Differential expression of the transcription factors MarA, Rob, and SoxS of Salmonella Typhimurium in response to sodium hypochlorite: Down-regulation of rob by MarA and SoxS

To survive, Salmonella enterica serovar Typhimurium (S. Typhimurium) must sense signals found in phagocytic cells and modulate gene expression. In the present work, we evaluated the expression and cross-regulation of the transcription factors MarA, Rob, and SoxS in response to NaOCl. We generated strains ΔsoxS and ΔmarA, which were 20 times more sensitive to NaOCl as compared to the wild-type strain; while Δrob only 5 times. Subsequently, we determined that marA and soxS transcript and protein levels were increased while those of rob decreased in a wild-type strain treated with NaOCl. To assess if changes in S. Typhimurium after exposure to NaOCl were due to a cross-regulation, as in Escherichia coli, we evaluated the expression of marA, soxS, and rob in the different genetic backgrounds. The positive regulation observed in the wild-type strain of marA and soxS was retained in the Δrob strain. As in the wild-type strain, rob was down-regulated in the ΔmarA and ΔsoxS treated with NaOCl; however, this effect was decreased. Since rob was down-regulated by both factors, we generated a ΔmarA ΔsoxS strain finding that the negative regulation was abolished, confirming our hypothesis. Electrophoretic mobility shift assays using MarA and SoxS confirmed an interaction with the promoter of rob.