18. The Methyl-Directed Mismatch Repair System in E. coli bacteria is an example of the inactivation of the repair system of DNA replication and, more generally, of a constitutive mutator mechanism provoking a global increase of mutation rate. Such a mechanism is usually present in stressful environmental conditions: it is due to a mutation in the genes of repair DNA polymerases (in E. coli bacteria, the mut genes) and provokes a permanent increase of mutation rate across the entire genome. If the mutator allele is selected because of its physical proximity (link) with a beneficial mutation occurred (“hitchhiking” or “second-order selection”), its frequency can increase in the population over time: once the mutator allele becomes fixed in the population, the mutator mechanism is considered as a mutational response to environmental challenges acquired by the population concerned (what biologists call a “constitutive mutator” population). See Radman & Wagner 1986, Taddei et al. 1997.

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