Regulation of cellular Mg²⁺ by Saccharomyces cerevisiae

Regulation of cellular Mg²⁺ by S. cerevisiae was investigated. The minimal concentration of Mg²⁺ that results in optimal growth of S. cerevisiae is about 30 μM and a half-maximum growth rate is attained at about 5 μM Mg²⁺. Since the plasma membrane has an electrical potential greater than 100 mV, passive equilibration of Mg²⁺ across the plasma membrane would provide sufficient cytosolic Mg²⁺ (0.1-1 mM). The total cellular Mg²⁺ of cells grown in synthetic medium containing 1 mM Mg²⁺ is about 400 nmol/mg protein, most of which is bound to polyphosphate, nucleic acids, and ATP. Total cellular Mg²⁺ decreases to about 80 nmol/mg protein as the Mg²⁺ in synthetic-growth medium is reduced to 0.02 mM, but remains relatively constant in growth medium containing 1 to 100 mM Mg²⁺. Cells shifted into Mg²⁺-free medium continue to grow by utilizing the vacuolar Mg²⁺ stores. Mg²⁺-starved cells replenish vacuolar Mg²⁺ stores with a halftime of 30 min. following the addition of 1 mM Mg²⁺ to the growth medium. The data indicate that cytosolic Mg²⁺ is maintained by the regulation of Mg²⁺ fluxes across both the vacuolar and plasma membranes.