Ag. Tkachenko et al., PUTRESCINE POTASSIUM EXCHANGE AS AN ADAPTIVE RESPONSE OF ESCHERICHIA-COLI TO HYPEROSMOTIC STRESS/, Microbiology, 66(3), 1997, pp. 274-278
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.