Silicate uptake by Nitzschia alba cells is higher in medium containing Na+ than in media lacking Na+ but containing K+, Rb+, NH4W, Li+, or choline+. The initial rate is inhibited by monensin and gramicidmi but not by valinomycin or nigericin and is less sensitive to inhibition by carbonyl cyanide m-chlorophenylhydrazone (CCCP). In isolated membrane vesicles, silicate is taken up when a Na+ gradient is imposed across the membrane or is generated by cytoplasmic Na4,K+-ATPase. H+ or K+ gradients in either direction do not stimulate uptake. Na+-gradient-dependent uptake is inhibited by monensin but not by CCCP, valinomycin, or vanadate, which inhibits the cytoplasmic Nav+a+-ATPase. Uptake increases if an internally negative potential is imposed across the membrane. The vesicular uptake shows saturation kinetics with a Km of 62 jiM and a Vmax of 4.1 nmol/mg of protein per min. In intact cells, the initial rate of silicate uptake increases with pH up to 9.5. Thus, in N. alba, silicate is symported with Na+, and the transport system is driven by the Na+ gradient that is generated and maintained across the membrane by the activity of Na+,K+-ATPase.Silicon is an essential trace element for basic processes ranging from DNA synthesis to bone formation, and in mammalian systems it acts as both a metabolite and a cytotoxic factor (1). But biological research on silicon is difficult, and what is known of its metabolic role has been gained from diatom studies. Diatoms use the silicate ion for wall formation (2), DNA replication (3, 4), nuclear DNA polymerase (5) and thymidylate kinase (4) synthesis, and cyclic AMP and cyclic GMP formation (6); hence diatoms possess specific active silicate transport system(s) (7-10). But little is known of how energy is coupled to silicate transport or of the energy-conserving processes in diatoms. Mitchell's chemosmotic theory of energy conservation in the form of ion gradients has proven to be true in various organisms (11)(12)(13)(14). In eukaryotic mammalian cells, the Na+ gradient, generated and maintained by plasma membranebound Na+,K+-ATPase, is coupled to most transport systems (15)(16)(17). Diatoms also possess a Na+,K+-ATPase in the plasma membrane, though it differs from the mammalian ATPase in many properties (18). Whether this enzyme or other energyyielding processes in the diatoms generate ion gradients has not been examined.In the fresh water diatom Nadculla pelliuulosa, Na+ is somewhat effective in promoting silicate uptake, K+ is half as effective as Na+, and NH+ and Li+ are ineffective (9). Glucose and amino acid transport systems in the marine diatom Cyclotella cryptica were found to be Na+ dependent (19), though it was suggested that the Na+ dependence may also be caused by the indirect effect of Na+ on the energy metabolism. On the The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. 6386other hand, K+-depend...
Osmotically shocked Escherichia coli and membrane vesicle ghosts from E. coli cells have lost the ability to accumulate potassium by active transport. The addition of valinomycin to the membrane ghosts restores the capacity to accumulate radioactive 42K and 86Rb by a temperature-and energy-dependent process. Membrane vesicles prepared from mutants of E. coli altered in potassium transport show defects in the valinomycin-stimulated accumulation of 42K that are related to the defects in the intact cells.Valinomycin is a cyclic depsipeptide antibiotic (1, 2) that acts by greatly increasing the permeability of membranes, specifically to potassium ions (3). Membranes whose permeability to potassium is altered by valinomycin include bacterial (3-5), erythrocyte (6-8), mitochondrial (9), and artificial black lipid membranes (10). The change is highly specific: only permeability to potassium and the related alkaline cations rubidium and cesium is affected (7, 10). The membranes remain impermeable to protons, sodium, lithium, and other ions in the presence of valinomycin. Evidence from relatively thick artificial membranes shows that valinomycin acts not by forming channels or pores through which potassium can travel, but by forming one-to-one complexes with K+ ions (8) which can then diffuse across the hydrophobic lipid membranes, with the K+ sheltered from the membrane within the cyclic folds of the valinomycin molecule (2). With mitochondria, at least, valinomycin facilitates the net accumulation of potassium (9) and this finding has led to models of valinomycin as a prototype for natural carriers of potassium in cell membranes. In this paper we present a similar finding with membranes prepared from cells of Escherichia coli-that is, valinomycin-facilitated uptake of potassium in an energy-dependent and apparently concentrative manner.E. coli has a number of advantages in the study of potassium transport because this organism possesses a transport system with high affinity for K (11,12), is able to maintain very large concentration gradients for K (13), and most of all because of the extensive knowledge of the genetics and metabolism of this organism. Lubin (14) first isolated and characterized specific potassium transport mutants in E. coli strain B. Later other potassium mutants were isolated in other E. coli strains (14-18) and Bacillus subtilis (14,19), and a number of mutants of E. coli K-12 with alterations in potassium transport have recently been isolated. A total of eight genes affecting potassium transport have been identified. One set of four closely linked kdp genes ("K+-dependent"; 15) and two other genes (trkA, trkD; "transport of K+") affect primarily uptake of potassium, while mutations in the trkB and trkC genes result in a defect in the retention of potassium (Epstein, in preparation). All these mutational defects appear to be specific for potassium because the mutants are normal in the ability to transport ,3-galactosides and proline (unpublished data). If we are to pursue the model from mit...
Colicin E1 blocks proline accumulation by membrane vesicles prepared from wild-type sensitive Escherichia coli. Two classes of mutant cells are unaffected by colicin. Vesicles from colicin-resistant strains are sensitive to colicin E1, whereas vesicles from colicin-tolerant strains are unaffected by colicin El. These results suggest that the colicin E1 receptor is on the cell membrane and that colicin-tolerant strains have altered membranes while colicin-resistant strains have altered cell walls.
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