Figure. Evolutionary distance in terms of synonymous and nonsynonymous changes per 100 sites (4) for 12 genes sequenced from Escherichia coli O157:H7, E. coli K-12, and Salmonella enterica Typhimurium. The points for synonymous sites are (left to right): gap, crr, mdh, icd, fliC (conserved 5' and 3' ends), trpB, putP, aceK, mutS, trpC, tonB, and trpA. Under the mutator hypothesis, the genetic distance between the pathogenic O157:H7 strain (or the closely related strain ECOR37) and the outgroup (Typhimurium) is expected to exceed the distance between the commensal K-12 and the outgroup. Prolonged periods of enhanced mutation rate should drive the points above the dotted line marking equal rates of molecular evolution. Two loci (tonB and trpA) show departure from the equal rate line, but neither has evolved differently from that expected by the molecular clock. The sequences of 12 genes were obtained from GenBank or the original sources as follows: aceK (5), crr (6-8), fliC (9), gap (10), icd (11), mdh (12), mutS (13,14), putP (15), tonB (16,17), trp (17-20).