Salmonella typhimurium contains two inducible Mg transport systems, MgtA and MgtB, the latter encoded by a two-gene operon, mgtCB. Mg deprivation of S. typhimurium increases transcription of both mgtA and mgtCB over a thousandfold and a similar increase occurs upon S. typhimurium invasion of epithelial cells. These increases are mediated by the phoPQ two-component signal transduction system, an essential system for S. typhimurium virulence. It was therefore hypothesized that expression of MgtA and MgtCB is increased upon invasion of eukaryotic cells because of a lack of intravacuolar Mg. However, when S. typhimurium was grown at pH 5.2, the capacity of the constitutive CorA transporter in mediating Mg was greater than that at pH 7.4. Furthermore, induction of mgtA and mgtCB transcription was greater in the presence of a wild-type corA allele than in its absence. This implies that intravacuolar S. typhimurium could obtain sufficient Mg via the CorA system. The effect of acid pH on mgtA and mgtCB transcription was also measured. Compared to induction at pH 7.4, exposure to pH 5.2 almost completely abolished induction of mgtA at low Mg concentrations but diminished induction of mgtCB only twofold. Adaptation of cells to acid pH by overnight growth resulted in normal levels of induction of mgtA and mgtCB at low Mg concentrations. These results imply an additional level of regulation for mgtA that is not present for mgtCB. Conversely, repression of mgtA and mgtCB expression by increased extracellular Mg was relatively insensitive to acid. Transcription of both loci was strongly induced upon invasion of the Hep-2 or CMT-93 epithelial-like or J774 macrophage-like cell lines. However, the presence or absence of functional alleles of either or both mgtA or mgtCB had no effect on invasion efficiency or short-term survival of S. typhimurium within the eukaryotic cells. It was concluded that the strong Mg-dependent induction of mgtA and mgtCB upon invasion of eukaryotic cells is not required because S. typhimurium lacks sufficient Mg during eukaryotic cell invasion and initial intravacuolar growth.


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