The invention relates to a process for producing one or more polyunsaturated fatty acids by means of heterologous gene expression comprising the steps of providing a production organism which comprises a heterologous gene cluster encoding a polyunsaturated fatty acid biosynthetic pathway encompassing a subsequence ER encoding an enoylreductase and a subsequence AT encoding an acyltransferase, growing the production organism in the presence of a fermentable carbon source whereby one or more polyunsaturated fatty acids are produced, and optionally recovering the one or more polyunsaturated fatty acids.