Cadmium Toxicity in Glutathione Mutants of Escherichia coli

The higher affinity of Cd 2+ for sulfur compounds than for nitrogen and oxygen led to the theoretical consideration that cadmium toxicity should result mainly from the binding of Cd 2+ to sulfide, thiol groups, and sulfur-rich complex compounds rather than from Cd 2+ replacement of transition-metal cations from nitrogen- or oxygen-rich biological compounds. This hypothesis was tested by using Escherichia coli for a global transcriptome analysis of cells synthesizing glutathione (GSH; wild type), γ-glutamylcysteine (Δ gshB mutant), or neither of the two cellular thiols (Δ gshA mutant). The resulting data, some of which were validated by quantitative reverse transcription-PCR, were sorted using the KEGG (Kyoto Encyclopedia of Genes and Genomes) orthology system, which groups genes hierarchically with respect to the cellular functions of their respective products. The main difference among the three strains concerned tryptophan biosynthesis, which was up-regulated in wild-type cells upon cadmium shock and strongly up-regulated in Δ gshA cells but repressed in Δ gshB cells containing γ-glutamylcysteine instead of GSH. Overall, however, all three E. coli strains responded to cadmium shock similarly, with the up-regulation of genes involved in protein, disulfide bond, and oxidative damage repair; cysteine and iron-sulfur cluster biosynthesis; the production of proteins containing sensitive iron-sulfur clusters; the storage of iron; and the detoxification of Cd 2+ by efflux. General energy conservation pathways and iron uptake were down-regulated. These findings indicated that the toxic action of Cd 2+ indeed results from the binding of the metal cation to sulfur, lending support to the hypothesis tested.