had a different range of sensitivities to interfering cations. This system was also repressible by high extracellular concentrations of Mg2+ . CorA The CorA gene and its corresponding protein are the most exhaustively studied Mg2+ transport system in any organism. Most of the published literature on the CorA gene comes from the laboratory of M. E. Maguire. Recently the group of R. J. Schweyen made a significant impact on the understanding of Mg2+ transport by CorA. The gene was originally named after the cobalt-resistant phenotype in E. coli that was caused by the gene's inactivation. The gene was genetically identified in E. coli by Park et al., but wasn't cloned until Hmiel et al. isolated the Salmonella enterica serovar Typhimurium (S. typhimurium) homologue. Later it would be shown by Smith and Maguire that the CorA gene was present in 17 gram-negative bacteria. With the large number of complete genome sequences now available for prokaryotes, CorA has been shown to be virtually ubiquitous among the Eubacteria, as well as being widely distributed among the Archaea. The CorA locus in E. coli contains a single open reading frame of 948 nucleotides, producing a protein of 316 amino acids. This protein is well conserved amongst the Eubacteria and Archaea. Between E. coli and S. typhimurium, the proteins are 98% identical, but in more distantly related species, the similarity falls to between 15 and 20%. In the more distantly related genes, the similarity is often restricted to the C-terminal part of the protein, and a short amino acid motif GMN within this region is very highly conserved. The CorA domain, also known as PF01544 in the pFAM conserved protein domain database (http://webarchive.loc.gov/all/20110506030957/http%3A//pfam.sanger.ac.uk/), is additionally present in a wide range of higher organisms, and these transporters will be reviewed below. The CorA gene is constitutively expressed in S. typhimurium under a wide range of external Mg2+ concentrations. However, recent evidence suggests that the activity of the protein may be regulated by the PhoPQ two-component regulatory system. This sensor responds to low external Mg2+ concentrations during the infection process of S. typhimurium in humans. In low external Mg2+ conditions, the PhoPQ system was reported to suppress the function of CorA and it has been previously shown that the transcription of the alternative Mg2+ transporters MgtA and MgtB is activated in these conditions. Chamnongpol and Groisman suggest that this allows the bacteria to escape metal ion toxicity