Identification of the Helicobacter pylori anti-σ28 factor

Flagellar motility is essential for colonization of the human gastric mucosa by Helicobacter pylori. The flagellar filament is composed of two subunits, FlaA and FlaB. Transcription of the genes encoding these proteins is controlled by the sigma (28) and sigma (54) factors of RNA polymerase respectively. The expression of flagellar genes is regulated, but no sigma (28)-specific effector was identified. It was also unclear whether H. pylori possessed a checkpoint for flagellar synthesis, and no gene encoding an anti-sigma (28) factor, FIgM, could be identified by sequence similarity searches. To investigate the sigma (28)-dependent regulation, a new approach based on genomic data was used. Two-hybrid screening with the H. pylori proteins identified a protein of unknown function (HP1122 interacting with the sigma (28) factor and defined the C-terminal part of HP1122 (residues 48-76) as the interaction domain. HP1122 interacts with region 4 of sigma (28) and prevents its association with the beta -region of H. pylori RNA polymerase. Thus, HP1122 presented the characteristics of an anti-sigma (28) factor. This was confirmed in H. pylori by RNA dot-blot hybridization and electron microscopy. The level of sigma (28)-dependent flaA transcription was higher in a HP1122-deficient strain and was decreased by the overproduction of HP1122. The overproduction of HP1122 also resulted in H. pylori cells with highly truncated flagella. These results demonstrate that HP1122 is the H. pylori anti-sigma (28) factor, FIgM, a major regulator of flagellum assembly. Potential anti-sigma (28) factors were identified in Campylobacter jejuni, Pseudomonas aeruginosa and Thermotoga maritima by sequence homology with the C-terminal region of HP1122.