SOS repair is one of the DNA repair mechanisms, which commonly refers to as “bypass” or “Emergency” repair. This system was first introduced in the year 1975, by a scientist named Miroslav Radman. It is a regulatory system which comprises of many complex inducer proteins that helps to repair the damaged DNA, caused due to environmental stress. SOS system also includes a repressor protein, namely LexA. The RecA protein floats around the cell, which regulates the activity of LexA protein. The RecA regulatory protein can mediate the repression or expression of LexA repressor.
SOS repair system is an error-prone mechanism, as it works without a DNA template. An organism initiates the production of activator protein (RecA), which results in the dissociation of LexA repressor and activates the SOS inducer proteins, is a process that refers as “SOS response system”.
Content: SOS Repair
Definition of SOS Repair
SOS repair can define as the DNA repair system, which makes the use of RecA regulatory protein, to inhibit the repressor activity and to activate the SOS inducer genes to treat the damage and stands for “Save Our Soul”. The SOS system remains repressed until the conversion of RecA protein into RecA protease. It does not repair the DNA damage completely and only provides tolerance to the organism affected.
In normal DNA, a bacterial cell does not need DNA repair genes to be activated. Thus there should be some controller that must control the expression of such genes. LexA acts as a repressor protein, that binds to the particular site of DNA refers as SOS box. The binding will repress the activity of SOS genes.
But in case of mutated DNA, the LexA repressor must be inactivated to induce the expression of SOS genes. In an SOS system, RecA acts as an activator of SOS genes, that cause proteolysis of the repressor protein and allow the expression of SOS genes into different DNA repairing inducer proteins.
An SOS system composed of the following components:
Regulator protein: It is encoded by “RecA” gene whose function is to activate the repressed SOS system, by inhibiting the binding of LexA to the SOS operator.
Repressor protein: It is encoded by “LexA” gene which causes inactivation of inducer proteins and binds to the operator to cause repression of the SOS system.
Inducer proteins: These are encoded by SOS-box genes that can activate the inducer proteins relative to the type of DNA damage.
|SOS proteins||Encoded by||Functions|
|Regulatory protein||RecA gene||It functions as regulator of SOS system|
|Repressor protein||LexA gene||It functions as inhibitor of SOS system|
|Inducer proteins||uvrA gene||Repairs short patch nucleotide damage, cross-links and long patch nucleotide damage|
|uvrD gene||Repairs cross-links, Me-directed mismatches, double stranded gaps and short patch nucleotide damage|
|umuC gene||It functions to bypass the lesion site of DNA and leads to mutagenesis|
|umuD gene||It functions to bypass the lesion site of DNA and leads to mutagenesis|
|ruv gene||Repairs recombinant DNA damage|
|recN gene||Repairs recombinant DNA damage and gaps between double stranded DNA|
|recQ gene||Repairs recombinant DNA damage|
|dinA gene||It functions to bypass the lesion site of DNA and leads to alter the normal sequence of DNA|
|sulA gene||It functions as an inhibitor of cell division|
Mechanism of SOS Repair
The mechanism of SOS repair is a complex cellular process mediated by the organism itself and includes the following steps:
- In case of excessive DNA damage, stress conditions etc. a cell produces a response by activating signal or RecA protein. It floats in the vicinity of the cell in search of any damage in the DNA.
- A RecA protein will specifically bind to the single stranded DNA. On binding with the single stranded DNA fragments, RecA will form a filament-like structure around the DNA.
- Then, a LexA repressor comes in contact with the nucleoprotein filament assembled by RecA protein. When RecA interacts with the repressor protein, it converts into RecA protease.
- The formation of RecA protease will cause autocatalytic proteolysis of LexA repressor protein. Thus, a LexA protein cannot further bind to the SOS operator.
- As the activity of LexA protein inactivates, the inducer proteins will activate to repair the DNA damage but alter the DNA sequence.
- When a DNA get repaired, a RecA protein will lose its efficiency for proteolysis and the LexA protein will again bind to the SOS operator or switch off the SOS system.
An SOS system always switches off when a DNA is healthy. The LexA repressor protein produced by the LexA promoter. LexA protein will attach to the consensus sequence having 20 base pairs of the SOS-box and inactivates the SOS system. Thus, LexA blocks the SOS box, which in turn arrests the activity of SOS genes that participate in the recovery of damaged DNA.
SOS repair system comes into action when the DNA is not in a normal condition and when all the repair system becomes failed. This system activates by an organism itself in response to the damage against UV-light or any other factors.
SOS system only activates when there is excessive DNA damage, leading to single-strand breakage at the replication fork. This DNA damage will activate the RecA regulatory protein, which will only attach to the single stranded DNA by the help of cellular energy ATP. The attachment of RecA protein and ssDNA will give rise to a right-handed nucleoprotein complex refers to as “RecA + ssDNA filament”.
The interaction of LexA repressor with the nucleoprotein complex will cause proteolytic cleavage of LexA dimer. Proteolytic cleavage is due to the conversion of RecA protein into protease that will suppress the activity of LexA protein. The SOS box genes will now express into different inducer proteins to recover the damaged DNA. The expression of inducer proteins will not occur on all at once but express relatively to the type of DNA damage.
Therefore, an SOS system switches on and off in the presence and absence of activator RecA protein, respectively.