SOS response

The SOS response was articulated by Evelyn Witkin. Later, by characterizing the phenotypes of mutagenised E. coli, she and post doctoral student Miroslav Radman detailed the SOS response to UV radiation in bacteria. The SOS response to DNA damage was a seminal discovery because it was the first coordinated stress response to be elucidated. ==Mechanism==

During normal growth, the SOS genes are under the repressor of the LexA protein. Under normal conditions, LexA binds to a consensus sequence (the SOS box) in the operator region of the SOS regulon genes. Some of these genes are expressed at certain levels even in the repressed state, according to the affinity of LexA for their SOS box. Activation of the SOS genes occurs after DNA damage by the accumulation of single stranded (ssDNA) regions generated at replication forks, where DNA polymerase is blocked. RecA forms a filament around these ssDNA regions in an ATP-dependent fashion, and becomes activated. The activated form of RecA interacts with the LexA repressor to facilitate the LexA repressor's self-cleavage from the operator. Once the pool of LexA decreases, repression of the SOS genes goes down according to the level of LexA affinity for the SOS boxes.. Instead, intrinsic promoter strength has been suggested to determine this order. ==Antibiotic resistance==

Research has shown that the SOS response system can lead to mutations which can lead to resistance to antibiotics. The increased rate of mutation during the SOS response is caused by three low-fidelity DNA polymerases: Pol II, Pol IV and Pol V. As well as genetic resistance the SOS response can also promote phenotypic resistance. Here, the genome is preserved whilst other non-genetic factors are altered to enable the bacteria to survive. The SOS dependent tisB-istR toxin-antitoxin system has, for example, been linked to DNA damage-dependent persister cell induction. ==Genotoxicity testing==

In Escherichia coli, different classes of DNA-damaging agents can initiate the SOS response, as described above. Taking advantage of an operon fusion placing the lac operon (responsible for producing beta-galactosidase, a protein which degrades lactose) under the control of an SOS-related protein, a simple colorimetric assay for genotoxicity is possible. A lactose analog is added to the bacteria, which is then degraded by beta-galactosidase, thereby producing a colored compound which can be measured quantitatively through spectrophotometry. The degree of color development is an indirect measure of the beta-galactosidase produced, which itself is directly related to the amount of DNA damage. The E. coli are further modified in order to have a number of mutations including a uvrA mutation which renders the strain deficient in excision repair, increasing the response to certain DNA-damaging agents, as well as an rfa mutation, which renders the bacteria lipopolysaccharide-deficient, allowing better diffusion of certain chemicals into the cell in order to induce the SOS response. Commercial kits which measures the primary response of the E. coli cell to genetic damage are available and may be highly correlated with the Ames Test for certain materials. ==Cyanobacteria==

Cyanobacteria, the only prokaryotes capable of oxygen evolving photosynthesis, are major producers of the Earth's oxygenic atmosphere. The marine cyanobacteria Prochlorococcus and Synechococcus appear to have an E. coli like SOS system for repair of DNA, since they encode genes homologous to key E. coli SOS genes such as lexA and sulA. ==Additional images==

File:Filamentation 1.jpg|The SOS response inhibits septum formation until bacterial DNA can be repaired and is observable as filamentation when cells are examined by microscopy (top right of image). ==See also==