An intragenic tandem repeat (TR) region has been previously reported in the tolA gene of Escherichia coli. In silico analysis of 123 E. coli tolA sequences from Genbank and PCR analysis of the tolA TR region from 111 additional E. coli strains revealed that this TR region is highly variable. Nine different TR sizes with 8 up to 16 repeat units were found in in silico analysis and 6 of these were also found by PCR analysis. The 13-unit TR emerged as the predominant type using both approaches (47.2% and 86.5%, respectively). Remarkably, TRs in pathogenic strains appeared to be more variable than those in non-pathogens. To demonstrate the occurrence of TR variation in a clonal population, a selection system for TR deletion events was constructed by inserting the 13-unit TR region of MG1655 in frame into a plasmid-borne chloramphenicol acetyltransferase (cat) gene. The resulting cat gene no longer conferred chloramphenicol resistance unless the insert size was reduced by TR contraction. Using this system, Cm-resistant revertants with a TR contraction were recovered at a frequency of 1.1 × 10(-7), and contraction was shown to be recA-dependent and enhanced in a DNA repair-deficient mutS background.