Magnesium transport in prokaryotes.

Possessing the largest hydrated radius, the smallest ionic radius, and the highest charge density among the biologically relevant cations, Mg(2+) provides an interesting problem for transport into living cells. Transport systems for Mg(2+) have been characterized primarily in Salmonella typhimurium because the well-developed genetics of Gram-negative bacteria make cloning and studying the transporters a viable proposition. The CorA transport system is expressed constitutively and is the major Mg(2+) transporter in Eubacteria and Archaea. It has three transmembrane domains, a uniquely large periplasmic domain, and no sequence homology to other proteins. The MgtE Mg(2+) transporter also lacks sequence homology to other proteins, and it is unclear if Mg(2+) transport is its primary function. The MgtA and MgtB Mg(2+) transporters have sequence homology to P-type ATPases. They are more closely related to the mammalian Ca(2+)-ATPases than to the prokaryotic P-type ATPases. MgtA and MgtB mediate Mg(2+) influx with, rather than against, the Mg(2+) electrochemical gradient. Unlike corA and mgtE, the mgtA and mgtC/mgtB loci are regulated, being induced by the two-component regulatory system PhoP/PhoQ. PhoQ is a Mg(2+) membrane sensor kinase that phosphorylates the transcription factor PhoP under Mg(2+)-limiting conditions. This factor then induces transcription of mgtA and mgtCB. MgtC, which is encoded by the first gene in the mgtCB operon, has no sequence homology to any known protein and is essential for S. typhimurium virulence in mice and macrophages, but does appear to be a Mg(2+) transporter. The physiological roles of these Mg(2+) transporters and their mechanisms are not yet completely clear, but initial data indicate that Mg(2+) transporters are unique transport systems with unusual mechanisms for mediating Mg(2+) movement through the membrane.