against their chemical gradient. Low levels of extracellular Mg are capable of increasing transcription of these transport proteins, which increases transport of Mg into Salmonella. The CorA system has three membrane-spanning segments. This system mediates Mg influx; however, at extremely high extracellular Mg concentrations, this protein can also mediate Mg efflux. Another cell membrane Mg transport protein exists in erythrocytes (RBCs). This RBC Na+-Mg2+ antiporter (not shown here) facilitates the outward movement of Mg from erythrocytes in the presence of extracellular Na+ and intracellular adenosine triphosphate (ATP) [4,5]. ADP—adenosine diphosphate; C—carbon; N—nitrogen. (From Smith and Maguire [4]).

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