MOLECULAR CHARACTERIZATION OF INTACT, BUT CRYPTIC, FLAGELLIN GENES IN THE GENUS SHIGELLA

Flagellin genes (fliC) were detected in two species of the genus Shigella. The fliC(SF) gene cloned from Shigella flexneri produced normal-type flagella in an Escherichia coli Delta fliC strain while the fliC(SS) genes from two Shigella sonnei strains produced curly-type flagella and their expression is repressible by Salmonella FljA repressor. The fliC(SF) gene (1650 bp) shared high similarity with the E. coli fliC(E) gene not only in the 5' and 3' constant sequences but also in the upstream and downstream sequences. The fliC(SS) genes (1572 bp) shared high similarity with the Salmonella typhimurium fliC(S) gene in the operator and 3' constant sequences and also shared high similarity with the fliC(E) gene in the downstream sequence, suggesting that the fliC(SS) gene has undergone horizontal transfer and recombination. Differences in nucleotide sequences of the central variable regions among the four fliC genes, including fliC(E) and fliC(S), suggest that they started differentiation in each lineage approximately 80 million years ago. Loss of motility in Shigella seems to be evolutionarily a recent event.