Magnesium and the role of MgtC in growth of Salmonella typhimurium.
Salmonella typhimurium has three distinct transport systems for Mg2+: 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 Mg2+ 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 Mg2+ via the two-component PhoPQ regulatory system important for virulence. To elucidate MgtC's role in a low-Mg2+ environment, we looked at growth and transport in strains lacking the CorA and MgtA Mg2+ transporters but expressing MgtB, MgtC, or both. mgtC mgtB+ and mgtC+ mgtB+ strains exhibited growth in N minimal medium without added Mg2+ with a 1- to 2-h lag phase. An mgtC+ mgtB strain was also able to grow in N minimal medium without added Mg2+ but only after a 24-h lag phase. In N minimal medium containing 10 mM Mg2+, 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 Mg2+ 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-Mg2+ environment, the growth of an mgtC+ mgtB strain was tested at low pH with and without added Mg2+. 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 Mg2+ was present. Finally, using 63Ni2+ and 57Co2+ as alternative substrates for the unavailable 28Mg2+, cation uptake could not be detected in an mgtC+ mgtB strain after Mg2+ starvation. We conclude that MgtC is not a Mg2+ transporter and that it does not have a primary role in the survival of S. typhimurium at low pH.
Moncrief
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MB
Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106-4965, USA. mcm6@po.cwru.edu
Maguire
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ME
D000251:Adenosine Triphosphatases / Q000235:genetics / Q000502:physiology; D001426:Bacterial Proteins / Q000502:physiology*; D001692:Biological Transport; D002352:Carrier Proteins / Q000235:genetics / Q000502:physiology*; D027682:Cation Transport Proteins; D003035:Cobalt / Q000378:metabolism; D003470:Culture Media; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000378:metabolism*; D009532:Nickel / Q000378:metabolism; D012486:Salmonella typhimurium / Q000254:growth & development* / Q000378:metabolism
D001426:Bacterial Proteins; D002352:Carrier Proteins; D027682:Cation Transport Proteins; D003470:Culture Media; D003035:Cobalt; D009532:Nickel; D000251:Adenosine Triphosphatases; C083836:MgtB protein, Salmonella typhimurium; C093586:MgtC protein, Salmonella typhimurium; D008274:Magnesium