PUTRESCINE POTASSIUM EXCHANGE AS AN ADAPTIVE RESPONSE OF ESCHERICHIA-COLI TO HYPEROSMOTIC STRESS/

Putrescine/potassium exchange in response to hyperosmotic stress was s tudied. The addition of 0.3 M NaCl or 0.44 M sucrose to an exponential ly growing E. coli culture induced potassium uptake and putrescine rel ease from the cell. Potassium added to an osmotically stressed potassi um-deficient culture was readily absorbed by cells; this was accompani ed by the loss of intracellular putrescine, both free and bound. Since DNA is the main binding site of putrescine, the loss of bound putresc ine caused a relaxation of DNA supercoiling. The increase in the intra cellular content of potassium not only restored but also enhanced DNA supercoiling as compared to the initial level. In vitro experiments sh owed the degree of plasmid DNA supercoiling to rise drastically at pot assium concentrations of 300-500 mM, while different putrescine concen trations affected this parameter differently. Thus, the physiological concentrations of putrescine (below 1 mM) greatly augmented DNA superc oiling, whereas higher concentrations (5-10 mM) exerted a relaxing eff ect. A change in DNA supercoiling in vivo in response to osmotic stres s is the result of competition between biogenic and abiogenic cations for the sites of binding to polyanionic DNA structures. A change in DN A topology serves as the regulatory factor controlling the expression of genes responsible for cell adaptation to osmotic stress.