Membrane ionic currents in Rhodobacter capsulatus. Evidence for electrophoretic transport of K+, Rb+ and NH4+.
1. The cytoplasmic membrane ionic current of cells of Rhodobacter capsulatus, washed to lower the endogenous K+ concentration, had a non-linear dependence on the membrane potential measured during photosynthetic illumination. Treatment of the cells with venturicidin, an inhibitor of the H(+)-ATP synthase, increased the membrane potential and decreased the membrane ionic current at values of membrane potential below a threshold. 2. The addition of K+ or Rb+, but not of Na+, led to an increase in the membrane ionic current and a decrease in the membrane potential in either the presence or absence of venturicidin. Approximately 0.4 mM K+ or 2.0 mM Rb+ led to a half-maximal response. At saturating concentrations of K+ and Rb+, the membrane ionic currents were similar. The membrane ionic currents due to K+ and Rb+ were not additive. The K(+)-dependent and Rb(+)-dependent ionic currents had a non-linear relationship with membrane potential: the alkali cations only increased the ionic current when the membrane potential lay above a threshold value. The presence of 1 mM Cs+ did not lead to an increase in the membrane ionic current but it had the effect of inhibiting the membrane ionic current due to either K+ or Rb+. 3. Photosynthetic illumination in the presence of either K+ or Rb+, and weak acids such as acetate, led to a decrease in light-scattering by the cells. This was attributed to the uptake of potassium or rubidium acetate and a corresponding increase in osmotic strength in the cytoplasm. 4. The addition of NH4+ also led to an increase in membrane ionic current and to a decrease in membrane potential (half-maximal at 2.0 mM NH4+). The relationship between the NH4(+)-dependent ionic currents and the membrane potential was similar to that for K+. The NH4(+)-dependent and K(+)-dependent ionic current were not additive. However, illumination in the presence of NH4+ and acetate did not lead to significant light-scattering changes. The NH4(+)-dependent membrane ionic current was inhibited by 1 mM Cs+ but not by 50 microM methylamine. 5. It is proposed that the K(+)-dependent membrane ionic current is catalysed by a low-affinity K(+)-transport system such as that described in Rb. capsulatus [Jasper, P. (1978) J. Bacteriol. 133, 1314-1322]. The possibility is considered that, as well as Rb+, this transport system can also operate with NH4+. However, in our experimental conditions NH4+ uptake is followed by NH3 efflux.(ABSTRACT TRUNCATED AT 400 WORDS)
Golby
P
P
School of Biochemistry, University of Birmingham, England.
Carver
M
M
Jackson
J B
JB
D000641:Ammonia / Q000378:metabolism / Q000494:pharmacology*; D000693:Anaerobiosis; D001426:Bacterial Proteins / Q000032:analysis / Q000378:metabolism*; D001692:Biological Transport / Q000528:radiation effects; D002384:Catalysis; D003593:Cytoplasm / Q000378:metabolism; D004553:Electric Conductivity / Q000528:radiation effects; D008027:Light; D008564:Membrane Potentials / Q000187:drug effects / Q000528:radiation effects; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins; D011188:Potassium / Q000378:metabolism / Q000494:pharmacology*; D012244:Rhodospirillaceae / Q000187:drug effects / Q000378:metabolism*; D012413:Rubidium / Q000378:metabolism / Q000494:pharmacology*; D014698:Venturicidins / Q000494:pharmacology
D001426:Bacterial Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D014698:Venturicidins; D000641:Ammonia; D012413:Rubidium; D011188:Potassium