Role of SdiA in Salmonella enterica serovar Typhimurium physiology and virulence

sdiA in Salmonella enterica serovar Typhimurium encodes a protein belonging to the LuxR family of transcriptional regulators. Initial computer analysis revealed the presence of a fur box 19 by upstream of the start codon of the sdiA gene and a helix-turn-helix motif in the carboxy-terminal part of the SdiA protein typical for transcriptional regulators. Deletion of the fur box resulted in twofold increase of sdiA transcription. Furthermore, addition of dipyridyl, an iron chelator, to culture media increased sdiA transcription to the level observed in the fur box mutant, confirming that sdiA is suppressed in the presence of iron. When S. enterica sv. Typhimurium was grown in conditioned medium, sdiA transcription was repressed to 30% of that in cells grown in fresh LB broth; this repression was independent of the fur box. Oral infection of mice with the strain lacking the helix-turn-helix domain of SdiA indicated increased virulence of this S. enterica sv. Typhimurium mutant. sdiA, dually controlled by iron concentration and culture-density-derived signals, may therefore play an important role in S. enterica sv. Typhimurium virulence regulation.