of 82 μM and a Vmax of 354 pmol min−1 108 cells−1. Mg2+ was a competitive inhibitor with a Ki of 50 μM—the Ki of Mg2+ inhibition of 60Co2+ uptake via CorA is 10 μM. A comparison of the available kinetic data for MgtA and CorA is shown in the table. Clearly, MgtE does not transport Co2+ to the same degree as CorA, and the inhibition of transport by Mg2+ is also less efficient, which suggests that the affinity of MgtE for Mg2+ is lower than that of CorA. The strongest inhibitor of Co2+ uptake was Zn2+, with a Ki of 20 μM. The transport of Zn2+ by this protein may be as important as that of Mg2+. The table shows a comparison of the transport kinetics of MgtE and CorA, and key kinetic parameter values for them are listed. As shown, the data has been generated at differing incubation temperatures. km and Ki are not significantly altered by the differing incubation temperature. Conversely, Vmax shows a strong positive correlation with temperature, hence the value of Co2+ Vmax for MgtE is not directly comparable with the values for CorA. Yeast Early research The earliest research showing that yeast takes up Mg2+ appears to be done by Schmidt et al. (1949). However, these authors only showed altered yeast Mg2+ content in a table within the paper, and the report's conclusions dealt entirely with the metabolism of phosphate. A series of experiments by Rothstein shifted the focus more towards the uptake of the metal cations, showing that yeast take up cations with the following affinity series; Mg2+, Co2+, Zn2+ > Mn2+ > Ni2+ > Ca2+ > Sr2+. Additionally, it was suggested that the transport of the different cations is mediated by the same transport system — a situation very much like that in bacteria. In 1998, MacDiarmid and Gardner finally identified the proteins responsible for the observed cation transport phenotype in Saccharomyces cerevisiae. The genes involved in this system and a second mitochondrial Mg2+ transport system, functionally identified significantly after the gene was cloned, are described in the sections below. ALR1 and ALR2 Two genes, ALR1 and ALR2, were isolated in a screen for Al3+ tolerance (resistance) in yeast. Over-expression constructs containing yeast genomic DNA were introduced into wild type yeast and the transformants were screened for growth on toxic levels of Al3+. ALR1 and ALR2 containing plasmids allowed the growth of