The Master Regulators of the Fla1 and Fla2 Flagella of Rhodobacter sphaeroides Control the Expression of Their Cognate CheY Proteins.

Rhodobacter sphaeroides is an alphaproteobacterium that has two complete sets of flagellar genes. The fla1 set was acquired by horizontal transfer from an ancestral gammaproteobacterium and is the only set of flagellar genes that is expressed during growth under standard laboratory conditions. The products of these genes assemble a single, subpolar flagellum. In the absence of the Fla1 flagellum, a gain-of-function mutation in the histidine kinase CckA turns on the expression of the fla2 flagellar genes through the response regulator CtrA. The rotation of the Fla1 and Fla2 flagella is controlled by different sets of chemotaxis proteins. Here, we show that the expression of the chemotaxis proteins that control Fla2, along with the expression of the fla2 genes, is coordinated by CtrA, whereas the expression of the chemotaxis genes that control Fla1 is mediated by the master regulators of the Fla1 system. The coordinated expression of the chemosensory proteins with their cognate flagellar genes highlights the relevance of integrating the expression of the horizontally acquired fla1 genes with a chemosensory system independently of the regulatory proteins responsible for the expression of fla2 and its cognate chemosensory system. IMPORTANCE Gene acquisition via horizontal transfer represents a challenge to the recipient organism to adjust its metabolic and genetic networks to incorporate the new material in a way that represents an adaptive advantage. In the case of Rhodobacter sphaeroides, a complete set of flagellar genes was acquired and successfully coordinated with the native flagellar system. Here we show that the expression of the chemosensory proteins that control flagellar rotation is dependent on the master regulators of their corresponding flagellar system, minimizing the use of transcription factors required to express the native and horizontally acquired genes along with their chemotaxis proteins.