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

<div>Bacterial polar flagella, comprised of flagellin, are essential for bacterial motility. <i>Pseudoalteromonas</i> sp. strain SM9913 is a bacterium isolated from deep-sea sediments. Unlike other <i>Pseudoalteromonas</i> strains that have a long polar flagellum, strain SM9913 has an abnormally short polar flagellum. Here, we investigated the underlying reason for the short flagellum 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, <i>PSM_A2281</i>, encoding the core segment and the C-terminal segment, and <i>PSM_A2282</i>, encoding the N-terminal segment, and only gene <i>PSM_A2281</i> is involved in the production of the short polar flagellum. When a chimeric gene of <i>PSM_A2281</i> and <i>PSM_A2282</i> 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 a normal length. Analyses of the simulated structures of A2281 and A2281::82 and that of the flagellar filament assembled with A2281::82 indicate that due to the lack of two α-helices, the core of the flagellar filament assembled with A2281 is incomplete and 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.</div>