Phosphate starvation is accompanied by disturbance of intracellular cysteine homeostasis in Escherichia coli
Metabolic rearrangements that occur during depletion of essential nutrients can lead to accumulation of potentially dangerous metabolites. Here we showed that depletion of phosphate (Pi), 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 Fe3þ to Fenton-reactive Fe2þ. 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 H2S with participation of 3- mercaptopyruvate sulfotransferase (3MST). The addition of Pi to starving cells led to a sharp recovery
of respiration and growth, GSH efflux into the medium and Kþ influx into the cells. A pronounced coupling of Pi, GSH, and Kþ
fluxes was shown upon Pi 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.
