Partially reciprocal replacement of FlrA and FlrC in regulation of Shewanella oneidensis flagellar biosynthesis

Abstract

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 gammaproteobacterium Shewanella 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 gene flaA. These differences largely account for the distinct phenotypes resulting from the loss or overproduction of FlrA and FlrC.