Downloaded from www.microbiologyresearch.org by IP: 54.160.81.51 On: Mon, 19 Sep 2016 14:13:47 Escherichia coli mutators: selection criteria and migration effect Ludovic Le Chat, 1 3 Michel Fons 2 and Francois Taddei 1 Correspondence Michel Fons michel.fons@univ.u-3mrs.fr 1 University of Paris 5, Faculty of Medicine, INSERM, U571, F-75730 Paris 15, France 2 Universite ´ Aix-Marseille III, Faculte ´ St Je ´ rome, IMRN service 342, F-13397 Marseille 20, France Received 9 August 2005 Revised 14 October 2005 Accepted 21 October 2005 In silico, it has been shown that mutator alleles that increase mutation rate can be selected for by generating adaptive mutations. In vitro and in vivo, competition between wild-type bacteria and isogenic mutator mutants is consistent with this view. However, in vivo, the gain of the mutator seems to be reduced when migration is allowed. In vitro, the advantage of mutators has been described as frequency-dependent, leading to mutator advantage only when they are sufficiently frequent. Using an in vitro system, it is demonstrated that (i) the selection of mutators is frequency- independent, yet depends on at least one mutator bacterium bearing an adaptive mutation (its presence depends on chance, mutation rates and population size of mutator bacteria); (ii) on average, the mutator gain is always equal to the ratio of the adaptive mutation frequency of the mutator versus wild-type; (iii) when migration into an empty niche is allowed, the mutator benefit is reduced if migration occurs after fixation of the adaptive mutation into the wild-type population. It is concluded that in all cases, mutator gain depends directly on the ratio of bacteria carrying a beneficial mutation in mutator versus wild-type lineages. INTRODUCTION Numerous natural isolates of Escherichia coli, Salmonella enterica, Pseudomonas aeruginosa and Neisseria meningitidis species appear to be ‘mutators’ (LeClerc et al., 1996; Matic et al., 1997; Oliver et al., 2000; Richardson & Stojiljkovic, 2001; Richardson et al., 2002). When compared to wild-type strains, mutators have an increased mutation rate through- out their genome, due to mutations that disrupt some aspect of DNA replication or repair functions (Friedberg et al., 1995). Most mutators from natural isolates are defective for the methyl-directed mismatch repair system (MMR). Among MMR-defective mutators, the majority are MutS- defective, a key component of the MMR (LeClerc et al., 1996; Matic et al., 1997; Oliver et al., 2002; Richardson et al., 2002). The MMR is the principal post-replicative pathway for DNA replication fidelity. In E. coli, the MMR uses dam DNA methylation (adenine methylation of GATC sequences) to discriminate between parental and neo-synthesized DNA strands. When replication errors occur, MutS recognizes base mismatches as well as insertions/deletions (up to 4 bases) and mediates the fixation of MutL. MutL recruits MutH, which cleaves the newly synthesized, unmethylated strand at the nearest GATC sequence. Consequently, the UvrD (MutU) helicase unwinds the strands and the newly synthesized one is degraded by exonucleases in the direction of the mismatch. Finally, DNA polymerase III fills the resultant gap (Modrich & Lahue, 1996). Many other loci, such as mutY (MutY removes adenine from 8-oxo-G and A-G mismatches) or xthA (XthA is a 59 abasic endonu- clease), have been described in the laboratory as conferring a mutator phenotype when affected (Horst et al., 1999). The frequency of mutators in natural populations is higher than that generated by mutation and counterselection equilibrium. They may be selected for directly or indirectly (Boe et al., 2000; Mao et al., 1997; Ninio, 1991). It has been proposed that mutator loci could be indirectly selected by ‘hitch-hiking’, together with the advantageous mutations generated (Chao & Cox, 1983). The selection for advanta- geous mutations leads to the selection of conserved gene linkages and hence the selection of the mutator allele. This beneficial effect would counter-balance the negative impact of deleterious and lethal mutations also overproduced in mutator lineages. This scenario is consistent with in silico, in vitro and in vivo observations (Taddei et al., 1997; Chao & Cox, 1983; Giraud et al., 2001). For the in vivo model, the mutS + genotype was restored in a mutS 2 strain that had previously evolved over 42 days in a mouse digestive tract (this strain was designated r-mutS + ). In competition with the mutS + ancestor strain, the r-mutS + strain showed a 3Present address: Department of Biological Sciences, Imperial College London, Sir Alexander Fleming Building, South Kensington Campus, London SW7 2AZ, UK. Abbreviations: f(Spc R ), frequency of spontaneously occurring spectinomycin-resistant mutants; Nal R , nalidixic-acid-resistant; Spc R , spectinomycin-resistant. 0002-8418 G 2006 SGM Printed in Great Britain 67 Microbiology (2006), 152, 67–73 DOI 10.1099/mic.0.28418-0