Elevation of cellular Mg2+ levels by the Mg2+ transporter, Alr1, supports growth of polyamine-deficient Saccharomyces cerevisiae cells

The polyamines putrescine, spermidine, and spermine are required for normal eukaryotic cellular functions. However, the minimum requirement for polyamines varies widely, ranging from very high concentrations (mm) in mammalian cells to extremely low in the yeast Saccharomyces cerevisiae. Yeast strains deficient in polyamine biosynthesis (spe1?, lacking ornithine decarboxylase, and spe2?, lacking SAM decarboxylase) require externally supplied polyamines, but supplementation with as little as 10(?8) m spermidine restores their growth. Here, we report that culturing a spe1? mutant or a spe2? mutant in a standard polyamine-free minimal medium (SDC) leads to marked increases in cellular Mg2+ content. To determine which yeast Mg2+ transporter mediated this increase, we generated mutant strains with a deletion of SPE1 or SPE2 combined with a deletion of one of the three Mg2+ transporter genes, ALR1, ALR2, and MNR2, known to maintain cytosolic Mg2+ concentration. Neither Alr2 nor Mnr2 was required for increased Mg2+ accumulation, as all four double mutants (spe1? alr2?, spe2? alr2?, spe1? mnr2?, and spe2? mnr2?) exhibited significant Mg2+ accumulation upon polyamine depletion. In contrast, a spe2? alr1? double mutant cultured in SDC exhibited little increase in Mg2+ content and displayed severe growth defects compared with single mutants alr1? and spe2? under polyamine-deficient conditions. These findings indicate that Alr1 is required for the up-regulation of the Mg2+ content in polyamine-depleted cells and suggest that elevated Mg2+ can support growth of polyamine-deficient S. cerevisiae mutants. Up-regulation of cellular polyamine content in a Mg2+-deficient alr1? mutant provided further evidence for a cross-talk between Mg2+ and polyamine metabolism.