Salt and oxidative stress tolerance in Debaryomyces hansenii and Debaryomyces fabryi.

We report the characterization of five strains belonging to the halotolerant highly related Debaryomyces hansenii/fabryi species. The analysis performed consisted in studying tolerance properties, membrane characteristics, and cation incell amounts. We have specifically investigated (1) tolerance to different chemicals, (2) tolerance to osmotic and salt stress, (3) tolerance and response to oxidative stress, (4) reactive oxygen species (ROS) content, (5) relative membrane potential, (6) cell volume, (7) K(+) and Na(+) ion content, and (8) membrane fluidity. Unexpectedly, no direct relationship was found between one particular strain, Na(+) content and its tolerance to NaCl or between its ROS content and its tolerance to H(2)O(2). Results show that, although in general, human origin D. fabryi strains were more resistant to oxidative stress and presented shorter doubling times and smaller cell volume than food isolated D. hansenii ones, strains belonging to the same species can be significantly different. Debaryomyces fabryi CBS1793 strain highlighted for its extremely tolerant behavior when exposed to the diverse stress factors studied.

Effect of mutation, electric membrane potential, and metabolic inhibitors on the accessibility of nucleic acids to ethidium bromide in Escherichia coli cells.

The uptake of ethidium bromide by Escherichia coli K 12 cells has been studied by using 14C-labeled ethidium and spectrofluorometry on three E. coli strains: the first one (AB1157) has an ethidium-resistant phenotype; the second one derives from the first one after a single mutation (at 10 min on the E. coli genetic map) and has an ethidium-sensitive (Ebs) phenotype; the third one is the acrA strain which appeared to have the same phenotype as the Ebs strain. When the cells are in exponential growth, no ethidium enters wild-type cells, and a very limited amount of ethidium enters Ebs and acrA cells. Massive quantities of ethidium enter AB1157, Ebs, and acrA cells treated by uncouplers and respiring Ebs cells treated by the membrane ATPase-inhibitor dicyclohexylcarbodiimide. A small amount of ethidium enters cells treated in M9 succinate medium by metabolic inhibitors such as KCN or cells starved with oxygen in the same M9 medium. The amount of ethidium and ethidium dimer retained at equilibrium by either type of cell, and by cells infected by T5 phage, as well as the kinetics of influx and efflux, has been measured under a variety of situations (membrane energized or not, and/or membrane ATPase inhibited or not). Furthermore, it was shown that ethidium binds to both RNA and DNA when it enters CCCP-treated wild-type E. coli cells, whereas it binds mainly to DNA when it enters Ebs and acrA cells in exponential growth. As it will be discussed, it is difficult to account for the EthBr uptake by invoking only membrane functions and active transport. Therefore, it is proposed that the variations of the nucleic acid accessibility in E. coli cells might play a role in the control of this uptake. Accordingly, in ethidium-sensitive cells, the mutation would have caused a significant part of the chromosomal DNA (10-20%) to become accessible to ethidium. Hansen [Hansen M. T. (1982) Mutat. Res. 106, 209-216], after a study of the photobinding of psoralen to nucleic acids in the acrA mutant, also suggested that DNA environment was modified in acrA cells.

The use of a potential-sensitive cyanine dye for studying ion-dependent electrogenic renal transport of organic solutes. Uptake of L-malate and D-malate by luminal-membrane vesicles.

The mechanisms of uptake of dicarboxylic acids by rabbit renal luminal-membrane vesicles were studied by the use of filtration and spectrophotometric techniques as described in an accompanying paper [Kragh-Hansen, Jørgensen & Sheikh (1982) Biochem. J.208, 359-368]. Addition of l- or d-malate to dye-membrane-vesicle suspensions in the presence of Na(+) gradients (extravesicular>intravesicular) resulted in spectral curves indicative of depolarization events. The renal uptake of dicarboxylic acids was dependent on the type of Na(+)-salt anion present and could be correlated with the ability of the anions to penetrate biological membranes (i.e. Cl(-)>SO(4) (2-)>gluconate). Identical results were obtained by a filtration technique with Sartorius membrane filters. The results indicate that the dicarboxylic acids are taken up by the membrane vesicles in an electrically positive form (i.e. Na(+)/substrate coupling ratio 3:1) by an Na(+)-dependent transport system. This proposal was further supported by spectrophotometric experiments with various ionophores such as valinomycin, gramicidin and nigericin. The absorbance changes associated with simultaneous addition of l- and d-malate and spectrophotometric competition studies revealed that the two isomers are taken up by a common transport system. Spectral changes of the dye induced by addition of increasing concentrations of l- or d-malate indicated that the transport system favours the unphysiological d-form rather than the l-form of malate. Furthermore, it was observed that the affinity of both isomers for the transport system was dependent on the concentration of Na(+) in the medium.

Electrical properties of the plasma membrane of microplasmodia of Physarum polycephalum.

Microplasmodia of Physarum polycephalum have been investigated by conventional electrophysiological techniques. In standard medium (30 mM K+, 4 mM Ca++, 3 mM Mg++, 18 mM citrate buffer, pH 4.7, 22 degrees C), the transmembrane potential difference Vm is around -100 mV and the membrane resistance about 0.25 omega m2. Vm is insensitive to light and changes of the Na+/K+ ratio in the medium. Without bivalent cations in the medium and/or in presence of metabolic inhibitors (CCCP, CN-, N3-), Vm drops to about 0 mV. Under normal conditions, Vm is very sensitive to external pH (pH0), displaying an almost Nernstian slope at pH0 = 3. However, when measured during metabolic inhibition, Vm shows no sensitivity to pH0 over the range 3 to 6, only rising (about 50 mV/pH) at pH0 = 6. Addition of glucose or sucrose (but not mannitol or sorbitol) causes rapid depolarization, which partially recovers over the next few minutes. Half-maximal peak depolarization (25 mV with glucose) was achieved with 1 mM of the sugar. Sugar-induced depolarization was insensitive to pH0. The results are discussed on the basis of Class-I models of charge transport across biomembranes (Hansen, Gradmann, Sanders and Slayman, 1981, J. Membrane Biol. 63:165-190). Three transport systems are characterized: 1) An electrogenic H+ extrusion pump with a stoichiometry of 2 H+ per metabolic energy equivalent. The deprotonated form of the pump seems to be negatively charged. 2) In addition to the passive K+ pathways, there is a passive H+ transport system; here the protonated form seems to be positively charged. 3) A tentative H+-sugar cotransport system operates far from thermodynamic equilibrium, carrying negative charge in its deprotonated states.