Ammonium depletion induces changes in low molecular weight thiol levels in Escherichia coli and Bacillus subtilis

High cytoplasmic cysteine levels are potentially dangerous because they can promote ROS formation. Here, we show that NH₄Cl depletion increases free cysteine concentrations in E. coli and B. subtilis and triggers mechanisms of its homeostasis. In E. coli, excess cysteine was reduced by a twofold increase in the rate of its incorporation into glutathione, export into the medium, and degradation to form H₂S. H₂S production was reduced fourfold in the starved E. coli mstA mutant, indicating an important role of 3-mercaptopyruvate sulfotransferase in cysteine degradation. In B. subtilis, intra- and extracellular cysteine concentrations were 2.2- and 4.8-fold higher than in E. coli. Ammonium depletion caused accelerated cysteine export and intense H₂S release; bacillithiol levels were unchanged. Overall, changes in low molecular weight thiols during nitrogen starvation were similar to their changes during other stresses we have studied previously, indicating that they may be part of a universal stress response.