Magnesium and the Role of mgtC in Growth of Salmonella typhimurium

Salmonella typhimurium has three distinct transport systems for Mg(2+): CorA, MgtA, and MgtB. The mgtCB operon encodes two proteins, MgtC, a hydrophobic protein with a predicted molecular mass of 22.5 kDa, and MgtB, a 102-kDa P-type ATPase Mg(2+) transport protein. The mgtCB locus has been identified as part of a new Salmonella pathogenicity island, SPI-3. Transcription of mgtCB is regulated by extracellular Mg(2+) via the two-component PhoPQ regulatory system important for virulence. To elucidate MgtC’s role in a low-Mg(2+) environment, we looked at growth and transport in strains lacking the CorA and MgtA Mg(2+) transporters but expressing MgtB, MgtC, or both. mgtC mgtB(+) and mgtC(+) mgtB(+) strains exhibited growth in N minimal medium without added Mg(2+) with a 1- to 2-h lag phase. An mgtC(+) mgtB strain was also able to grow in N minimal medium without added Mg(2+) but only after a 24-h lag phase. In N minimal medium containing 10 mM Mg(2+), all strains grew after a short lag phase; the mgtC(+) mgtB strain grew to a higher optical density at 600 nm than an mgtC(+) mgtB(+) strain and was comparable to wild type. The lengthy lag phase before growth in an mgtC(+) mgtB strain was not due to lack of expression of MgtC. Western blot analysis indicated that substantial MgtC protein is present by 2 h after suspension in N minimal medium. Surprisingly, in an mgtC(+) mgtB(+) strain, MgtC was undetectable during Mg(2+) starvation, although large amounts of MgtB were observed. The lack of expression of MgtC is not dependent on functional MgtB, since a strain carrying a nonfunctional MgtB with a mutation (D379A) also did not make MgtC. Since, during invasion of eukaryotic cells, S. typhimurium appears to be exposed to a low-pH as well as a low-Mg(2+) environment, the growth of an mgtC(+) mgtB strain was tested at low pH with and without added Mg(2+). While significant quantities of MgtC could be detected after suspension at pH 5.2, the mgtC(+) mgtB strain was unable to grow at pH 5.2 whether or not Mg(2+) was present. Finally, using (63)Ni(2+) and (57)Co(2+) as alternative substrates for the unavailable (28)Mg(2+), cation uptake could not be detected in an mgtC(+) mgtB strain after Mg(2+) starvation. We conclude that MgtC is not a Mg(2+) transporter and that it does not have a primary role in the survival of S. typhimurium at low pH.