Switching of Flagellar Motility in Helicobacter pylori by Reversible Length Variation of a Short Homopolymeric Sequence Repeat in fliP , a Gene Encoding a Basal Body Protein

The genome ofHelicobacter pylori contains numerous simple nucleotide repeats that have been proposed to have regulatory functions and to compensate for the conspicuous dearth of master regulatory pathways in this highly host-adapted bacterium.H. pylori strain 26695, whose genomic sequence was determined by The Institute for Genomic Research (TIGR), contains a repeat of nine cytidines in thefliP flagellar basal body gene that splits the open reading frame in two parts. In this work, we demonstrate that the 26695C9 strain with a splitfliP gene as sequenced by TIGR was nonflagellated and nonmotile. In contrast, earlier isolates of strain 26695 selected by positive motility testing as well as pig-passaged derivatives of 26695 were all flagellated and highly motile. All of these motile strains had a C8 repeat and consequently a contiguousfliP reading frame. By screening approximately 50,000 colonies of 26695C9 for motility in soft agar, a motile revertant with a C8 repeat could be isolated, proving that the described switch is reversible. ThefliP genes of 20 motile clinicalH. pylori isolates from different geographic regions possessed intactfliP genes with repeats of eight cytidines or the sequence CCCCACCC in its place. IsogenicfliP mutants of a motile, C8 repeat isolate of strain 26695 were constructed by allelic exchange mutagenesis and found to be defective in flagellum biogenesis. Mutants produced only small amounts of flagellins, while the transcription of flagellin genes appeared unchanged. These results strongly suggest a unique mechanism regulating motility inH. pylori which relies on slipped-strand mispairing-mediated mutagenesis offliP .