[Role of putrescine and potassium transport in the adaptation of Escherichia coli to ammonium starvation].

Ammonium depletion in the nutrient medium induced an active transport of putrescine into the cell, which was not associated with its utilization as a nitrogen source. The uptake of putrescine was accompanied by a proportional release of potassium from the cells. Quantitative analysis of free as well as weakly and tightly bound pools of putrescine showed that this diamine was bound mainly to DNA. Topological studies by the plasmid method indicated an increase in the DNA supercoiling in response to the putrescine transport. In vitro experiments made it possible to establish an ambiguous dependence of DNA topology on putrescine content--its physiological concentrations (0.5-1.0 mM) enhanced DNA supercoiling, while higher concentrations (2-10 mM) caused gradual relaxation of DNA. A possible physiological significance of these effects in adaptive response of cells to ammonium deficiency is discussed.