REGULATION OF CELLULAR MG2-CEREVISIAE( BY SACCHAROMYCES)

Regulation of cellular Mg2+ by S. cerevisiae was investigated. The min imal concentration of Mg2+ that results in optimal growth of S. cerevi siae is about 30 mu M and a half-maximum growth rate is attained at ab out 5 mu M 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 nm ol/mg protein as the Mg2+ in synthetic growth medium is reduced to 0.0 2 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 vacuo lar 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+ i s maintained by the regulation of Mg2+ fluxes across both the vacuolar and plasma membranes.