Magnesium transport in Salmonella typhimurium: biphasic magnesium and time dependence of the transcription of the mgtA and mgtCB loci

This study examined magnesium (Mg²⁺) transport regulation in Salmonella typhimurium, which possesses three Mg²⁺ transport systems: the constitutive CorA transporter and two P-type ATPases (MgtA and MgtB) whose genes are normally repressed. Using luciferase reporter assays, researchers tracked transcription of mgtA and mgtCB promoters in response to varying Mg²⁺ concentrations over 24 hours. The transcriptional response exhibited two distinct phases: an early phase (1-6 hours) showing 200-400-fold increases with a half-maximal response (K₀.₅) at 0.5 mM Mg²⁺, and a later phase (6-24 hours) showing up to 2000-10,000-fold increases with K₀.₅ at 0.01 mM Mg²⁺. The mgtA locus exhibited delayed kinetics compared to mgtCB. Increased transcription correlated with 1000-fold increases in functional Mg²⁺ uptake. The phoPQ two-component regulatory system was essential for mgtCB transcription at both phases but only partially controlled the late phase of mgtA transcription, suggesting a second, phoPQ-independent regulatory mechanism for mgtA. These findings reveal complex, biphasic Mg²⁺-sensing mechanisms governing magnesium transporter expression.

Key findings

• Mg²⁺-dependent transcription of mgtA and mgtCB shows biphasic kinetics with early (0-6 h) and late (6-24 h) phases having distinct Mg²⁺ sensitivities
• Early transcriptional increases (200-400-fold) have K₀.₅ ~0.5 mM Mg²⁺; late increases (up to 10,000-fold) have K₀.₅ ~0.01 mM Mg²⁺
• The phoPQ two-component system is absolutely required for mgtCB regulation but only partially controls the late phase of mgtA induction
• Large transcriptional increases translate into functional protein, with Mg²⁺ uptake increasing over 1000-fold between 8-24 hours of incubation
• At least two distinct transcriptional regulatory mechanisms control mgtA expression, while mgtCB appears to rely primarily on phoPQ-mediated regulation