Null trk1 trk2 mutants of Saccharomyces cerevisiae exhibit a low-affinity uptake of K ؉ and Rb ؉ . We show that this low-affinity Rb ؉ uptake is mediated by several independent transporters, and that trk1⌬ cells and especially trk1⌬ trk2⌬ cells are highly hyperpolarized. Differences in the membrane potentials were assessed for sensitivity to hygromycin B and by flow cytometric analyses of cellular DiOC 6 (3) fluorescence. On the basis of the latter analyses, it is proposed that Trk1p and Trk2p are involved in the control of the membrane potential, preventing excessive hyperpolarizations. K ؉ starvation and nitrogen starvation hyperpolarize both TRK1 TRK2 and trk1⌬ trk2⌬ cells, thus suggesting that other proteins, in addition to Trk1p and Trk2p, participate in the control of the membrane potential. The HAK1 K ؉ transporter from Schwanniomyces occidentalis suppresses the K ؉ -defective transport of trk1⌬ trk2⌬ cells but not the high hyperpolarization, and the HKT1 K ؉ transporter from wheat suppresses both defects, in the presence of Na ؉ . We discuss the mechanism involved in the control of the membrane potential by Trk1p and Trk2p and the causal relationship between the high membrane potential (negative inside) of trk1⌬ trk2⌬ cells and its ectopic transport of alkali cations.

Deletion of the potassium transporter genes TRK1 and TRK2 impairs potassium uptake in Saccharomyces cerevisiae, resulting in a greatly increased requirement for the ion and the inability to grow on low pH medium. Selection for mutations that restored growth of trk1⌬ trk2⌬ cells on low pH (3.0) medium led to the isolation of a dominant suppressor that also partially suppressed the increased K ؉ requirement of these cells. Molecular analysis revealed the suppressor to be an allele of BAP2 that encodes a permease for branched chain amino acids. The suppressor mutation (BAP2-1) converts a phenylalanine codon, highly conserved among the amino acid permease genes, to a serine codon in a region predicted to lie within the sixth membrane-spanning domain. Generation of the analogous mutation in the histidine permease produced an allele, HIP1-293, that similarly suppressed the low pH sensitivity of trk1⌬ trk2⌬ cells. Suppression of trk1⌬ trk2⌬ phenotypes by BAP2-1 or HIP1-293 was correlated with increased Rb ؉ uptake. The presence of the substrate amino acids enhanced but was not essential for suppression of trk1⌬ trk2⌬ phenotypes and increased Rb ؉ uptake. The conserved site altered by the suppressor mutations appears to be important; his4 HIP1-293 cells show an increased requirement for histidine compared with his4 HIP1 cells.

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The capacity to transport potassium and to discriminate between the different alkali cations has been found to affect sodium tolerance in Saccharomyces cerevisiae. Mutants with a defective capacity to transport K+ were more sensitive to high concentrations of Na+ because they accumulated more Na+ and less K+ than wild-type cells which showed high discrimination between K+ and Na+.

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RESUMEN:El posible deterioro de la calidad de las aguas subterráneas, causado por la aplicación de lodos de depuradora a un suelo agrícola, es evaluado en términos del incremento de la salinidad del agua de recarga y de su contenido en nutrientes. Para ello, se obtuvieron las curvas de lavado de cada elemento en estudio, a partir de un ensayo de lixiviación en columnas de suelo, el cual permitió estimar un intervalo de variación de la dosis de aplicación a adoptar para que hiciera mínima la posibilidad de contaminación del medio. El estudio de los valores de la Relación de Adsorción de Sodio (RAS) en los lixiviados obtenidos, así como de la variación de la conductividad eléctrica (CE) y concentraciones de distintos solutos, hizo posible evaluar la calidad de las aguas de percolación. Estas no presentan ningún riesgo de alcalinización del suelo, presentando valores de CE a tener en cuenta únicamente en los casos correspondientes a dosis del residuo extremas, y de forma transitoria. El fósforo aportado con los lodos de depuradora aumenta con la dosis de aplicación de los mismos, quedando parte fijado en el suelo. La fracción lixiviada podría condicionar la concentración de las aguas subterráneas, si bien sólo para las dosis extremas. En cuanto a la dinámica del nitrógeno inorgánico, las condiciones anaerobias en las que se realizó el ensayo conducen a una concentración de nitratos despreciable en los lixiviados. El aporte directo es en forma amoniacal, quedando retenido en el suelo y siendo asimismo transportado a lo largo del perfil. Las curvas de transporte de este ión para cada dosis adoptada completan el estudio. La evaluación de los aspectos mencionados, esto es, salinidad y aportes de fósforo y nitrógeno, arroja resultados favorables para la aplicación de lodos dé depuradora a suelos agrícolas, para las dosis de aplicación indicadas.ABSTRACT: Groundwater quality damage caused by land application of sewage sludge is discussed, focussing on water salinity increase and eutrophication risk. Solute transport curves through the soil profile were obtained for soil columns under laboratory conditions, and an environmentally safe range for the sludge application rate was established. Leached water quality was estimated with regard to SAR and EC values reached, and solute concentrations found. Results showed no alkalinity risk for either soil or groundwater. EC values were moderately high only for extreme application rates. Soil phosphorus content increased with higher application rates. Predominant inorganic nitrogen species was ammonium due to the anaerobic conditions the experiment was carried out under. Salinity evaluation and phosphorus and nitrogen increase in percolation water showed positive results for the application of sewage sludge to soils. INTRODUCCIÓNLa necesidad de depurar las aguas residuales de forma previa a su vertido es una realidad incuestionable contemplada en la legislación y planes de gestión del agua. Sin embargo, esta actividad genera unos residuos, los lodos o fangos de depuradora, cuyo des...