A KDP-LIKE, HIGH-AFFINITY, K+-TRANSLOCATING ATPASE IS EXPRESSED DURING GROWTH OF RHODOBACTER-SPHAEROIDES IN LOW POTASSIUM MEDIA - DISTRIBUTION OF THIS K+-ATPASE AMONG PURPLE NONSULFUR PHOTOTROPHIC BACTERIA

Cells of the purple non-sulphur bacterium Rhodobacter sphaeroides express a high-affinity K+ uptake system when grown in media with low K+ concentrations. Antibodies against the catalytic KdpB protein or the whole KdpABC complex of Escherichia coli cross-react with a 70.0 kDa R. sphaeroides protein that was expressed only in cells grown in media with low K+ concentrations. In membranes derived from R. sphaeroides cells grown with low K+ concentrations (induced cells), a high ATPase activity could be detected when assayed in Tris-HCI pH 8.0 containing 1 mM MgSO4. This ATPase activity increased upon addition of 1 mM KCl from 166 to 289 mumol ATP hydrolysed x min-1 x g protein-1 (1.7-fold stimulation). The K+-stimulated ATPase activity was inhibited approximately 93% by 0.5 mM vanadate but hardly by NN-dicyclohexylcarbodiimide (DCCD). These results indicate that the inducible K+-ATPase in R. sphaeroides resembles the Kdp K+-translocating ATPase of Escherichia coli. This Kdp-like transport system is also expressed in R. capsulatus and Rhodospirillum rubrum during growth in media with low K+ concentrations suggesting a wide distribution of this transport system among phototrophic bacteria.