Characterization of the cytoplasm of Escherichia coli K-12 as a function of external osmolarity. Implications for protein-DNA interactions in vivo.

The water-accessible volumes, the amounts of all significant osmolytes, and the protein concentration in the cytoplasm of aerobically grown Escherichia coli K-12 have been determined as a function of the osmolarity of the minimal growth medium. The volume of cytoplasmic water (Vcyto) decreases linearly with increasing osmolarity from 2.23(+/- 0.12) microliters/mg dry weight in cells grown at 0.10 OSM to 1.18(+/- 0.06) microliters/mg dry weight at 1.02 OSM. Above 0.28 OSM, growth rate decreases linearly with increasing osmolarity. The growth rate extrapolates to zero at an osmolarity of approximately 1.8, corresponding to an estimated Vcyto of 0.5(+/- 0.2) microliters/mg dry weight. Measurements of Vcyto in titrations of non-growing cells with the plasmolyzing agent NaCl were used to obtain volumes of "bound" water (presumably water of macromolecular hydration) and cytoplasmic osmotic coefficients for cells grown in medium of low (0.10 OSM) and moderate (0.28 OSM) osmolarity. The volume of bound water Vb is similar in the two osmotic conditions (Vb = 0.40(+/- 0.04) microliters/mg dry wt), and corresponds to approximately 0.5 g H2O/g cytoplasmic macromolecule. Since Vcyto decreases with increasing osmolarity, whereas Vb appears to be independent of osmolarity, water of hydration becomes a larger fraction of Vcyto as the osmolarity of the growth medium increases. Growth appears to cease at the osmolarity where Vcyto is approximately equal to Vb. K+ and glutamate (Glu-) are the only significant cytoplasmic osmolytes in cells grown in medium of low osmolarity. The amount of K+ greatly exceeds that of Glu-. Analysis of cytoplasmic electroneutrality indicates that the cytoplasm behaves like a concentrated solution of the K+ salt of cytoplasmic polyanions, in which the amount of additional electrolyte (K+ Glu-) increases with increasing osmolarity. As the osmolarity of the growth medium becomes very low, the cytoplasm approaches an electrolyte-free K+-polyanion solution. In vivo osmotic coefficients were determined from the variation of Vcyto with external osmolarity in plasmolysis titrations of non-growing cells. The values obtained (phi = 0.54(+/- 0.06) for cells grown at 0.10 OSM and phi = 0.71(+/- 0.11) at 0.28 OSM) indicate a high degree of non-ideality of intracellular ions arising from coulombic interactions between K+ and cytoplasmic polyanions. Analysis of these osmotic coefficients using polyelectrolyte theory indicates that the thermodynamic activity of cytoplasmic K+ increases from approximately 0.14 M in cells grown at an external osmolarity of 0.10 OSM to approximately 0.76 M at 1.02 OSM.(ABSTRACT TRUNCATED AT 400 WORDS)