FIGURE 3. Expression of the mmaK renders the wild-type E. coli cells sensitive to externally added K⁺ ion.

A, phenotyping of the cells carrying the empty control plasmid (top row) or the pB13d-MmaK plasmids (bottom row) on tryptone-based agarose plates. On the plate containing only tryptone (plate 1) or tryptone with an additional 10 or 50 mM NaCl (plates 2 and 3), the colonies of mmaK-bearing cells are more translucent (see “Discussion”), but the colony-forming units of the mmaK-bearing cells are similar in plates 1, 2, and 3, indicating that these media do not restrict growth. However, on the plate containing an additional 10 or 50 mM KCl (plates 4 and 5), the colony-forming units of the mmaK-bearing cells were reduced by >10⁵-fold. Each spot was inoculated from 5 μl of a series of 10-fold dilution of stationary cells (10²-, 10³-, 10⁴-, 10⁵-, and 10⁶-fold from left to right). B, phenotype in the liquid medium and effects of adding Na⁺ or K⁺ to exponentially growing cultures. In the tryptone medium (panel 1), the growth rate of the mmaK-bearing cells (filled circles) is similar to that of the control cells (open circles); however, the cell density of the stationary mmaK-bearing cells is lower than that of the control cells. Addition of 10 mM KCl (arrow in panel 3), but not 10 mM NaCl (panel 2), immediately stops the growth of the mmaK-bearing cells, but not the control cells without mmaK. For the growth curve measurements, stationary cells in modified Luria Bertani medium were washed once with fresh tryptone medium and inoculated into fresh tryptone medium to A₆₀₀ ~ 0.02 as the time “0”, and the cultures were incubated aerobically by rotating at 275 rpm in 37 °C. Growth was monitored by measuring the optical densities at 600 nm. Data were averaged from duplicate experiments of independent transformants (standard deviations are shown).