FIG. 2.

Influence of the nine single amino acid substitutions in KdpD on the regulation of kdpFABC expression under K⁺ limitation and osmotic stress. E. coli strain HAK006 was transformed with plasmid pBD5-9 or pBD5-9*, coding for KdpD or its derivatives carrying the indicated replacements. Cells were grown under K⁺-limiting conditions (A and B) or osmotic stress conditions induced by NaCl (C and D) or sucrose (E and F) (for details, see Materials and Methods). The corresponding K⁺, NaCl, and sucrose concentrations in the media are indicated. It should be noted that under osmotic stress conditions the medium still contains 6.6 mM K⁺. β-Galactosidase activities were determined as described in Materials and Methods and are given in Miller units. The data presented in graphs A, C, and E represent average values obtained in at least three independent experiments. In graphs B, D, and F, the corresponding induction ratios (inducing/noninducing conditions) are shown: 0.1 mM K⁺/115 mM K⁺ (B), 0.6 M NaCl/0 M NaCl (D), and 0.6 M sucrose/0 M sucrose (F).