Effect of uncouplers on the bioenergetic properties of a carbonyl cyanide m-chlorophenylhydrazone-resistant mutant Escherichia Coli UV6

Divergent Roles of Escherichia Coli Encoded Lon Protease in Imparting Resistance to Uncouplers of Oxidative Phosphorylation: Roles of marA, rob, soxS and acrB

Uncouplers of oxidative phosphorylation dissipate the proton gradient, causing lower ATP production. Bacteria encounter several non-classical uncouplers in the environment, leading to stress-induced adaptations. Here, we addressed the molecular mechanisms responsible for the effects of uncouplers in Escherichia coli. The expression and functions of genes involved in phenotypic antibiotic resistance were studied using three compounds: two strong uncouplers, i.e., Carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and 2,4-Dinitrophenol (DNP), and one moderate uncoupler, i.e., Sodium salicylate (NaSal). Quantitative expression studies demonstrated induction of transcripts encoding marA, soxS and acrB with NaSal and DNP, but not CCCP. Since MarA and SoxS are degraded by the Lon protease, we investigated the roles of Lon using a lon-deficient strain (Δlon). Compared to the wild-type strain, Δlon shows compromised growth upon exposure to NaSal or 2, 4-DNP. This sensitivity is dependent on marA but not rob and soxS. On the other hand, the Δlon strain shows enhanced growth in the presence of CCCP, which is dependent on acrB. Interestingly, NaSal and 2,4-DNP, but not CCCP, induce resistance to antibiotics, such as ciprofloxacin and tetracycline. This study addresses the effects of uncouplers and the roles of genes involved during bacterial growth and phenotypic antibiotic resistance. Strong uncouplers are often used to treat wastewater, and these results shed light on the possible mechanisms by which bacteria respond to uncouplers. Also, the rampant usage of some uncouplers to treat wastewater may lead to the development of antibiotic resistance.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Bacterial resistance to uncouplers

Uncoupler resistance presents a potential challenge to the conventional chemiosmotic coupling mechanism. In E. coli, an adaptive response to uncouplers was found in cell growing under conditions requiring oxidative phosphorylation. It is suggested that uncoupler-resistant mutants described in the earlier literature might represent a constitutive state of expression of this "low energy shock" adaptive response. In the environment, bacteria are confronted by nonclassical uncoupling factors such as organic solvents, heat, and extremes of pH. It is suggested that the low energy shock response will aid the cell in coping with the effects of natural uncoupling factors. The genetic analysis of uncoupler resistance has only recently began, and is yielding interesting and largely unexpected results. In Bacillus subtilis, a mutation in fatty acid desaturase causes an increased content of saturated fatty acids in the membrane and increased uncoupler resistance. The protonophoric efficiency of uncouplers remains unchanged in the mutants, inviting nonorthodox interpretations of the mechanism of resistance. In E. coli, two loci conferring resistance to CCCP and TSA were cloned and were found to encode multidrug resistance pumps. Resistance to one of the uncouplers, TTFB, remained unchanged in strains mutated for the MDRs, suggesting a resistance mechanism different from uncoupler extrusion.

Cardiolipins and biomembrane function

Evidence is discussed for roles of cardiolipins in oxidative phosphorylation mechanisms that regulate State 4 respiration by returning ejected protons across and over bacterial and mitochondrial membrane phospholipids, and that regulate State 3 respiration through the relative contributions of proteins that transport protons, electrons and/or metabolites. The barrier properties of phospholipid bilayers support and regulate the slow proton leak that is the basis for State 4 respiration. Proton permeability is in the range 10(-3)-10(-4) cm s-1 in mitochondria and in protein-free membranes formed from extracted mitochondrial phospholipids or from stable synthetic phosphatidylcholines or phosphatidylethanolamines. The roles of cardiolipins in proton conductance in model phospholipid membrane systems need to be assessed in view of new findings by Hübner et al. [313]: saturated cardiolipins form bilayers whilst natural highly unsaturated cardiolipins form nonlamellar phases. Mitochondrial cardiolipins apparently participate in bilayers formed by phosphatidylcholines and phosphatidylethanolamines. It is not yet clear if cardiolipins themselves conduct protons back across the membrane according to their degree of fatty acyl saturation, and/or modulate proton conductance by phosphatidylcholines and phosphatidylethanolamines. Mitochondrial cardiolipins, especially those with high 18:2 acyl contents, strongly bind many carrier and enzyme proteins that are involved in oxidative phosphorylation, some of which contribute to regulation of State 3 respiration. The role of cardiolipins in biomembrane protein function has been examined by measuring retained phospholipids and phospholipid binding in purified proteins, and by reconstituting delipidated proteins. The reconstitution criterion for the significance of cardiolipin-protein interactions has been catalytical activity; proton-pumping and multiprotein interactions have yet to be correlated. Some proteins, e.g., cytochrome c oxidase are catalytically active when dimyristoylphosphatidylcholine replaces retained cardiolipins. Cardiolipin-protein interactions orient membrane proteins, matrix proteins, and on the outerface receptors, enzymes, and some leader peptides for import; activate enzymes or keep them inactive unless the inner membrane is disrupted; and modulate formation of nonbilayer HII-phases. The capacity of the proton-exchanging uncoupling protein to accelerate thermogenic respiration in brown adipose tissue mitochondria of cold-adapted animals is not apparently affected by the increased cardiolipin unsaturation; this protein seems to take over the protonophoric role of cardiolipins in other mitochondria. Many in vivo influences that affect proton leakage and carrier rates selectively alter cardiolipins in amount per mitochondrial phospholipids, in fatty acyl composition and perhaps in sidedness; other mitochondrial membrane phospholipids respond less or not at all.(ABSTRACT TRUNCATED AT 400 WORDS)

Uncoupler resistance in E. coli Tuv and Cuv is due to the exclusion of uncoupler by the outer membrane

The uncoupler resistant bacterial strains E. coli Tuv and Cuv share the high deoxycholate sensitivity of the parent strain, Doc S. However, both Tuv and Cuv show greater resistance than Doc S to other detergents. Measurement of the periplasmic volume indicates that the outer membrane of Doc S is freely permeable to both TPP+ and hydroxymethylinulin. Tuv and Cuv are able to exclude these compounds. EDTA treatment was necessary prior to measuring membrane potential in Tuv and Cuv. Under conditions where delta phi could be measured, uncouplers acted to dissipate delta phi with equal potency in all strains. Uncoupler resistant proline uptake in Tuv and Cuv was abolished by EDTA treatment. Transduction experiments with phage P1 showed that uncoupler resistance could be transferred from Tuv to Doc S. Such transductants were no longer sensitive to novabiocin. The gene for uncoupler resistance cotransduced with the gene pyrE (82 min). Plating efficiency experiments with P1 suggests that detergent sensitivity in Doc S arises from an rfa (81 min) mutation. This mutation is no longer present in Tuv.

The protonophore resistance of Bacillus megaterium is correlated with elevated ratios of saturated to unsaturated fatty acids in membrane phospholipids

Growth of the protonophore-resistant strain of Bacillus megaterium, strain C8, in the presence of oleic acid markedly reduced its resistance to low concentrations of carbonylcyanide m-chlorophenylhydrazone (CCCP). Growth of the CCCP-sensitive wild-type strain in the presence of stearic acid increased the resistance of that strain to growth inhibition by protonophore. Studies of the membrane lipids indicated that in the absence of additions to the medium, membranes from C8 contained greatly reduced levels of monounsaturated fatty acids relative to the wild type; wild-type levels were restored by growth of C8 in the presence of oleic acid, concomitant with the loss of resistance. Conversely, growth of the wild type on stearic acid increased the ratio of saturated/unsaturated fatty acids in the membrane, concomitant with a modest increase in the resistance of the wild-type strain to CCCP. The exogenous oleic acid was preferentially incorporated into phosphatidylethanolamine, diphosphatidylglycerol, and 1,2-diacylglycerol, whereas stearic acid was incorporated preferentially into phosphatidylglycerol, and into the small component of free fatty acids. Depending upon the growth conditions, changes in membrane lipid-to-membrane protein ratio and in the ratios of polar lipid components were observed, but none of those changes correlated as did the changes in saturated fatty-acid-to-unsaturated fatty-acid ratio with protonophore resistance. This latter correlation was further suggested by experiments in which the protonophore resistance of wild type B. megaterium was shown to increase with increasing growth temperature without any temperature-dependent loss of protonophore efficacy. The experiments here support the hypothesis developed from work with Bacillus subtilis that changes in the fatty acid composition of the membrane phospholipids affect energy coupling, and make it clear that simple increases or decreases in the hydrolytic activity of ATPase in the uncoupler-resistant mutants of bacilli are not correlated with resistance in some direct way.

Incorporation of specific exogenous fatty acids into membrane lipids modulates protonophore resistance in Bacillus subtilis

Attempts to manipulate the level of C16:1 fatty acids in membrane phospholipids were made by using Bacillus subtilis and its protonophore-resistant mutants to test the hypothesis that C16:1 fatty acid levels relate to the bioenergetic properties of the mutant strains. Growth of the three mutants in the presence of palmitoleic acid restored the level of C16:1 fatty acids in the membrane lipids to somewhat above those found in the wild type. The palmitoleic acid was preferentially incorporated into diphosphatidylglycerol (cardiolipin) and phosphatidylethanolamine and was associated with increased levels of these phospholipids. These membrane preparations showed no increase in the levels of free fatty acids. The increase in C16:1 fatty acids achieved by growth in the presence of palmitoleic acid was accompanied by secondary changes in membrane lipids as well as a pronounced diminution in the protonophore resistance of growth and ATP synthesis. Other membrane-associated properties that had been observed in these mutants, e.g., elevated ATPase levels, were not altered coordinately with protonophore resistance and C16:1 fatty acid levels. Growth of the wild type in the presence of palmitic acid caused a modest elevation of the C16:0 of the membrane lipids and a modest increase in the protonophore resistance of growth and ATP synthesis. Growth of the wild type at elevated temperatures, in the absence of fatty acid supplementation, also enhanced its resistance to protonophores. The results support the hypothesis that specific changes in membrane lipid composition underlie the bioenergetic changes associated with protonophore resistance.

Isolation and characterization of uncoupler-resistant mutants of Bacillus subtilis

Three mutant strains of Bacillus subtilis were isolated on the basis of their ability to grow in the presence of 5 microM carbonyl cyanide m-chlorophenylhydrazone (CCCP). The mutants (AG2A, AG1A3, and AG3A) were also resistant to 2,4-dinitrophenol, and AG2A exhibited resistance to tributyltin and neomycin. The mutants all exhibited (i) elevated levels of membrane ATPase activity relative to the wild type; (ii) slightly elevated respiratory rates, with the cytochrome contents of the membranes being the same as or slightly lower than those of the wild type; (3) a passive membrane permeability to protons that was indistinguishable from that of the wild type in the absence of CCCP and that was increased by addition of CCCP to the same extent as observed with the wild type; and (4) an enhanced sensitivity to valinomycin with respect to the ability of the ionophore to reduce the transmembrane electrical potential. Finally and importantly, starved whole cells of all the mutants synthesized more ATP than the wild type did upon energization in the presence of any one of several agents that lowered the proton motive force. Studies of revertants indicated that the phenotype resulted from a single mutation. Since a mutation in the coupling membrane might produce such pleiotropic effects, an analysis of the membrane lipids was undertaken with preparations made from cells grown in the absence of CCCP. The membrane lipids of the uncoupler-resistant strains differed from those of the wild type in having reduced amounts of monounsaturated C16 fatty acids and increased ratios of iso/anteiso branches on the C15 fatty acids. Correlations between protonophore resistance and the membrane lipid compositions of the wild type, mutants, and revertants were most consistent with the hypothesis that a reduction in the content of monounsaturated C16 fatty acids in the membrane phospholipids is related, perhaps casually, to the ability to synthesize ATP at low bulk transmembrane electrochemical gradients of protons.

Quantitative relationship between protonophoric and uncoupling activities of analogs of SF6847 (2,6-di-t-butyl-4-(2',2'-dicyanovinyl)phenol)

Uncoupling activity with rat liver mitochondria and protonophoric activity across the lecithin liposomal membranes were measured for a series of non-classical uncouplers related to the most potent uncoupler known until now, SF6847 (2,6-di-t-butyl-4-(2',2'-dicyanovinyl)phenol). The correlation between uncoupling and protonophoric activities for a number of uncouplers, both non-classical and classical (simply substituted phenols), was examined quantitatively. Correlation was excellent when such factors as the stability of anionic species in the membrane phase and the difference in the pH conditions of the extramembranous aqueous phase were taken into account. Carbonylcyanide m-chlorophenylhydrazone (CCCP) and carbonylcyanide p-trifluoromethoxyphenylhydrazone (FCCP), which are structurally different, were correlated in a way that resembled the correlation of phenolic compounds, so we think that the mode of action of weakly acidic uncouplers was the same regardless of the structural type. Our findings were evidence for the shuttle-type mechanism of uncoupling action.

Uncoupler-induced relocation of elongation factor Tu to the outer membrane in an uncoupler-resistant mutant of Escherichia coli

Escherichia coli UV6, a mutant which is resistant to the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP), when grown in the presence of CCCP, but not in its absence, incorporated a new protein (Mr, 42 000) into the cell envelope. This protein was found in both cytoplasmic and outer-membrane fractions. In the outer membrane it was one of three or four most abundant proteins. The protein was tightly bound to the membranes and was not solubilized by several detergents. Solubilization was achieved with sodium lauroylsarcosinate (sarkosyl). The protein was purified close to homogeneity by affinity chromatography on a column of GDP-Sepharose. It was identified as elongation factor Tu (EF-Tu) on the basis of electrophoretic mobility, profiles of peptide fragments produced by proteolysis, and by its ability to bind to GDP-Sepharose. Disruption of cells in the presence of CCCP or incubation of envelopes with EF-Tu did not result in incorporation of EF-Tu into the membranes. It is suggested that this protein is incorporated into the outer membrane as a consequence of an alteration in the normal protein biosynthetic mechanisms of the mutant induced by the presence of CCCP.

Electron spin resonance studies of lipid fluidity changes in membranes of an uncoupler-resistant mutant of Escherichia coli

The fluidity of the lipids in membrane preparations from a mutant of Escherichia coli resistant to the uncoupler CCCP, grown at different temperatures with and without CCCP, was examined by electron spin resonance using the spin probe 5-doxyl stearic acid. The fluidity of the membrane lipids at the growth temperature, as estimated using electron spin resonance, was less in cells grown at lower temperatures. Precise homeoviscous adaptation was not observed. Growth in the presence of CCCP resulted in a decrease in membrane lipid fluidity, particularly in the inner (cytoplasmic) membrane. There was no change in the proportion of phosphatidylethanolamine, phosphatidylglycerol and cardiolipin in the cell envelope. However, there was an increase in the proportion of unsaturated fatty acids in membranes from cells grown with uncoupler. This was reflected in the increased fluidity of the lipids extracted from these membranes. This result is contrary to that expected from measurements of the fluidity of the lipid in these membranes. The decreased fluidity of the lipid in these membranes may be a consequence of the observed increase in the ratio of protein to phospholipid.