SoxS represses flagellar gene expression through RflP in Salmonella enterica Serovar Typhimurium

Flagellar gene expression is subject to regulation by many global transcription factors in response to environmental and nutritional signals. One of the primary ways this occurs in Salmonella enterica serovar Typhimurium, and its close relatives, is through controlling levels of FlhD4C2 (the flagellar master regulator) via transcriptional, post-transcriptional, and post-translational mechanisms. Recently, we found the homologous transcription factors MarA, Rob, and SoxS repress flhDC expression by distinct mechanisms. MarA and Rob, regulators involved in inducible multidrug resistance, repressed flhDC transcription by interacting directly with the flhDC promoter. Alternatively, SoxS, the oxidative stress response regulator, altered FlhD4C2 levels independent of flhDC transcription by post-transcriptional or post-translational mechanism. Here, using a forward genetic screen, we discovered that SoxS-dependent repression of flagellar gene expression occurs through RflP, an anti-FlhD4C2 factor that targets FlhD4C2 for proteolytic degradation. Elevated soxS expression resulted in concomitant increases in rflP expression, indicating SoxS may work through RflP at the level of rflP transcription. Mapping of the rflP promoter and a bioinformatic search yielded a putative SoxS binding site proximal to the rflP transcription start site. Comparison of the rflP promoter region in S. Typhimurium and Escherichia coli indicate substantial differences, providing a possible mechanism for differential expression of rflP between these species. IMPORTANCESalmonella enterica is a major cause of foodborne illness. Understanding environmental and intracellular signals used by Salmonella to control expression of virulence-associated traits is critical to advancing treatment and prevention of Salmonella-related disease. Reduced expression of flagella at key points during Salmonella infection aids in evasion of the host innate immune system. Within macrophages Salmonella is non-flagellated and exposed to oxidative stress. SoxS-dependent repression of flagellar genes may provide a potential link between oxidative stress and reductions in flagellar expression.