20. The “SOS system”, discovered by Radman (1973) in E. coli bacteria, consists of more than twenty genes whose expression is induced by the presence of DNA breakages or strong metabolic stresses (e.g., starvation). These genes code for inhibitor and activator protein factors (e.g., LexA and RecA in E. coli) and for special kinds of “error-prone lesion by pass” DNA-polymerases: some kinds of DNA-polymerases (e.g., Pol V in E. coli) can perform the so-called translesion-synthesis (TLS), i.e., can bypass the replication blocks caused by lesions, but have a reduced copying fidelity, so they produce localized mutations in the damaged regions; other kinds of DNA-polymerases (e.g., Pol IV in E. coli) can also act on undamaged DNA regions and so provoke genome-wide mutations. The result is a global and/or local increase of the mutation rate. Once the stress stimulus wears off, there is no more induction of the SOS genes and the mutation rate decreases to its normal average value. See Taddei 1995, Taddei et al. 1997, Tenaillon et al. 2004.

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