The development of biofuels has received much attention due to the global warming and limited resources associated with fossil fuels. Butanol has been identified as a potential option due to its advantages over ethanol, for example higher energy density, compatibility with current infrastructure and its possibility to be blended with gasoline at any ratio. Yeast Saccharomyces cerevisiae can be used as a producer of butanol. However, butanol toxicity to the host limits the yield produced. In this study, four strains of yeast isolated from the habitats of lager beer, ale, wine and baker ́s yeast were grown in YPD media containing isobutanol concentrations of 1.5 %, 2 %, 3 % and 4 %. Growth was measured to determine the most tolerant strain. Gene expression for the genes RPN4, RTG1 and ILV2 was also measured, to determine its involvement in butanol stress. The genes have in previous studies seen to be involved in butanol tolerance or production, and the hypothesis was that they all should be upregulated in response to butanol exposure. It was found that the yeast strain isolated from lager beer was most tolerant to isobutanol concentrations of 2 % and 3 %. It was also found that the gene RPN4 was upregulated in response to isobutanol stress. There was no upregulation of RTG1 or ILV2, which was unexpected. The yeast strain isolated from lager beer and the gene RPN4 is proposed to be investigated further, to be able to engineer a suitable producer of the biofuel butanol.

The global warming on earth has been obvious since the 1950’s. Fossil fuels have a big impact on the observed warming and it is time to replace them with more environmentally friendly fuels. Biobutanol has been proven to be a preferred substitute to fossil fuels. The yeast Saccharomyces cerevisiae is a potential butanol producer. A problem in the biobutanol production is that the product, butanol, is toxic to the producer. In this study four S. cerevisiae strains were investigated for 1- and 2-butanol tolerance with spot tests and growth measurements with different concentrations of 1- and 2-butanol.  One of the four strains, an ale yeast, showed a higher tolerance for 1- and 2-butanol. 2-butanol was overall more tolerated by the yeast. The gene expression for the genes TMC1, LPL1, FLR1 and RPN4 was also investigated at exposure of 3 % 2-butanol. RPN4 is important in the proteasome protein degradation, which is associated with butanol tolerance. TMC1, LPL1 and FLR1 are associated to RPN4, which make them potential genes coupled to butanol tolerance. The genes TMC1 and RPN4 showed an up-regulation when exposed to 3 % 2-butanol. In conclusion, 2-butanol is preferred as a biofuel produced by ale yeast and the ideal genes to use in genetic engineering to achieve a higher butanol tolerance is TMC1 and RPN4. These results contribute to the development of an effective production of biobutanol by S. cerevisiae.

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