Bacterial strains, plasmids, primers, and growth conditions

S. enterica serovar Typhimurium strains were derived from the wild-type strain 14028s. Unless otherwise stated, bacteria were grown at 37°C in LB broth or in N-minimal medium (pH 7.7) (44) supplemented with 0.1% casamino acids, 38 mM glycerol, and 10 mM or 10 μM of MgCl2. When necessary, antibiotics were added at the following final concentrations: ampicillin (50 μg/ml), chloramphenicol (20 μg/ml), kanamycin (50 μg/ml), and tetracycline (10 μg/ml). P22 transduction of Salmonella strains was performed as described (45). E. coli DH5a was used as a host for the preparation of plasmid DNA. Bacterial strains and plasmids used in this study are listed in table S3. Primers used in this study are listed in tables S4 and S5.

All experiments were carried out with wild-type S. enterica serovar Typhimurium strain 14028s, wild-type E. coli strains MG1655 or W3110, and mutant derivatives. LB (Becton, Dickinson and Co.), Super Optimal Broth (SOB) (Becton, Dickinson and Co.), or Super Optimal broth with Catabolite repression (SOC) (SOB supplemented with 20 mM glucose) media were used for cloning and strain construction, as noted. When necessary, ampicillin was used at 50 μg/ml, and tetracycline was used at 12.5 μg/ml. Fusaric acid (FA) plates were used for selection against tetracycline-resistant bacteria [LB agar (40 g/liter; Becton, Dickinson and Co.), NaH2PO4·H2O (10 g/liter) (Sigma-Aldrich), chlortetracycline hydrochloride (60 mg/liter) (Sigma-Aldrich), FA (12 mg/liter) (Acros Organics), and 0.1 mM ZnCl2 (Sigma-Aldrich)] (46). For experiments in which gene expression was measured, bacteria were grown in N-minimal medium (pH 7.7 or pH 4.9) (47) and the indicated concentration of MgCl2. For time course experiments, cells were grown in N-minimal medium (pH 7.7; 10 mM Mg2+) to log phase and then either washed with fresh N-minimal medium (pH 7.7; no Mg2+) twice and resuspended with N-minimal medium (pH 7.7; 10 μM Mg2+) or resuspended with N-minimal medium (pH 4.9; 10 mM Mg2+) before being incubated for the indicated times. All incubations were carried out at 37°C with shaking at 250 rpm.

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