Abstract
Cobalt is toxic for cells, but mechanisms of this toxicity are largely unknown. The biochemical and genetic experiments reported here demonstrate that iron-sulfur proteins are greatly affected in cobalt-treated Escherichia coli cells. Exposure of a wild-type strain to intracellular cobalt results in the inactivation of three selected iron-sulfur enzymes, the tRNA methylthio-transferase, aconitase, and ferrichrome reductase. Consistently, mutant strains lacking the [Fe-S] cluster assembly SUF machinery are hypersensitive to cobalt. Last, expression of iron uptake genes is increased in cells treated with cobalt. In vitro studies demonstrated that cobalt does not react directly with fully assembled [Fe-S] clusters. In contrast, it reacts with labile ones present in scaffold proteins (IscU, SufA) involved in iron-sulfur cluster biosynthesis. We propose a model wherein cobalt competes out iron during synthesis of [Fe-S] clusters in metabolically essential proteins.
Highlights
Cobalt is required as a trace element in procaryotes and eucaryotes to fulfill a variety of metabolic functions
In this report we show that (i) exposure of E. coli cells to intracellular cobalt results in the inactivation of [Fe-S] enzymes, (ii) E. coli mutant strains lacking suf genes are much more sensitive to Co, and inactivation of aconitase is more severe in a sufC mutant, (iii) Co-treated cells respond by increased expression of Fur-repressed genes, and (iv) in vitro cobalt has a direct and specific effect on clusters chelated by scaffold proteins involved in [Fe-S] cluster biosynthesis
Soluble extracts of E. coli cells grown in LB containing 200 M cobalt were analyzed for cobalt content by colorimetric assays and atomic absorption; they contained 105–130 M Co, whereas untreated cells contained 5–10 M (Table 2)
Summary
In this report we show that (i) exposure of E. coli cells to intracellular cobalt results in the inactivation of [Fe-S] enzymes, (ii) E. coli mutant strains lacking suf genes are much more sensitive to Co, and inactivation of aconitase is more severe in a sufC mutant, (iii) Co-treated cells respond by increased expression of Fur-repressed genes, and (iv) in vitro cobalt has a direct and specific effect on clusters chelated by scaffold proteins involved in [Fe-S] cluster biosynthesis. Together these observations indicate that cobalt toxicity is. Related to its effect on iron metabolism and, in particular, on the [Fe-S] cluster assembly process during de novo synthesis or repair
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