Lecure 17: SOS Response In E.coli Flashcards
What is the SOS response system?
In e.coli, SOS response system is the regulatory system that controls cells response to DNA damage
What is a regulon?
A group of genes/openings that are under the control of the same regulator (LexA)
> 30 genes in SOS regulator, and have different positions on the chromosome and different functions
What are the different roles of SOS genes in e.coli?
Homologous recombination
Nucleotide excision repair
Polymerase
Other action on DNA
Cell division
SAS regulator
Toxic
What happens to LexA when there is no DNA damage?
No DNA damage in cell, LexA protein binds a specific consensus sequence in the limiter of SAS genes = LexA box/SOSbox
As LexA is transcriptional repressor, binding to the promoter impedes transcription
Where is LexA box located?
In promoter near or inside RNA polymerase binding sites
How does LexA repress expression of SOS genes?
RNA polymerase recognises -35 and -10 boxes. LexA box overlaps with -10 box.
When LexA dimer binds to LexA box, it interferes with the action of RNA polymerase and represses expression of SOS genes
How is SOS response initiated?
- E.coli experiences inside or outside stress that damages its DNA
- double strand break is formed
- RecBCD processes DNA ends and loads RecA to form filament
- active RecA filament acts of LexA as co-protease and induces self-cleavage of cytoplasmic LexA
- once cleaved, cytoplasmic LexA is further degraded by cytoplasmic proteases
- less concentration of LexA to bind to LexA box, so cannot bind boxes with most different sequence
- absence of LexA to boxes facilitates access of RNA polymerase to promoter
- longer LexA is degraded, more SOS genes are transcribed
- gene transcription increases
What is LexA?
Transcriptional repressor that controls expression of SOS regulon
What is RecA?
The protein that when bound to single strand DNA, permits repression of system and expression of SOS regulon
What is positive feedback loop?
Amplify appropriate response when problem is not fixed
What is a negative feedback loop?
Stop the response when problem is solved
How does SOS response create feedback loops?
By directly controlling the expression of RecA and LexA
What happens if there is production of more RecA in presence of damaged DNA?
What happens if there is production of more RecA in absence of damaged DNA?
What happens if there is production of more LexA in presence of damaged DNA?
