Lecture 17 - The SOS repsonse In E.coli Flashcards
What does an organism need to do after DNA damage?
Efficiently and swiftly repair it so the cell cycle can continue. The cell cycle will pause so the damage is not given to the next generation
How can the stress response be an issue?
Can sometimes lead to errors and overproduction if repair proteins can be a problem for the cells as you are using up all the source and if it’s not needed it could cause harm
The SOS response - what is a regulon?
Group of genes/operons that are under the control of the same regulator (LexA)
Is the sos response specific?
Yes
Do all regullon have different functions and can move throughout the cell?
Yes but they are all involved in cell survival
Where is the sos regulator found in the chromosome?
90 minutes
Are all regulons spread across the cell?
Yes
If there is no DNA damage where does the LexA bind?
Specific regions in the promotor of the SOS genes this is a LexA box or a SOS box
Is LexA a repressor?
Yes
What does the LexA gene do when bound to the SOS promotor?
It impedes transcription
Where is the LexA box near?
The RNA polymerase binding site
What happens when a LexA dimer binds to the LexA box which is in an RNA polymerase binding site?
It interferes with RNA polymerase repressing the expression of the sos genes
When the sos response is not needed are the sos genes expressed?
No
Do most genes have a basal expression level and what does this mean?
even when turned off they will still excrete a minimum expression
What genes would be expressed first from a Lexa box?
If a lexA box is bound to a sequence of DNA that is most different to its consensus sequence of DNA as this will be a weaker binding.
Are genes which are expressed later stronger in a LexA box?
Yes as they bind more tightly to the consensus strand as there is more sequences in common
What is also important in the timing of expression of genes and also the strength of the genes?
The location and their place in the promotor
The SOS system - what happens in a double strand break?
RECBCD processes DNA and loads RECA recombinase onto the strand to make a recA ss DNA filament.
The active RECA filament can then act of Lexa
How does the active RECA filament act on Lexa?
Acts as a co-protease and induces self-cleavage of cytoplasmic LexA.
What happens to LexA once it is cleaved?
Cytoplasmic LexA is further degraded by cytoplasmic proteases and the concentration of LexA decreases meaning less LexA can bind to the LexA boxes the most different from the consensus sequence
What does the absence of the LexA boxes facilitate?
Allows the RNA polymerase into the promotor and the transcription of the genes increase.
What does the level of SOS response activate?
The transcription of genes expressed in early SOS response
What would happen if the damage was not repaired and RECA was still present?
The concentration of cytoplasmic LexA decreases further leading to the expression of more genes. The longer the cell experiences DNA damage the more SOS genes are transcribed until the expression of later SOS genes
Why are feedback loops used?
They regulate and can amplify the response.
What is a positive feedback loops role?
To amplify if the problem isn’t solved
What is the negative feedback loops response?
To stop the response if the problem is solved
How does the SOS response create feedback loops?
The direct influence of LexA and recA
What further regulates the feedback loop?
Other proteins as some stabilise RecA ssDna and some destabilise it.
What does nucleotide excision repair do?
Faithfully fixes nucleotides onto broken DNA strands
What do all the genes involved in the SOS response do?
homologous repair
Nucleotide excision repair
Polymerase
Other action on DNA
Cell division
SOS regulators
Toxic
How does translation synthesis work when repairing DNA?
They use specific polymerase to replicate through DNA lesions this can be either faithful or not
What is the general timeline of SOS response? early response
During early response the integrity of the DNA is trying to be kept and things such as polymerase high-fidelity, homologous recombination and nucleotide excision repair takes place. This needs to be fast and faithful
What is the general timeline of SOS response? What happens if the response has repaired the DNA?
More LexA is produced to stop the response if damage is repaired. However if its not the late response takes place
What is the general timeline of SOS response? The late response?
The cell cycle needs to be stopped at this point and so the cell uses toxic genes which impair cell division, cell growth and DNA replication. The SOS response ten starts sending out error prone polymerase high-fidelity to try and repair the cell through translation synthesis - this is slow and can produce mutations
What are the consequences of the SOS response?
Homologous recombination = genome rearragement
Increase mutation rate
Stop cell cycle
What induces the SOS response?
Horizontal gene transfer
Why could slowing down cell cycle be good?
Antimicrobials target fast growing cells
What do Antimicrobials do which would start the SOS response?
They damage DNA
