PMC

Alignment of a portion of the mutL sequence of the experimental system's wild-type ancestor, REL606, with the mutations found in the populations Ara⁻2 and Ara⁻4. Also shown is the amino acid sequence of the corresponding section of the MutL protein. The numbering given is for the REL606 nucleic acid sequence.

Rate of mutation to nalidixic acid resistance determined by fluctuation test. The mutation rate is shown on a log scale. Strains PS2348 and PS2350 exhibited mutation rates consistent (as defined in Results) with those measured previously for PS17 and PS19, the strains from which the mutL mutations were obtained. The four strains derived from PS2348 and PS2350 and complemented with uvrD⁺ (PS2353 and PS2355) or mutL⁺ (PS2358 and PS2360) all showed drastically reduced mutation rates. Error bars represent 95% confidence intervals calculated as described in Materials and Methods. The dashed line represents the upper 95% confidence interval calculated previously for REL606, the wild-type ancestor of PS2348 and PS2350 (); this line was used as the cutoff to determine whether complementation had restored the wild-type mutation rate.

Alignment of the mutL repeat sequence in the experimental E. coli B strain (REL606) with homologous sequences from mutL genes of other bacterial species. Positions at which the sequence of another species or strain is identical to that of E. coli B are indicated by periods. Only E. coli O157:H7 shares complete sequence identity with E. coli B within this stretch. E. coli K-12, the two Salmonella species, and Shigella flexneri all contain only two complete repeats. The sequences of more distantly related bacterial species, including many not shown here, were found to contain no trace of any repeated elements in this region. The E. coli B sequence obtained from REL606 and all other sequences shown here are available from GenBank. The numbering given is for the E. coli B nucleic acid sequence.