L7 Lambda C1 C2 C3 Int vs Xis Flashcards
Early and delayed early genes
required for both lytic and lysogenic cycle
Delayed early gene products
c2/c3 regulators - required for establishment of lysogenic cycle
7 recom. genes
2 replication genes
Q protein
Late gene products
10 head genes
11 tail genes
2 lysis genes
C1 binding
c1 binding at OR1 and Or2 represses genes from PL and PR - including cro
C1 acts as the repressor of the cro gene by binding to OR1 and OR2, cooperative, initial binding of OR1 brings a second complex into OR2. Activates its own expression at OR3.
Stable C1 binding
tetramerization between dimers of OR1, OR2, OL1, OL2 leads to very stable association => maintains lysogenic cycle
Induction of lambda lysogen
- cleavage of C1
- Occasional spontaneous conversion into the lytic cycle just because it is in a biological environment.
- UV irradiation: RecA mediated cleavage of C1 repressor protein => destruction of C1 allows cro and N protein to be expressed
Summary of competition between C1 and cro
OneNote Summary
Paradox of C1 expression
See OneNote diagram
PRM expression requires C1 for activation so…PRE (repressor establishment)
PRE: gets the process started, not involved in maintenance
PRE
- weak promoter, requires C2 protein
- stabilised by C3
- C2 and C2 required for establishing lysogeny but not its maintenance
C3 produced from PL promoter
C2 and C3 able to drive expression from PRE
Sigma70 does not recognise PRE, PRE is entirely dependent on C2 protein
C2 protein binds around -35 region, RNA pol binds at -10&-35 region
Integration
requires Int protein
recombination at attB and attP site
Excision
requires both Int and Xis protein
BUT needs more Xis than Int
Lysogeny requirements
- need more Int than Xis
- transcripts from P1 express Int not Xis
- not nut site, not anti-teriminated by N and DO terminate at T1
Expression of int and xis is related to the PI promoter, PI sits in the middle of the xis gene, so when it is expressed it only expresses the int gene
PI requires C2 protein to activate expression and C3 too since C2 and C3 work together
C2 and C3 responsible for pushing the process into the lysogenic state
Lysis requirements
- need more Xis than Int (prevent integration)
- PL not repressed by C1
- transcripts from PL subject to anti-termination, do not terminate at T1 => sib sequence picked up in extended transcript
Sib site
- Contains site for RNAse3 degradation
RNASE3 capable of bringing about 3’ to 5’ degradation (retro-regulation)
It will chew back the int gene before it gets to the xis gene, less Int expressed than Xis, this is what you want in the lytic cycle
Induction Requirements
- excise prophage from e.coli genome
- need Int = Xis
- PL and PR derepressed with destruction of C1
- both Int and Xis expressed from PL with anti-termination by N
- no retro-regulation
