The flagellar set Fla2 in Rhodobacter sphaeroides is controlled by the CckA pathway and is repressed by organic acids and the expression of Fla1.

Rhodobacter sphaeroides has two different sets of flagellar genes. Under the growth conditions commonly used in the laboratory, the expression of the fla1 set is constitutive, whereas the fla2 genes are not expressed. Phylogenetic analyses have previously shown that the fla1 genes were acquired by horizontal transfer from a gammaproteobacterium and that the fla2 genes are endogenous genes of this alphaproteobacterium. In this work, we characterized a set of mutants that were selected for swimming using the Fla2 flagella in the absence of the Fla1 flagellum (Fla2(+) strains). We determined that these strains have a single missense mutation in the histidine kinase domain of CckA. The expression of these mutant alleles in a Fla1(-) strain allowed fla2-dependent motility without selection. Motility of the Fla2(+) strains is also dependent on ChpT and CtrA. The mutant versions of CckA showed an increased autophosphorylation activity in vitro. Interestingly, we found that cckA is transcriptionally repressed by the presence of organic acids, suggesting that the availability of carbon sources could be a part of the signal that turns on this flagellar set. Evidence is presented showing that reactivation of fla1 gene expression in the Fla2(+) background strongly reduces the number of cells with Fla2 flagella.