Fig. 2.

Schematic model of the sequential production and export of FlaA and FlaB. The figure summarizes our current working model of how spatial arrangement of the two flagellins is achieved in Shewanella putrefaciens, depicted in four subsequent stages of the flagellar assembly. a The current model of flagellin expression suggests that transcription is inhibited as long as the alternative sigma factor 28 (FliA) is blocked by the anti-sigma factor FlgM. In S. putrefaciens, this only applies to FlaB while FlaA production does not depend on FliA (see Supplementary Fig. 2b). Therefore, a pool of FlaA is already present while the basal body is still being assembled. b Once the hook-basal-body complex is completed the export apparatus changes its specificity from hook proteins (FlgE) to the flagellins and other late assembly proteins. At this stage also the anti-sigma factor FlgM gets exported and FliA-dependent transcription of FlaB is initiated. c The already produced FlaA monomers get exported first and assemble at the base of the flagellar filament. d The increasing production of FlaB monomers passes that of FlaA and potentially FlaA transcription and/or translation may be reduced or even completely terminated (indicated by the question marks). Thus, FlaB constitutes the majority residual part of the flagellar filament. (Pol, RNA polymerase; Rib, ribosome; σ, sigma factor)