Functional screening of a novel Δ15 fatty acid desaturase from the coccolithophorid Emiliania huxleyi

The coccolithophorid Emiliania huxleyi is a bloom-forming marine phytoplankton thought to play a key role as a biological pump that transfers carbon from the surface to the bottom of the ocean, thus contributing to the global carbon cycle. This alga is also known to accumulate a variety of polyunsaturated fatty acids. At 25 °C, E. huxleyi produces mainly 14:0, 18:4n − 3, 18:5n − 3 and 22:6n − 3. When the cells were transferred from 25 °C to 15 °C, the amount of unsaturated fatty acids, i.e. 18:1n − 9, 18:3n − 3 and 18:5n − 3, gradually increased. Among the predicted desaturase genes whose expression levels were up-regulated at low temperature, we identified a gene encoding novel ∆15 fatty acid desaturase, EhDES15, involved in the production of n − 3 polyunsaturated fatty acids in E. huxleyi. This desaturase contains a putative transit sequence for localization in chloroplasts and a ∆6 desaturase-like domain, but it does not contain a cytochrome b5 domain nor typical His-boxes found in ∆15 desaturases. Heterologous expression of EhDES15 cDNA in cyanobacterium Synechocystis sp. PCC 6803 cells increased the level of n − 3 fatty acid species, which are produced at low levels in wild-type cells grown at 30 °C. The orthologous genes are only conserved in the genomes of prasinophytes and cryptophytes. The His-boxes conserved in orthologues varied from that of the canonical ∆15 desaturases. These results suggested the gene encodes a novel ∆15 desaturase responsible for the synthesis of 18:3n − 3 from 18:2n − 6 in E. huxleyi.