FIGURE 3.

Transport activity of NhaS3. A, pH dependence of Na⁺/H⁺ antiporter activities of NhaS3 in inverted membrane vesicles prepared from nhaS3-expressing E. coli. Fluorescence from acridine orange was monitored. The percentage of fluorescence dequenching observed after the addition of 5 mm NaCl was plotted relative to that of lactate-induced quenching. EV, empty vector; *, not detected. B, complementation of an E. coli K⁺ uptake mutant (LB2003) by NhaS3. Growth of E. coli LB2003 expressing nhaS3 or Synechocystis K⁺ uptake transporter KtrABE on synthetic solid medium containing 10 mm KCl. C, K⁺ uptake activity by NhaS3 in E. coli. Cells containing NhaS3 or the empty vector were incubated with 1 mm KCl in HEPES-NaOH, pH 7.5, and aliquots were withdrawn at the indicated times. D, measurement of K⁺ or Mg²⁺ efflux mediated by NhaS3. K⁺, Mg²⁺, or Li⁺, instead of Na⁺ were added to membrane vesicles prepared from nhaS3-expressing E. coli cells. Antiporter activity was determined by the same procedure as in A. *, not detected.