Transmembrane Proton Electrochemical Gradients in Dark Aerobic and Anaerobic Cells of the Cyanobacterium (Blue-Green Alga) Anacystis nidulans

Publisher Website
Google Scholar
Abstract
The transmembrane proton electrochemical potential gradient .delta..mu.H+ in whole cells of Anacystis nidulans was measured in aerobic and anaerobic dark conditions using the distribution, between external medium and cell interior, of radioactivity labeled weak acids (acetylsalicyclic acid, 5,5-dimethyloxazolidine-2,4-dione) or bases (imidazole, methylamine), and permeant ions (tetraphenylphosphonium cation, thiocyanate anion), as determined by flow dialysis. Alternatively, the movements across the plasma membrane of .delta.pH-indicating atebrin or 9-aminoacridine, and of .DELTA..psi.-indicating 8-anilino-1-naphthalenesulfonate were qualitatively followed by fluorescence measurements. Attempts were made to discriminate between the individual chemiosmotic gradients across the cytoplasmic (plasmalemma) and the intracytoplasmic (thylakoid) membranes. By use of the ionophore nigericin, monensin, and valinomycin, the components of the protonmotive force, namely the proton concentration gradient .DELTA.pH and the electric membrane potential .DELTA..psi. were shown to be mutually exchangeable within the range of external pH vaues tested (3.2-11.0). Both components were depressed by the uncoupler carbonylcyanide m-chlorophenylhydrazone, though inhibition of .DELTA.pH was much more pronounced than that of .DELTA..psi., notably in the alkaline pHo range. The total proton electrochemical gradient across the plasma membrane was significantly higher in aerobic than in anaerobic cells and increased markedly (i.e. became more negative) towards lower pHo values. This increase was paralleled by a similar increase in the rate of endogenous respiration of the cells. At the same time the ATPase inhibitor dicyclohexylcarbodiimide only slightly affected the proton motive force across the plasma membrane of aerobic cells. The results will be discussed in terms of a respiratory competent plasma membrane in Anacystis nidulans.