[Adaptation strategies of halophilic microorganisms and Debaryomyces hansenii (halophilic yeast)]. / Estrategias de adaptación de microorganismos halófilos y Debaryomyces hansenii (Levadura halófila).

The term halophile is used for all those organisms belonging to hypersaline habitats; they constitute an interesting class of organisms able to compete successfully in salt water and to resist its denaturing effects. A wide diversity of microorganisms, prokaryotic and eukaryotic belong to this category. Halophile organisms have strategies allowing them not only to withstand osmotic stress, but also to function better in the presence of salt, in spite of maintaining high intracellular concentrations of salt, partly due to the synthesis of compatible solutes that allow them to balance their osmotic pressure. We describe the characteristics of some halophile organisms and D. hansenii (halophile yeast), that allow them to resist high concentrations of salt. The interest to know the great diversity microorganisms living in hypersaline habitats is growing, and has begun to be the center of recent investigations, since halophile organisms produce an wide variety of biomolecules that can be used for different applications. In this review we describe some mechanisms with which some halophile organisms count to resist the high concentration of salts, mainly NaCl.

Enzyme kinetics of the prime K+ channel in the tonoplast of Chara: selectivity and inhibition.

The prime potassium channel from the tonoplast of Chara corallina has been analyzed in terms of an enzymatic kinetic model (Gradmann, Klieber & Hansen 1987, Biophys. J. 53:287) with respect to its selectivity for K+ over Rb+ and to its blockage by Cs+ and by Ca2+. The channel was investigated by patch-clamp techniques over a range of membrane voltages (Vm, referred to an extracytoplasmic electrical potential of zero) from -200 mV to +200 mV under various ionic conditions (0 to 300 mM K+, Rb+, Cs+, Ca2+, and Cl-) on the two sides of isolated patches. The experimental data are apparent steady-state current-voltage relationships under all experimental conditions used and amplitude histograms of the seemingly noisy open-channel currents in the presence of Cs+. The used model for K+ uniport comprises a reaction cycle of one binding site through four states, i.e., (1) K(+)-loaded and charged, facing the cytoplasm, (2) K(+)-loaded and charged facing the vacuole, (3) empty, facing the vacuole, and (4) empty, facing the cytoplasm. Vm enters the system in the form of a symmetric Eyring barrier between state 1 and 2. The numerical results for the individual rate constants are (in 10(6)s-1 for zero voltage and 1 M substrate concentration): k12: 1,410, k21: 3,370, k23: 105,000, k32: 10,600, k34: 194, k43: 270, k41: 5,290, k14: 15,800. For the additional presence of an alternate transportee (here Rb+), the model can be extended in an analog way by another two states ((5) Rb(+)-loaded and charged, facing cytoplasm, and (6) Rb(+)-loaded and charged, facing vacuole) and six more rate constants (k45: 300, k54: 240, k56: 498, k65: 4,510, k63: 4,070, k36: 403). This six-state model with its unique set of fourteen parameters satisfies the complete set of experimental data. If the competing substrate can be bound but not translocated (here Cs+ and Ca2+). k56 and k65 of the model are zero, and the stability constants Kcyt (= k36/k63) and Kvac (= k45/k54) turn out to be Kcyt(Ca2+): 250 M-1 x exp(Vm/(64 mV)), kvac(Ca2+): 10 M-1 x exp(-Vm/(66 mV)), Kcyt(Cs+): 0, and Kvac(Cs+): 46 M-2 x exp(-Vm/(12.25 mV)).(ABSTRACT TRUNCATED AT 400 WORDS)