Extracellular Mg2+ controls the activation of the Salmonella PhoP/PhoQ regulatory system. One of the adaptive responses driven by PhoP/PhoQ includes the transcriptional induction of mgtA and mgtCB, which encode two P-type Mg2+ transporters. Mg2+ also controls mgtA expression by a riboswitch located in its 5′-untranslated region (5′UTR). In this work, it was shown that the 5′UTR of both mgtA and mgtCB is responsible for a fine-tuned Mg2+-dependent regulation of these genes. Evidence was also provided that the Mg2+ riboswitch targets the mgtA transcript for degradation by RNase E when cells are grown in high Mg2+ environments.