The small protein MgtS and small RNA MgrR modulate the PitA phosphate symporter to boost intracellular magnesium levels.

In response to low levels of magnesium (Mg2+ ), the PhoQP two component system induces the transcription of two convergent genes, one encoding a 31-amino acid protein denoted MgtS and the second encoding a small, regulatory RNA (sRNA) denoted MgrR. Previous studies showed that the MgtS protein interacts with and stabilizes the MgtA Mg2+ importer to increase intracellular Mg2+ levels, while the MgrR sRNA base pairs with the eptB mRNA thus affecting lipopolysaccharide modification. Surprisingly, we found overexpression of the MgtS protein also leads to induction of the PhoRB regulon. Studies to understand this activation showed that MgtS forms a complex with a second protein, PitA, a cation-phosphate symporter. Given that the additive effect of ∆mgtA and ∆mgtS mutations on intracellular Mg2+ concentrations seen previously is lost in the ∆pitA mutant, we suggest that MgtS binds to and prevents Mg2+ leakage through PitA under Mg2+ -limiting conditions. Consistent with a detrimental role of PitA in low Mg2+ , we also observe MgrR sRNA repression of PitA synthesis. Thus, PhoQP induces the expression of two convergent small genes in response to Mg2+ limitation whose products act to modulate PitA at different levels to increase intracellular Mg2+ .

Increasing intracellular magnesium levels with the 31-amino acid MgtS protein.

Synthesis of the 31-amino acid, inner membrane protein MgtS (formerly denoted YneM) is induced by very low Mg2+ in a PhoPQ-dependent manner in Escherichia coli Here we report that MgtS acts to increase intracellular Mg2+ levels and maintain cell integrity upon Mg2+ depletion. Upon development of a functional tagged derivative of MgtS, we found that MgtS interacts with MgtA to increase the levels of this P-type ATPase Mg2+ transporter under Mg2+-limiting conditions. Correspondingly, the effects of MgtS upon Mg2+ limitation are lost in a ∆mgtA mutant, and MgtA overexpression can suppress the ∆mgtS phenotype. MgtS stabilization of MgtA provides an additional layer of regulation of this tightly controlled Mg2+ transporter and adds to the list of small proteins that regulate inner membrane transporters.