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SOS chromotest
The SOS chromotest is a biological assay to assess the genotoxic potential of chemical compounds. The test is a colorimetric assay which measures the expression of genes induced by genotoxic agents in ''Escherichia coli'', by means of a fusion with the structural gene for β-galactosidase. The test is performed over a few hours in columns of a 96-well microplate with increasing concentrations of test samples. This test was developed as a practical complement or alternative to the traditional Ames test assay for genotoxicity, which involves growing bacteria on agar plates and comparing natural mutation rates to mutation rates of bacteria exposed to potentially mutagenic compounds or samples. The SOS chromotest is comparable in accuracy and sensitivity to established methods such as the Ames test and is a useful tool to screen genotoxic compounds, which could prove carcinogenic in humans, in order to single out chemicals for further in-depth analysis.
As with other bacterial gentoxicity and mutagenicity assays, compounds requiring metabolic activation for activity can be investigated with the addition of S9 microsomal rat liver extract.
==Mechanism==
The SOS response plays a central role in the response of ''E. coli'' to genotoxic compounds because it responds to a wide array of chemical agents. Triggering of this system can and has been used as an early sign of DNA damage. Two genes play a key role in the SOS response: lexA encodes a repressor for all the genes in the system, and recA encodes a protein able to cleave the LexA repressor upon activation by an SOS inducing signal (caused in this case by the presence of a genotoxic compound). Although the exact mechanism of the SOS response is still unknown, it is induced when DNA lesions perturb or stop DNA replication. .
Various end-points are possible indicators of the triggering of the SOS system; activation of the RecA protein, cleavage of the LexA repressor, expression of any of the SOS genes, etc. One of the simplest assays consists of monitoring the expression of an SOS gene by means of fusion with lacZ, the structural gene for ''E. coli'' β-galactosidase.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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