Increased expression of Mg(2+) transport proteins enhances the survival of Salmonella enterica at high temperature

Abstract

Mg(2+) homeostasis is important for Salmonella pathogenesis. In Salmonella enterica, the transcription of the mgtA gene, which encodes a Mg(2+) transporter, is regulated by a Mg(2+)-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, DeltaUTR(re-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 Mg(2+) concentration. High-level expression of the heterologous MgtE Mg(2+) transport protein of Bacillus subtilis also enhanced the thermotolerance of S. enterica. We hypothesize that increased Mg(2+) accumulation might enhance thermotolerance by protecting the integrity of proteins or membranes, by mitigating oxidative damage or acting as an inducer of thermoprotective functions.

Fig. 1.

The thermotolerance of the strains TL1 (wild type; A) and TL3290 (chr-1; B) was determined at 53 °C in exponentially growing cells in M63 + 10 mM glucose containing the indicated supplements, as described in Materials and Methods. GB, 1 mM glycine betaine.

Fig. 2.

The chr-1 mutation caused a C to U change at position +98 of the mgtA riboswitch. The stem and loop structures formed at low and high Mg²⁺ were proposed by Cromie et al. (31). The beginning and endpoints of the interval from positions 148 to 247 that was replaced by an unrelated sequence of 84 nt in the ΔUTR^re-100 mutation (31) are indicated by the arrows. [Reproduced with permission from ref. (Copyright 2006, Elsevier).]

Fig. 3.

The effect of the ΔUTR^re-100 mutation on thermotolerance. This experiment with strain TL4391 was carried out at the same time and under the same conditions as those shown in Fig. 1. GB is 1 mM glycine betaine.

Fig. 4.

High-level expression of the MgtE protein also confers increased thermotolerance. Strain TL4445 carrying the empty cloning vector pBADMycHis(a) (A) and strain TL4449 carrying the B. subtilis mgtE gene under the control of the arabinose-inducible pBAD promoter (B) were grown in M63 + 20 mM glycerol without (−Ara) or with 1 mM L-arabinose (+Ara) and their heat resistance determined at 53 °C, as described in Materials and Methods.