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Appl. Environ. Microbiol. doi:10.1128/AEM.01360-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Fluidization of Membrane Lipids Enhances Tolerance of Freezing and Salt Stress by Saccharomyces cerevisiae

Department of Biotechnology, Instituto de Agroquímica y Tecnología de los Alimentos (CSIC), P. O. Box 73, E-46100-Burjassot Valencia, Spain; Department of Physiology and Technology of Plant Products, Instituto de la Grasa (CSIC), P.O. Box 1078, E- 41080 Sevilla, Spain

^* To whom correspondence should be addressed. Email: randez{at}iata.csic.es.

	Abstract

Unsaturated fatty acids play an essential role in the biophysical characteristics of cell membranes and determine the proper function of membrane-attached proteins. Thus, the ability of cells to alter the degree of unsaturation in their membranes is an important factor in cellular acclimatization to environmental conditions. Many eukaryotic organisms can synthesize dienoic fatty acids, but S. cerevisiae can introduce only a single double bond at the ⁹ position. We expressed two sunflower (Helianthus annuus) oleate ¹² desaturases, FAD2-1 and FAD2-3, in yeast cells of the wild-type W303-1A strain (trp1) and analyzed their effects on growth and stress tolerance. Production of the heterologous desaturases increased the content of dienoic fatty acids, especially 18:2^9,12, the unsaturation index and the fluidity of the yeast membrane. The total fatty acid content remained constant and the level of mono-unsaturated fatty acids decreased. Growth at 15°C was reduced in the FAD2 strains, probably due to tryptophan auxotrophy since the trp1 (TRP1) transformants that produced the sunflower desaturases grew as the control strain. Our results suggest that changes in the fluidity of the lipid bilayer affect tryptophan uptake and/or the correct targeting of tryptophan transporters. The expression of the sunflower desaturases, in either Trp⁺ or Trp^- strains, increased NaCl tolerance. Production of dienoic fatty acids increased tolerance to freezing of wild-type cells pre-incubated at 30°C or 15°C. Thus, membrane fluidity is an essential determinant of stress resistance in S. cerevisiae and engineering of membrane lipids has the potential to be a useful tool of increasing freeze tolerance in industrial strains.

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