Partially Reciprocal Replacement of FlrA and FlrC in Regulation of Shewanella oneidensis Flagellar Biosynthesis

In some bacteria with a polar flagellum, an established regulatory hierarchy controlling stepwise assembly of the organelle consists of four regulators: FlrA, σ54 , FlrBC, and σ28 . Because all of these regulators mediate the expression of multiple targets, they are essential to the assembly of a functional flagellum and therefore to motility. However, this is not the case for the gammaproteobacteriumShewanella oneidensis : cells lacking FlrB, FlrC, or both remain flagellated and motile. In this study, we unravel the underlying mechanism, showing that FlrA and FlrC are partially substitutable for each other in regulating flagellar assembly. While both regulators are bacterial enhancer binding proteins (bEBPs) for σ54 , FlrA differs from FlrC in its independence of σ54 for its own transcription and its inability to activate the flagellin geneflaA . These differences largely account for the distinct phenotypes resulting from the loss or overproduction of FlrA and FlrC.IMPORTANCE The assembly of a polar flagellum in bacteria has been characterized as relying on four regulators, FlrA, σ54 , FlrBC, and σ28 , in a hierarchical manner. They all are essential to the process and therefore to motility, except inS. oneidensis , in which FlrB, FlrC, or both together are not essential. Here we show that FlrA and FlrC, as bEBPs, are partially reciprocal in functionality in this species. As a consequence, the presence of one allows flagellar assembly and motility in the other's absence. Despite this, there are significant differences in the physiological roles played by these two regulators: FlrA is the master regulator of flagellar assembly, whereas FlrC fine-tunes motility. These intriguing observations open up a new avenue to further exploration of the regulation of flagellar assembly.