MINIREVIEW Get the Message Out: Cyclic-Di-GMP Regulates Multiple Levels of Flagellum-Based Motility

Flagella are complex organelles whose synthesis depends upon approximately 50 gene products. Not surprisingly, mul-tiple mechanisms control and direct flagellar biogenesis (re-viewed in references 1, 15, and 55). Perhaps best understood is the control exerted at the level of transcription: hierarchies exist, such that the flagellar structural proteins are synthesized only under proper conditions and at the time each is needed for assembly. Many bacteria also possess posttranscriptional mechanisms that control translation or protein stability. As-sembly itself also occurs in a highly ordered fashion, with the insertion of one component building on the insertion of a previous subunit. In the best-characterized bacteria, flagellar biogenesis begins with the insertion of an inner membrane protein called FliF (Fig. 1). Building on the assembled FliF subunits are proteins that comprise the flagellar type III secre