains
SEE ONENOTE
- has two domains
- forms dimers which can interact to form a tetramer
- some C1 mutations prevent positive regulation without affecting negative regulation
- C1 dimers bind to operator site and can bind cooperatively
- C1 can also bind to OL1, OL2 and can complex with C1 at OR1 and OR2 (and OR3)
