results lack comparability with the bacterial Mg2+ transport proteins. The alternative techniques of 28Mg2+ radiotracer analysis and mag-fura 2 to measure Mg2+ uptake have not yet been used with Alr1p. 28Mg2+ is currently not available and the mag-fura 2 system is unlikely to provide simple uptake data in yeast. The yeast cell maintains a heterogeneous distribution of Mg2+ suggesting that multiple systems inside the yeast are transporting Mg2+ into storage compartments. This internal transport will very likely mask the uptake process. The expression of ALR1 in S. typhimurium without Mg2+ uptake genes may be an alternative, but, as stated earlier, the effects of a heterologous expression system would need to be taken into account. MNR2 The MNR2 gene encodes a protein closely related to the Alr proteins, but includes conserved features that define a distinct subgroup of CorA proteins in fungal genomes, suggesting a distinct role in Mg2+ homeostasis. Like an alr1 mutant, growth of an mnr2 mutant was sensitive to Mg2+-deficient conditions, but the mnr2 mutant was observed to accumulate more Mg2+ than a wild-type strain under these conditions. These phenotypes suggested that Mnr2 may regulate Mg2+ storage within an intracellular compartment. Consistent with this interpretation, the Mnr2 protein was localized to the membrane of the vacuole, an internal compartment implicated in the storage of excess mineral nutrients by yeast. A direct role of Mnr2 in Mg2+ transport was suggested by the observation that increased Mnr2 expression, which redirected some Mnr2 protein to the cell surface, also suppressed the Mg2+-requirement of an alr1 alr2 double mutant strain. The mnr2 mutation also altered accumulation of other divalent cations, suggesting this mutation may increase Alr gene expression or protein activity. Recent work supported this model, by showing that Alr1 activity was increased in an mnr2 mutant strain, and that the mutation was associated with induction of Alr1 activity at a higher external Mg concentration than was observed for an Mnr2 wild-type strain. These effects were observed without any change in Alr1 protein accumulation, again indicating that Alr1 activity may be regulated directly by the Mg concentration within the cell. MRS2 and Lpe10 Like the ALR genes, the MRS2 gene was cloned and sequenced before it was identified as a Mg2+ transporter. The MRS2 gene was identified in the nuclear genome of yeast in a screen for suppressors of a mitochondrial gene RNA splicing mutation, and was cloned and sequenced by Wiesenberger