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Journal of Bacteriology, January 1999, p. 153-160, Vol. 181, No. 1
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Sequence Diversity of Flagellin (fliC) Alleles in Pathogenic Escherichia coli

Sean D. Reid, Robert K. Selander, and Thomas S. Whittam*

Institute of Molecular Evolutionary Genetics and Department of Biology, Pennsylvania State University, University Park, Pennsylvania 16802

Received 20 July 1998/Accepted 14 October 1998

To study the molecular evolution of flagellin, the protein subunit specifying flagellar (H) antigens, the fliC genes from 15 pathogenic strains of Escherichia coli were amplified by PCR and sequenced. Comparison of fliC sequences of H6 and H7 strains revealed that alleles have a mosaic structure indicating the occurrence of past horizontal transfer of DNA segments between strains. The close similarity of H7 sequences also indicates the exchange of an entire fliC H7 allele between distant clonal lineages. In addition, the ratio of silent substitutions to amino acid replacements suggests that a short segment in the central region of fliC has been under positive selection in the divergence of H6 and H7 alleles. Phylogenetic analysis demonstrates that the fliC sequences of O157:H7 and O55:H7 serotypes are nearly identical and highly divergent from those of E. coli strains expressing H6 and H2 flagellar antigens. A nonmotile clone of sorbitol-fermenting O157 has rapidly accumulated multiple mutations in fliC, presumably as a result of the silencing of flagellin expression.

* Corresponding author. Mailing address: IMEG, Dept. Biology, 208 Mueller Laboratory, Pennsylvania State University, University Park, PA 16802. Phone: (814) 863-1970. Fax: (814) 865-9131. E-mail: tsw1{at}psu.edu.

Journal of Bacteriology, January 1999, p. 153-160, Vol. 181, No. 1
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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