[Cloning and expression of two elongase genes involved in the biosynthesis of docosahexaenoic acid in Thraustochytrium sp. FJN-10].

Docosahexaenoic acid (DHA C22:6n-3), a typical long chain polyunsaturated fatty acids (PUFAs) has many positive effects on diseases such as artherosclerosis, hypertriglyceridemia, hypertension and cancers. Marine fungi, especially Thraustochytrium spp. producing much DHA can serve as model organisms for explaining the mechanism on the biosynthesis of PUFA. We described two elongase genes (TFD6 and TFD5) involved in the biosynthesis of DHA in Thraustochytrium sp. FJN-10 was cloned by using reverse transcription PCR and rapid amplification of cDNA ends. TFD6 cDNA was 816 bp in length and encoded a protein of 271 amino acids. TFD5 cDNA was 831 bp in length and encoded a protein of 276 amino acids. Transmembrane analysis revealed that TFD6 contained five transmembrane domains while TFD5 contained seven. Tertiary structures of TFD6, TFD5 elongases were predicted by HHMMSTR (Hidden markov model for local sequence-structure) model and Rosetta program. Alignment of TFD6, TFD5 with other elongases showed that both of them shared an HXXHH conserved histidine-rich motif. Phylogenetic analysis showed that TFD6 was the closest to Thraustochytrium 66 elongase, while TFD5 was the closest to Thraustochytrium sp. delta5 elongase. TFD6 and TFD5 were subcloned into the Hind III/Xba I restriction site of pYES2 vector respectively. Recombined plasmids were transformed into Saccharomyces cerevisiae using lithium acetate method. Gas chromatography analysis showed that TFD6 could elongate C18:3n-3 to C20:3n-3 while TFD5 could elongate C20:5n-3 to C22:5n-3.