Giant flagellins form thick flagellar filaments in two species of marine γ-proteobacteria

Abstract

Flagella, the primary means of motility in bacteria, contain thousands of copies of the protein flagellin in self-assembled helical filaments that function as microscopic propellers. Evolution has presented a wide range of different sizes of flagellin, but the upper reaches of the size distribution have barely been explored. We show here that two species of marine Gammaproteobacteria, Bermanella marisrubri Red65 and Oleivorans marinus 2O1, are motile due to the production of thick, monopolar flagellar filaments. In each case, a 'giant' flagellin of more than 1,000 amino acids is the only predicted flagellin protein. Two species of Methylobacterium from the leaves of Arabidopsis thaliana also both possess genes for giant flagellins. However, their flagellar filaments are of similar thickness to bacteria with flagellins half the size. This may be explained by the presence of multiple, smaller, flagellin genes in the Methylobacterium species. This work further illustrates how "the" bacterial flagellum is not a single, ideal structure, but a continuum of evolved machines adapted to a wide range of niches.