Identification of a Δ4 fatty acid desaturase from Thraustochytrium sp involved in the biosynthesis of docosahexanoic acid by heterologous expression in Saccharomyces cerevisiae and Brassica juncea

The existence of Delta4 fatty acid desaturation in the biosynthesis of docosahexanoic acid (DHA) has been questioned over the years. In this report we describe the identification from Thraustochytrium sp. of two cDNAs, Fad4 and Fad5, coding for Delta4 and Delta5 fatty acid desaturases, respectively. The Delta4 desaturase, when expressed in Saccharomyces cerevisiae, introduced a double bond at position 4 of 22:5(n-3) and 22:4(n-6) resulting in the production of DHA and docosapentanoic acid. The enzyme, when expressed in Brassica juncea under the control of a constitutive promoter, desaturated the exogenously supplied substrate 22:5(n-3), resulting in the production of DHA in vegetative tissues. These results support the notion that DHA can be synthesized via Delta4 desaturation and suggest the possibility that DHA can be produced in oilseed crops on a large scale.