Zeocin resistance suppresses mutation in hypermutable Escherichia coli

The ability to introduce random mutations into genes cloned in bacterial vectors is particularly useful for bioengineering and molecular biological studies. The increasing importance of hypermutable bacteria for these purposes is emphasized by the commercial availability of the E. coli strain XL1-Red (Stratagene), which containsmutS, mutT andmutDmutations. Defects inmutS confer a hypermutable phenotype because this gene is part of the methyl directed mismatch repair (MMR) system, a post-replicative DNA repair pathway that identifies and corrects mismatched DNA duplexes (LeClerc et al., 1996). MutT mutator strains are unable to hydrolyse 8-oxodGTP and display an increase in transversions (Fowlera & Schaaper, 1997). Defects in mutD also contribute to the hypermutability of strain XL1-Red because the frequency of transitions and transversions is enhanced by the loss of the DNA polymerase proofreading function (Selifonova et al., 2001).