E. coli strains, plasmid DNAs and oligonucleotides used in this study.

aThe complete annotated genotype for strain RW118 is: rpsL31 xyl-5 mtl-1 galK2 lacY1 tsx-33 supE44 thi-1 hisG4(Oc) argE3(Oc) araD139 thr-1 Δ(gpt-proA)62 sulA211.bCGSC: E. coli Genetic Stock Center, Yale University, New Haven, CT 06520, USA.cThese strains were generated by plasmid shuffle; see Material and Methods for a detailed description of the dnaN plasmid shuffle assay. For strains MS202-MS208, the sequence of each plasmid encoded dnaN allele was verified by automated nucleotide sequence analysis, and the –1 frameshift mutation in the dnaN–1FS allele was confirmed by diagnostic PCR and XhoI restriction analysis.dThe complete annotated genotype for strain AB1157 is: xyl-5 mtl-1 galK2 rpsL31 kdgK51 lacY1 tsx-33 supE44 thi-1 leuB6 hisG4(Oc) mgl-51 argE3(Oc) rfbD1 proA2 ara-14 thr-1 qsr-9 qin-111.eThe complete annotated genotype for strain MG1655 is: ilvG rfb-50 rph-1.fThe sequence corresponding to the XhoI restriction endonuclease site (CTCGAG) within the dnaN–1FS allele, which contains a C→T substitution and −1 dG frameshift (CTTAG), is shown in lower case italics.