SOS response

The SOS response is a technical term in biology for a reaction prokaryotic cells to severe DNA damage and one of the repair systems of the cell. Such damage is caused for example by ionizing radiation, heat or DNA -altering chemical substances.

Heavily damaged DNA can not replicate properly and leads to the following cell division to death of both daughter cells. The SOS response is a program that allows the affected cell to first repair the damaged DNA, and only thereafter to continue the cell cycle.

First, by single-stranded DNA ( the case of larger DNA damage almost always occurs ), the RecA protein is activated. RecA ie "measures " the state of the constant DNA in the cell. The activated RecA cleaves his turn the LexA repressor. LexA binds in the intact condition to specific regions of DNA and blocks transcription of the SOS - specific genes. With the cleavage of the LexA transcription of these genes is released and starts the DNA repair. The activated genes include, inter alia, sfiA, which inhibits cell division and thus the cell is time for repair. Furthermore, so-called uvr genes are transcribed coding as the ABC exonuclease for DNA repair proteins. In addition, the LexA repressor regulates its own transcription and that of RecA, which means that as soon as the damage of the DNA is removed and the concentration of RecA activated decreases LexA again accumulates in the cell and the system returns to its stable output condition.

The SOS response is a common bacteria -specific response to greater DNA damage. In contrast to " normal" DNA repair processes, the repair is effected at the SOS response only inaccurate, that is, with a significantly increased error rate, thereby resulting in a number of mutations. Ionizing radiation and short wavelength UV radiation are therefore also used for randomized mutagenesis.

If the lysogenic bacterium, thus leading an SOS response in the propagation and lysis of an integrated in the bacterial genome virus. For example, the bacteria containing the DNA genome of the bacteriophage lambda. The phage lambda cI protein 's repressor prevents the multiplication and lysis of the phage. Runs in a bacterium SOS response from, the RecA protein not only cleaves its substrate LexA but also cI and forwards as the lytic pathway of phage lambda a. The phage can then leave the weakened host cell and does not have to go with her to reason.

Eukaryotes have a similar system, in which case the protein p53 plays a crucial role. Is in a eukaryotic cell the DNA damage repair to a large, apoptosis is initiated.