Fig 4.

A partner-switching mechanism involving B. subtilis CsrA, FliW and Hag proteins create a morphogenetic checkpoint for flagellum biosynthesis. CsrA protein represses translation of hag mRNA encoding flagellin. FliW competitively binds to Hag or CsrA, depending on the intracellular concentration of Hag. Prior to the completion of the flagellum secretion channel or upon capping of the pore, high intracellular levels of Hag occupy FliW and CsrA is free to block Hag translation. When the flagellum channel is open and Hag protein is being actively secreted, intracellular Hag levels drop and FliW is free to bind to CsrA, derepressing the synthesis of Hag protein (Mukherjee et al., 2011).