© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 343 Biotechnol. J. 2011, 6, 343–356 DOI 10.1002/biot.201000316 www.biotechnology-journal.com 1 Introduction The yeast Saccharomyces cerevisiae is a widely used model organism for studying eukaryal cellu- lar and molecular principals [1]. This yeast is also an industrial workhorse used for the production of fuels, chemicals, food ingredients and pharmaceu- ticals [2–4]. It is particularly well known for its role as a cell factory for bioethanol production, where its high ethanol tolerance allows for efficient pro- duction, reaching very high titers. Also a number of recombinant proteins (i.e., hormones and vaccines) are being commercially produced by S. cerevisiae, and here efficient protein production, folding and secretion are of major importance [5]. There is also much interest in exploiting yeast as a production Research Article Heterologous production of polyunsaturated fatty acids in Saccharomyces cerevisiae causes a global transcriptional response resulting in reduced proteasomal activity and increased oxidative stress Rawisara Ruenwai 2,3 , Andrea Neiss 1 , Kobkul Laoteng 4 , Wanwipa Vongsangnak 1 , Arastoo Badoei Dalfard 1 * , Supapon Cheevadhanarak 2,3 , Dina Petranovic 1 and Jens Nielsen 1 1 Department of Chemical and Biological Engineering, Chalmers University of Technology, Gothenburg, Sweden 2 School of Bioresources and Technology, King Mongkut’s University of Technology Thonburi, Bangkok, Thailand 3 Pilot Plant Development and Training Institute, King Mongkut’s University of Technology Thonburi, Bangkhuntien, Bangkok, Thailand 4 Biochemical Engineering and Pilot Plant Research and Development Unit, National Center for Genetic Engineering and Biotechnology, National Sciences and Technology Development Agency at King Mongkut’s University of Technology Thonburi, Bangkhuntien, Bangkok , Thailand Due to their health benefits there is much interest in developing microbial processes for efficient production of polyunsaturated fatty acids (PUFAs). In this study we co-expressed Mucor rouxii Δ 12 - and Δ 6 -desaturase genes in Saccharomyces cerevisiae, which resulted in a yeast strain that accumu- lated linoleic acid and γ-linolenic acid in the different lipid species. Additionally, the strain contained higher levels of phospholipids and lower levels of ergosterol than the reference strain. Integrated analysis of the transcriptome revealed decreased expression of genes involved in ergosterol biosyn- thesis, but more unexpectedly it also pointed towards attenuated activity of the ubiquitin-protea- some system and a reduced oxidative stress response. In vitro and in vivo measurements showed reduced levels of all three proteasomal activities and also increased levels of reactive oxidative species in the PUFA-producing strain. Overall our results clearly show that PUFAs in yeast can be detrimental for several key cellular pathways, such as the oxidative stress response and proteaso- mal activity, suggesting that the membrane composition is of vital importance for these processes. Keywords: Desaturase · Lipid biosynthesis · PUFAs · yeast · Stress response Correspondence: Professor Jens Nielsen, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden E-mail: nielsenj@chalmers.se Fax: +46-31-772-3801 Abbreviations: AZE, L-azetidine-2-carboxylic acid; GLA, γ-linolenic acid; HNE, 4-hydroxy-2-nonenal; LA, linoleic acid; PUFAs, polyunsaturated fatty acids; ROS, reactive oxidative species; UPS, ubiquitin-proteasome system Received 21 September 2010 Revised 2 November 2010 Accepted 10 November 2010 Supporting information available online * Current address: Department of Biology, Faculty of Science, Shahid Bahenar University of Kerman, Kerman, Iran