The herbicide paraquat and the antibiotic nitrofurantoin (redox-active compounds that can transfer electrons singly to oxygen) induced intracellular accumulation of the regulatory inhibitor guanosine tetraphosphate (stringency) in Escherichia coli. This mechanism is sufficient to account for the rapid bacteristasis produced in minimal medium by these agents. The growth inhibition and stringency induction were prevented by inclusion of specific amino acids in the medium. Stringency was first reported to result from amino acid starvation, with unloaded transfer ribonucleic acids (tRNAs) acting as the trigger. Previously, inhibition of growth of E. coli by paraquat and hyperbaric oxygen were shown to be prevented by inclusion in the medium of a nearly identical profile of specific amino acids, including branched-chain amino acids, which were required because of poisoning of their biosynthesis at the dihydroxyacid dehydratase site, and stringency has been induced by hyperbaric oxygen poisoning. Thus, stringency induction via a common poisoned site in branched-chain amino acid biosynthesis appears to be a shared mechanism of toxicity for these agents and hyperbaric oxygen, which also share the propensity for one-electron-transfer, free-radical reactions in cells.
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