Many bacteria possess flagella, helical proteinaceous rotating filaments, which enable active motility in various environments. The flagellar apparatus is an intricate nanomachine, which is synthesized from a range of different building blocks of various stoichiometries. A complex regulation pattern underlies the stepwise formation of flagellar subunits, resulting in an inside-out assembly of the flagellar apparatus.
Notably, flagellation occurs in species-specific pattern. In many environments, the monopolar flagellation (a single flagellum at one cell pole) is the predominant pattern. In these cells, flagellar synthesis has to be targeted to the appropriate cell pole, and, additionally, the number of flagella has to be restricted to a single one. For many bacterial species, this flagellation pattern is governed by the SRP-type GTPase FlhF as the polarity factor and the MinD-like ATPase FlhG as the flagella counting factor. In recent studies, we developed a model describing how onset of flagella synthesis is spatially controlled and how flagella formation is shut down to restrict the number of flagella to a single one. This model along with the implications and many still open questions will be presented here.