Interdependence of K+ and glutamate accumulation during osmotic adaptation of Escherichia coli

Escherichia coli responds to an increase in medium osmolarity by accumulating K+ and glutamate. At low osmolarity a large fraction of cytoplasmic K+ serves to balance charge on macromolecular anions. That fraction of K+ is here referred to as "bound," as distinguished from "free" K+ that serves to balance charge of small anions. At higher osmolarity where cytoplasmic K+ increases markedly, the bound fraction decreases but the absolute amount of bound K+ expressed per unit of dry weight increases. The increase in bound K+ can be explained largely by the reduction of cytoplasmic putrescine at high osmolarity. At high osmolarity, glutamate is the major cytoplasmic anion, equal to at least 70% of free cytoplasmic K+. A sudden increase in the osmolarity of the medium stimulates glutamate synthesis with a lag of only about a minute; glutamate synthesis is almost totally dependent on K+ uptake. The high rate of flow of nitrogen through the glutamate pool under control conditions of growth at low osmolarity indicates that glutamate accumulation immediately after shift to high osmolarity must be due to inhibition of utilization of glutamate in the synthesis of other nitrogen-containing compounds rather than stimulation of glutamate synthesis. In agreement with this reasoning we find the kinetics of glutamate accumulation to be independent of the specific path of synthesis, whether by glutamate dehydrogenase or by glutamate synthase. Synthesis of glutamate appears to be required to attain normal values of the electrical membrane potential after shift to high osmolarity.