Background: Two ancestral nucleoid-associated proteins called Hha and YdgT contribute to the negative regulation of several virulence-associated genes in Salmonella enterica serovar Typhimurium. Our previous work showed that Hha and YdgT proteins are required for negative regulation of Salmonella Pathogenicity Island-2 and that hha ydgT double mutants are attenuated for murine infection. Interestingly, hha ydgT mutant bacteria exhibited a non-motile phenotype suggesting that Hha and YdgT have a role in flagellar regulation.
Results: In this study we show that the non-motile phenotype of hha ydgT mutants is due to decreased levels of the master transcriptional regulator FlhD4C2 resulting in down-regulation of class II/III and class III flagellar promoters and lack of surface flagella on these cells. The horizontally acquired pefI-srgD region was found to be partially responsible for this phenotype since deletion of pefI-srgD in a hha ydgT deletion background resulted in transient restoration of class II/III and III transcription, expression of surface flagella, and motility in the quadruple mutant.
Conclusion: These data extend our current understanding of the mechanisms through which Hha and YdgT regulate flagellar biosynthesis and further describe how S. Typhimurium has integrated horizontal gene acquisitions into ancestral regulatory networks.