In 1968, Lusk described the limitation of bacterial (Escherichia coli) growth on Mg-poor media, suggesting that bacteria required Mg and were likely to actively take this ion from the environment. The following year, the same group and another group, Silver, independently described the uptake and efflux of Mg in metabolically active E. coli cells using Mg. By the end of 1971, two papers had been published describing the interference of Co, Ni and Mn on the transport of Mg in E. coli and in Aerobacter aerogenes and Bacillus megaterium. In the last major development before the cloning of the genes encoding the transporters, it was discovered that there was a second Mg uptake system that showed similar affinity and transport kinetics to the first system, but had a different range of sensitivities to interfering cations. This system was also repressible by high extracellular concentrations of Mg