Evolutionary changes of the flhDC flagellar master operon in Shigella strains

Shigella strains are nonmotile. The master operon of flagellar synthesis, flhDC, was analyzed for genetic damage in 46 Shigella strains representing all known serotypes. In 11 strains (B1, B3, B6, B8, B10, B18, D5, F1B, D10, F3A, and F3C) the flhDC operon was completely deleted. PCR and sequence analysis of the flhDC region of the remaining 35 strains revealed many insertions or deletions associated with insertion sequences, and the majority of the strains were found to be defective in their flhDC genes. As these genes also play a role in regulation of nonflagellar genes, the loss may have other consequences or be driven by selection pressures other than those against flagellar motility. It has been suggested that Shigella strains fall mostly into three clusters within Escherichla coli, with five outlier strains, four of which are also within E. coli (G. M. Pupo, R. Lan, and P. R. Reeves, Proc. Natl. Acad. Sci. USA 97:10567-10572, 2000). The distribution of genetic changes in the flhDC region correlated very well with the three clusters and outlier strains found using housekeeping gene DNA sequences, enabling us to follow the sequence of mutational change in the flhDC locus. Two cluster 2 strains were found to have unique flhDC sequences, which are most probably due to recombination during the exchange of the adjacent O-antigen gene clusters.