Functional divergence of flagellar type III secretion system: A case study in a non-flagellated, predatory bacterium.

The lack of functional flagella and the ability to prey upon other microorganisms are well-known traits of , a plant beneficial bacterial species. Here, we report a possible link between these two traits in the model strain OH11 (OH11). The genome of OH11 encompasses several homologous genes involved in the flagellum formation but it lacks a functional , encoding the flagellin. Despite the lack of the main component of the flagellum, OH11 genome includes genes involved in the flagellar type III secretion system (FT3SS), which is commonly deployed by flagellated bacteria to transport flagellar subunit proteins. To understand the role played by FT3SS in OH11, we showed that the remaining FT3SS genes were expressed under laboratory conditions. Subsequently, we showed that the identified FT3SS genes involved in the secretion of the hook-capping protein FlgD, suggesting OH11 likely possessed a functional FT3SS system. Blocking FT3SS in OH11 via inactivation of the ATPase FliI impaired the secretion of the proteins Le3970 (protease), Le4493 (ß-1,3-glucanase A) and Le1659 (halo acid dehalogenase family), that showed a toxic activity against the yeast . The possible link between FT3SS and OH11 antagonism towards was also confirmed by loss of toxicity in both mutants of Δ and Δ that lacks the FT3SS structural gene when co-cultured with the yeast strain. The design of synthetic proteins toxic against the Gram-negative bacterium further supported the involvement of FT3SS in the ability of OH11 to parasitize other microorganisms. Overall, these results revealed a possible cooption of components of FT3SS system in the competition with other microorganisms in the plant beneficial bacterium OH11 and highlighted a functional divergence of FT3SS between flagellated and non-flagellated bacteria.