The regulation of polar flagella assembly in Pseudomonas putida

The Gram-negative bacterium Pseudomonas putida bears a tuft of flagella at a single cell pole. New flagella must be assembled de novo every cell cycle to secure motility of both daughter cells. Here we show that the coordinated action of FimV, FlhF and FleN sets the location, timing and number of flagella assembled. The polar landmark protein FimV recruits FleN and enables stable association of FlhF to the cell poles. FlhF determines flagellar position by recruiting FliF to the cell pole and preventing its nucleation at non-polar sites. FlhF is also required for assembly of the flagellar type III secretion system, enabling Class III flagellar promoter activation and export of the external flagellar components. FleN regulates flagellar number by repressing flhF expression and a second, posttranscriptional mechanism. Flagellar assembly starts with FlhF recruitment to the new pole shortly after cell division and finishes with the emergence of FliC filaments during the following cell division. FimV and FleN regulate the onset of flagellar assembly by preventing premature polar targeting of FlhF. Our results shed new light on the mechanisms that ensure the timely assembly of the appropriate number of flagella at the correct polar location in polarly flagellated bacteria. ### Competing Interest Statement The authors have declared no competing interest.