The frequency of mutators in populations of Escherichia coli

Owing to occasional spontaneous mutations in genes encoding DNA repair, any population of a reasonable size is expected to harbor a sub-population of genetic mutators. Using a genetically modified strain of Escherichia coil K -12, we have estimated the frequency of mutators to be about 3 x 10(-5). By and large, this corresponds to a mutation rate from non-mutators to mutato rs of 5 x 10(-6) per bacterium per generation. Using a mutS::Tn10 derivativ e as representative for mutators, we estimated the increase in mutation rat es in mutators to be 19- to 82-fold, depending on the test-mutation under c onsideration. The load associated with this increase in mutation rate resul ted in a growth inhibition of 1%. From these data, we estimated that the ra te of detrimental mutations in the non-mutators to be 2 x 10(-4)-8 x 10(-4) . The situations where adaptive mutations may result in an increase in the frequency of mutators are discussed. (C) 2000 Elsevier Science B.V. All rig hts reserved.