Identification and characterization of fatty acid desaturases in Schizochytrium sp. HX-308

As a promising species of the thraustochytrids, Schizochytrium sp. has attracted extensive attention due to its ability to produce high levels of polyunsaturated fatty acids. In Schizochytrium sp. HX-308, there was a complete anaerobic polyketide synthase (PKS) pathway and an incomplete aerobic desaturase/elongase pathway. However, it is still quite unclear which fatty acid desaturases are involved in fatty acid desaturation in Schizochytrium sp. and how they act in concert. In this work, we firstly identified five putative desaturases in Schizochytrium sp. HX-308, including Δ12, two Δ8, Δ6, and Δ5 fatty acid desaturase genes. They were heterologously expressed in Yarrowia lipolytica and the first four fatty acid desaturases were verified as functional. Δ12 desaturase contributes to a significant increase in the content of C18:2Δ9, 12 fatty acids, from 17.26 % to 28.46 %. The expression of two Δ8 desaturases resulted in an increase in C20:3Δ8, 11, 14 content, from 3.84 % to 7.29 % and 6.82 %, respectively. Δ6 desaturase enables the PO1F strain to produce a new fatty acid C18:3Δ6, 9, 12, reaching 9.73 % of the total fatty acids. Although the expression of Δ5 desaturase didn't produce C20:4 Δ5, 8, 11, 14 fatty acids, it significantly decreased the content of C18:1Δ9 and C18:2Δ9, 12 fatty acids and largely significantly increased C18:3Δ6, 9, 12 and C20:2Δ11, 14 fatty acid content. This work will provide valuable insights into the optimization of the polyunsaturated fatty acid composition of Schizochytrium sp. through genetic engineering in the future.