Increased expression of Mg2+ transport proteins enhances the survival of Salmonella enterica at high temperature

Mg2+ homeostasis is important for Salmonella pathogenesis. In Salmonella enterica, the transcription of the mgtA gene, which encodes a Mg2+ transporter, is regulated by a Mg2+-sensing riboswitch [Cromie MJ, Shi Y, Latifi T, Groisman EA (2006) Cell 125: 71-84]. In a genetic analysis of the determinants of thermotolerance in S. enterica serovar Typhimurium, we isolated the chr-1 mutation that increased the resistance of exponential phase cells to killing by high temperature. This mutation is a single base change in the mgtA riboswitch that causes high-level constitutive expression of mgtA. We showed that another mgtA riboswitch mutation, Delta UTRre-100, which had been constructed by Cromie et al., also confers similar increased thermotolerance. Surprisingly, the chr-1 mutation is located at a position that would not be predicted to be important for the regulatory function of the riboswitch. We obtained physiological evidence suggesting that the chr-1 mutation increases the cytosolic free Mg2+ concentration. High-level expression of the heterologous MgtE Mg2+ transport protein of Bacillus subtilis also enhanced the thermotolerance of S. enterica. We hypothesize that increased Mg2+ accumulation might enhance thermotolerance by protecting the integrity of proteins or membranes, by mitigating oxidative damage or acting as an inducer of thermoprotective functions.