Systems metabolic engineering of the industrial fungus Ashbya gossypii turns it into an oleaginous microorganism

Ashbya gossypii is a paradigm of the white biotechnology and its metabolic engineering permitted the switch from the industrial chemical synthesis to the current biotechnological production of riboflavin (vitamin B2). We recently reconstructed and validated the genome-scale metabolic model of A. gossypii iRL766, which includes 766 genes involved in 1,595 reactions and comprising 799 unique metabolites distributed in four different compartments. This model, as compared to iTO977, a genome-scale model of the close relative S. cerevisiae, showed a stronger lipid metabolism with a more complex fatty acid desaturation system and many extracellular lipases. Indeed, A. gossypii utilizes oils as carbon source in both its natural habitat and industrial fermentations. Nonetheless, its capability to accumulate lipids is not enough to consider it an oleaginous fungus since it is not able to produce more than 20% of its cell dry weight as fatty acids. We therefore performed computer-guided metabolic engineering of both the lipid catabolism and anabolism. Firstly, beta oxidation pathway was blocked making A. gossypii unable to degrade fatty acids and secondly the ATP citrate lyase from the oleaginous yeast Yarrowia lipolytica was heterologously expressed in order to enhance fatty acid synthesis. The combinational approach generated a strain capable of accumulating up to 70% of its cell dry weight as fatty acids, and thus, undoubtedly making A. gossypii an oleaginous organism. This novel feature in addition to its well-established genetic tools and its industrial suitability make A. gossypii a good candidate to produce high value lipids.