A Tradeoff Drives the Evolution of Reduced Metal Resistance in Natural Populations of Yeast

Various types of genetic modification and selective forces have been implicated in the process of adaptation to novel or adverse environments. However, the underlying molecular mechanisms are not well understood in most natural populations. Here we report that a set of yeast strains collected from Evolution Canyon (EC), Israel, exhibit an extremely high tolerance to the heavy metal cadmium. We found that cadmium resistance is primarily caused by an enhanced function of a metal efflux pump, PCA1. Molecular analyses demonstrate that this enhancement can be largely attributed to mutations in the promoter sequence, while mutations in the coding region have a minor effect. Reconstruction experiments show that three single nucleotide substitutions in the PCA1 promoter quantitatively increase its activity and thus enhance the cells' cadmium resistance. Comparison among different yeast species shows that the critical nucleotides found in EC strains are conserved and functionally important for cadmium resistance in other species, suggesting that they represent an ancestral type. However, these nucleotides had diverged in most Saccharomyces cerevisiae populations, which gave cells growth advantages under conditions where cadmium is low or absent. Our results provide a rare example of a selective sweep in yeast populations driven by a tradeoff in metal resistance

Large Volume ZnWO4 Crystal Scintillators With Excellent Energy Resolution and Low Background

Large volume zinc tungstate crystal scintillators with improved scintillation characteristics were produced. Energy resolution and relative light output of samples with various sizes and shapes (cylinders, rectangular, and hexagonal prisms) were investigated. The energy resolutions of ZnWO 4 scintillator modules for the 662 keVγ line of 137Cs were 8.5% for a 1 cm3 sample and 10.7% for the large hexagonal module of dimensions ◇ 40 × 40 mm. A variation of the light output by 7%, depending on direction of observation was found for the 1 cm3 cubic crystal. Afterglow,measured 20 ms after termination of irradiation, was 0.002%. The relative light output of the scintillator and the decay kinetics were studied over the temperature range 7-300 K. The level of radioactive contamination of the ZnWO4 scintillator was determined in the Solotvina Underground Laboratory at a sensitivity of ≈0.1-10 mBq/kg. © 2006 IEEE

developmwnts of enriched cadmium tungstate crystal scintillators to search for double beta decay processes of 106Cd

R&D of cadmium tungstate (CdWO 4 ) crystal scintillators with cadmium enriched in 106 Cd is in progress with aim to realize an experiment to search for double beta processes in 106 Cd. Samples of cadmium (with natural isotopic composition and enriched in 106 Cd) were purified by vacuum distillation. Cadmium tungstate compounds (natural and enriched) to grow CdWO 4 crystals were synthesized from solutions. Contamination of the cadmium samples and synthesized compounds were measured by mass spectrometry. Concentration of iron was determined by atomic absorption spectroscopy. CdWO 4 crystal boule was grown by Czochralsky method from the natural cadmium tungstate compound. The total losses of cadmium on the stages of purification, raw material synthesis, crystal growth and scintillator production do not exceed 4 %. An excellent energy resolution FWHM = 8.1 % was measured by ∅22 × 29 mm CdWO 4 scintillator for 662 keV γ quanta of 137 Cs source