MAGNESIUM TRANSPORT IN SALMONELLA-TYPHIMURIUM - REGULATION OF MGTA AND MGTCB DURING INVASION OF EPITHELIAL AND MACROPHAGE CELLS

Salmonella typhimurium contains two inducible Mg2+ transport systems, MgtA and MgtB, the latter encoded by a two-gene operon, mgtCB. Mg2+ de privation of S. typhimurium increases transcription of both mgtA and m gtCB over a thousandfold and a similar increase occurs upon S. typhimu rium invasion of epithelial cells. These increases are mediated by the phoPQ two-component signal transduction system, an essential system f or S. typhimurium virulence. It was therefore hypothesized that expres sion of MgtA and MgtCB is increased upon invasion of eukaryotic cells because of a lack of intravacuolar Mg2+. However, when S typhimurium w as grown at pH 5.2. the capacity of the constitutive CorA transporter in mediating Mg2+ was greater than that: at pH 7.4. Furthermore, induc tion of mgtA and mgtCB transcription was greater in the presence of a wild-type corA allele than in its absence. This implies that intravacu olar 5 typhimurium could obtain sufficient Mg2+ via the CorA system. T he 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 completel y abolished induction of mgtA at low Mg2+ concentrations but diminishe d induction of mgtCB only twofold. Adaptation of cells to acid pH by o vernight growth resulted in normal levels of induction of mgtA and mgt CB at low Mg2+ concentrations. These results imply an additional level of regulation for mgtA that is not present for mgtCB. Conversely, rep ression of mgtA and mgtCB expression by increased extracellular Mg2+ w as relatively insensitive to acid. Transcription of both loci was stro ngly induced upon invasion of the Hep-2 or CMT-93 epithelial-like or J 774 macrophage-like cell lines. However, the presence or absence of fu nctional alleles of either or both mgtA or mgtCB had no effect on inva sion efficiency or short-term survival of 5 typhimurium within the euk aryotic cells. It was concluded that the strong Mg2+-dependent inducti on of mgtA and mgtCB upon invasion of eukaryotic cells is not required because 5. typhimurium lacks sufficient Mg2+ during eukaryotic cell i nvasion and initial intravacuolar growth.