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E. coli exposure to ciprofloxacin disturbs cysteine homeostasis; an increase in the intracellular concentration of cysteine is dangerous due to its ability to enhance ROS generation. Unlike wild-type bacteria, in which the cysteine content did not exceed the control level, cells of the gshA mutant lacking glutathione are characterized by increased concentration of intracellular cysteine in proportion to the concentrations of the antibiotic, despite the intensive export of cysteine into the medium. At low concentrations of ciprofloxacin, the mutant strain formed half as many colonies as the parent strain in the survival test. These findings attest to the important role of the incorporation of excess cysteine into glutathione as one of the mechanisms of cysteine homeostasis during the stress response to antibiotic. [ABSTRACT FROM AUTHOR]

Increased intracellular cysteine poses a potential danger to cells due to the high ability of cysteine to reduce free iron and promote the Fenton reaction. Here, we studied ways to maintain cysteine homeostasis in E. coli cells while inhibiting protein synthesis with valine or chloramphenicol. When growing wild-type bacteria on minimal medium with sulfate, an excess of cysteine resulting from the inhibition of protein synthesis is mainly incorporated into glutathione (up to 90%), which, therefore, can be considered as cysteine buffer. The share of hydrogen sulfide, which is the product of cysteine degradation by cysteine synthase B (CysM), does not exceed 1–3%, the rest falls on free cysteine, exported from cells. As a result, intracellular free cysteine is maintained at a low level (about 0.1 mM). The lack of glutathione in the gshA mutant increases H2S production and excretion of cysteine and leads to a threefold increase in the level of intracellular cysteine in response to valine and chloramphenicol. The relA mutants, exposed to valine, produce more H2S, dramatically accelerate the export of glutathione and accumulate more cysteine in the cytoplasm than their parent, which indicates that the regulatory nucleotide (p)ppGpp is involved in maintaining cysteine homeostasis. Disruption of cysteine homeostasis in gshA and relA mutants increases their sensitivity to peroxide stress. [ABSTRACT FROM AUTHOR]

Metabolic rearrangements that occur during depletion of essential nutrients can lead to accumulation of potentially dangerous metabolites. Here we showed that depletion of phosphate (P i), accompanied by a sharp inhibition of growth and respiration, caused a transient excess of intracellular cysteine due to a decrease in the rate of protein synthesis. High cysteine level can be dangerous due to its ability to produce ROS and reduce Fe3+ to Fenton-reactive Fe2+. To prevent these negative effects, excess cysteine was mainly incorporated into glutathione (GSH), the intracellular level of which increased by 3 times, and was also exported to the medium and partially degraded to form H 2 S with participation of 3-mercaptopyruvate sulfotransferase (3MST). The addition of P i to starving cells led to a sharp recovery of respiration and growth, GSH efflux into the medium and K+ influx into the cells. A pronounced coupling of P i , GSH, and K+ fluxes was shown upon P i depletion and addition, which may be necessary to maintain the ionic balance in the cytoplasm. We suggest that processes aimed at restoring cysteine homeostasis may be an integral part of the universal response to stress under different types of stress and for different types of bacteria. [ABSTRACT FROM AUTHOR]