Lack of N-terminal segment of the flagellin protein results in the production of a shortened polar flagellum in a deep-sea sedimentary bacterium Pseudoalteromonas sp. SM9913

Bacterial polar flagella, comprised of flagellin, are essential for bacterial motility. Pseudoalteromonas sp. SM9913 is a bacterium isolated from deep-sea sediments. Unlike other Pseudoalteromonas strains that have a long polar flagellum, strain SM9913 has an abnormally short polar flagellum. Here, we investigated the underlying reason for the short flagellar length and found that a single base mutation was responsible for the altered flagellar assembly. This mutation leads to the fragmentation of the flagellin gene into two genes, PSM_A2281 , encoding the core segment, and the C-terminal segment, and PSM_A2282 , encoding the N-terminal segment, and only gene PSM_A2281 is involved in the production of the short polar flagellum. When a chimeric gene of PSM_A2281 and PSM_A2282 encoding an intact flagellin A2281::82 was expressed, a long polar flagellum was produced, indicating that the N-terminal segment of flagellin contributes to the production of a polar flagellum of normal length. Analysis of the simulated structures of A2281 and A2281::82 and that of the flagellar filament assembled with A2281::82 indicates that, due to the lack of two α-helices, the core of the flagellar filament assembled with A2281 is incomplete, which is likely too weak to support the stability and movement of a long flagellum. This mutation in strain SM9913 had little effect on its growth and only a small effect on its swimming motility, implying that strain SM9913 can live well with this mutation in natural sedimentary environments. This study provides a better understanding of the assembly and production of bacterial flagella. Importance Polar flagella, which are an essential organelle for bacterial motility, are comprised of multiple flagellin subunits. A flagellin molecule contains an N-terminal segment, a core segment and a C-terminal segment. Results of this investigation of the deep-sea sedimentary bacterium Pseudoalteromonas sp. SM9913 demonstrate that a single base mutation in the flagellin gene leads to the production of an incomplete flagellin without the N-terminal segment and that the loss of the N-terminal segment of the flagellin protein results in the production of a shortened polar flagellar filament. Our results shed light on the important function of the N-terminal segment of flagellin in the assembly and stability of bacterial flagellar filament.