uptake by yeast. However, a recent report indicates that several of the observations made by Stadler et al. were not reproducible. For example, regulation of ALR1 mRNA accumulation by Mg2+ supply was not observed, and the stability of the Alr1 protein was not reduced by exposure to excess Mg2+. The original observation of Mg-dependent accumulation of the Alr1 protein under steady-state low-Mg conditions was replicated, but this effect was shown to be an artifact caused by the addition of a small peptide (epitope) to the protein to allow its detection. Despite these problems, Alr1 activity was demonstrated to respond to Mg supply, suggesting that the activity of the protein is regulated directly, as was observed for some bacterial CorA proteins. A functional Alr1p (wild type) or Alr2p (overexpressed) is required for S. cerevisiae growth in standard conditions (4 mM Mg2+), and Alr1p can support normal growth at Mg2+ concentrations as low as 30 μM. 57Co2+ is taken up into yeast via the Alr1p protein with a km of 77 – 105 μM (; C. MacDiarmid and R. C. Gardner, unpublished data), but the Ki for Mg2+ inhibition of this transport is currently unknown. The transport of other cations by the Alr1p protein was assayed by the inhibition of yeast growth. The overexpression of Alr1p led to increased sensitivity to Ca2+, Co2+, Cu2+, La3+, Mn2+, Ni2+ and Zn2+, an array of cations similar to those shown to be transported into yeast by a CorA-like transport system. The increased toxicity of the cations in the presence of the transporter is assumed to be due to the increased accumulation of the cation inside the cell. The evidence that Alr1p is primarily a Mg2+ transporter is that the loss of Alr1p leads to a decreased total cell content of Mg2+, but not of other cations. Additionally, two electrophysiological studies where Alr1p was produced in yeast or Xenopus oocytes showed a Mg2+-dependent current in the presence of the protein; Salih et al., in prep. The kinetics of Mg2+ uptake by Alr1p have been investigated by electrophysiology techniques on whole yeast cells. The results suggested that Alr1p is very likely to act as an ion-selective channel. In the same paper, the authors reported that Mg2+ transport by Alr1p varied from 200 pA to 1500 pA, with a mean current of 264 pA. No quantification of the amount of protein producing the current was presented, so the