Leonor Pereira

Genetic characterization of commercial Saccharomyces cerevisiae isolates recovered from vineyard environments.

One hundred isolates of the commercial Saccharomyces cerevisiae strain Zymaflore VL1 were recovered from spontaneous fermentations carried out with grapes collected from vineyards located close to wineries in the Vinho Verde wine region of Portugal. Isolates were differentiated based on their mitochondrial DNA restriction patterns and the evaluation of genetic polymorphisms was carried out by microsatellite analysis, interdelta sequence typing and pulsed‐field gel electrophoresis (PFGE). Genetic patterns were compared to those obtained for 30 isolates of the original commercialized Zymaflore VL1 strain. Among the 100 recovered isolates we found a high percentage of chromosomal size variations, most evident for the smaller chromosomes III and VI. Complete loss of heterozygosity was observed for two isolates that had also lost chromosomal heteromorphism; their growth and fermentative capacity in a synthetic must medium was also affected. A considerably higher number of variant patterns for interdelta sequence amplifications was obtained for grape‐derived strains compared to the original VL1 isolates. Our data show that the long‐term presence of strain VL1 in natural grapevine environments induced genetic changes that can be detected using different fingerprinting methods. The observed genetic changes may reflect adaptive mechanisms to changed environmental conditions that yeast cells encounter during their existence in nature. Copyright

Functional analysis of Kluyveromyces lactis carboxylic acids permeases: heterologous expression of KlJEN1 and KlJEN2 genes.

The present work describes a detailed physiological and molecular characterization of the mechanisms of transport of carboxylic acids in Kluyveromyces lactis. This yeast species presents two homologue genes to JEN1 of Saccharomyces cerevisiae: KlJEN1 encodes a monocarboxylate permease and KlJEN2 encodes a dicarboxylic acid permease. In the strain K.lactis GG1888, expression of these genes does not require an inducer and activity for both transport systems was observed in glucose-grown cells. To confirm their key role for carboxylic acids transport in K. lactis, null mutants were analyzed. Heterologous expression in S. cerevisiae has been performed and chimeric fusions with GFP showed their proper localization in the plasma membrane. S. cerevisiae jen1Δ cells transformed with KlJEN1 recovered the capacity to use lactic acid, as well as to transport labeled lactic acid by a mediated mechanism. When KlJEN2 was heterologously expressed, S. cerevisiae transformants gained the ability to transport labeled succinic and malic acids by a mediated mechanism, exhibiting, however, a poor growth in malic acid containing media. The results confirmed the role of KlJen1p and KlJen2p as mono and dicarboxylic acids permeases, respectively, not subjected to glucose repression, being fully functional in S. cerevisiae.

Remanence plots technique extended to exchange bias systems

Efforts have been recently made to use remanence plots in exchange-bias studies. However, since the two remnant magnetizations of a biased loop may differ, this technique cannot be applied in its classical form. This work extends it to systems with shifted loops and shows that the number of distinct plots is significantly increased. The approach was probed on Co/IrMn exchange bias films. Possible discrepancies between experiment and theory are pointed out and discussed. The adaptation of the model presented here enables it to become one of the few accessible techniques for rapid and accurate evaluation of magnetic interactions in biased systems.

Abrupt suppression of the exchange bias across a non-magnetic insulator spacer

This paper reports results obtained on exchange-biased IrMn/Al2O3/Co films deposited by magnetron sputtering, where the thickness of the non-magnetic insulator layer, tAl2O3, was varied. Ferromagnetic resonance and static magnetization measurements were used to study the exchange interaction between the antiferromagnet (IrMn) and ferromagnet (Co) layers. X-ray diffractometry and x-ray reflectometry as well as high-resolution transmission electron microscopy were employed for structural characterization of the films. It was found that the IrMn/Co exchange coupling decreases very abruptly with tAl2O3 being the exponential decay length at least four times smaller than the values previously reported for exchange-bias systems. Such a rapid suppression of the coupling is explained in terms of the prompt loss of the direct contact between the antiferromagnet and the ferromagnet with tAl2O3.