may be due to the inhibition of Mg2+ loss that would otherwise occur in the presence of high levels of Co2+. It is currently unknown whether Mg2+ is more toxic when the CorBCD genes are deleted. It has been speculated that the Mg2+ ion will initially interact with any transport protein through its hydration shell. Cobalt (III) hexaammine, Co(III)Hex, is a covalently bound (non-labile) analog for the first shell of hydration for several divalent cations, including Mg2+. The radius of the Co(III)Hex molecule is 244 pm, very similar to the 250 pm radius of the first hydration shell of Mg2+. This analog is a potent inhibitor of the CorA transport system, more so than Mg2+, Co2+ or Ni2+. The additional strength of the Co(III)Hex inhibition might come from the blocking of the transport pore due to the inability of the protein to ‘dehydrate’ the substrate. It was also shown that Co(III)Hex was not transported into the cells, suggesting that at least partial dehydration would be required for the transport of the normal substrate (Mg2+). Nickel (II) hexaammine, with a radius of 255 pm, did not inhibit the CorA transport system, suggesting a maximum size limit exists for the binding of the CorA substrate ion. These results suggest that the important property involved in the recognition of Mg2+ by CorA is the size of the ion with its first shell of hydration. Hence, the volume change generally quoted for the bare to hydrated Mg2+ ion of greater than 500-fold, including the second sphere of hydration, may not be biologically relevant, and may be a reason for the first sphere volume change of 56-fold to be more commonly used. MgtA and MgtB The presence of these two genes was first suspected when Nelson and Kennedy (1972) showed that there were Mg2+-repressible and non-repressible Mg2+ uptake systems in E. coli. The non-repressible uptake of Mg2+ is mediated by the CorA protein. In S. typhimurium the repressible Mg2+ uptake was eventually shown to be via the MgtA and MgtB proteins. Both MgtA and MgtB are regulated by the PhoPQ system and are actively transcribed during the process of infection of human patients by S. typhimurium. Although neither gene is required for pathogenicity, the MgtB protein does enhance the long-term survival of the pathogen in the cell. The genes are also upregulated in vitro when the Mg2+ concentration falls below 50 μM (Snavely et al.,