Structural and regulatory determinants of flagellar motility in - The archetypal flagellum of DSM 17395.

Flagellar motility is crucial for the swim-and-stick lifestyle of and plays an important role for bacterial-algal interactions. This alphaproteobacterial order contains three distinct types of flagellar gene clusters (FGCs) for the formation of a functional flagellum. Our phylogenetically broad taxon sampling of more than 300 genomes revealed that the most common FGC, the -type, was probably already present in the common ancestor of and was strictly vertically inherited, while the other two FGC types, 2 and , were spread via horizontal operon transfers. Swimming of the marine model organism DSM 17395 () is mediated by the archetypal -type flagellum. Screening of 13,000 transposon mutants of on soft agar plates revealed that 40 genes, including four genes encoding conserved but not yet characterized proteins (CP1-4) within the FGC, are essential for motility. Exoproteome analyses indicated that CP1-4 are required at different stages of flagellar assembly. Only eight genes outside the FGC were identified as essential for swimming motility, including all three genes of the CtrA phosphorelay. Using comparative transcriptomics of Δ, Δ and Δ mutants of the distantly related model organisms and DFL 12, we identified genes for the flagellum and cyclic di-GMP turnover as core targets of the CtrA phosphorelay and a conserved connection with quorum sensing across members of the .