Heterologous production of free dihomo-γ-linolenic acid by Aspergillus oryzae and its extracellular release via surfactant supplementation.

Free dihomo-γ-linolenic acid (DGLA) and its desaturated form, free arachidonic acid (ARA) are polyunsaturated free fatty acids (FFAs). They are useful raw materials to produce eicosanoid pharmaceuticals. In this study, we aimed at their production by the oleaginous filamentous fungus Aspergillus oryzae via metabolic engineering. Three genes encoding enzymes involved in the synthesis of DGLA and ARA, were isolated from the filamentous fungus Mortierella alpina that produces ARA in a triacylglycerol form. These genes were concatenated to promoters and terminators of highly expressed genes of A. oryzae, and the concatenated DNA fragments were further concatenated with each other to generate a single DNA fragment in the form of a biosynthetic gene cluster. By homologous recombination, the resulting DNA fragment was integrated to the chromosome of the A. oryzae acyl-CoA synthetase gene disruptant whose FFA productivity was enhanced at 9.2-fold more than the wild-type strain. The DNA-integrated disruptant produced free DGLA but did not produce free ARA. Thus, focusing on free DGLA, after removal of the gene for converting DGLA to ARA, the constructed strain produced free DGLA at 145 mg/l for 5 d. Also, by supplementing Triton X-100 surfactant at 1% to the culture, over 80% of free DGLA was released from cells without inhibiting the growth. Consequently, the constructed strain will be useful for attempting production of free DGLA-derived eicosanoids because it bypasses excision of free DGLA from triacylglycerols by lipase. To our knowledge, this is the first report on microbial production of free DGLA and its extracellular release.