Organization of genes responsible for bacterial EPA and DHA biosynthesis and domain structures of individual genes. The organization of gene clusters is divided into three types. Type I, which is represented by Shewanella pneumatophori SCRC-2738, is a gene cluster including all five pfa genes in a similar vicinity. Type II consists of a cluster of the four genes pfaABCD, with pfaE separate from the other genes. This type of cluster is represented by Moriella marina MP-1. The relative direction of pfaE has not been determined for this bacterium. In type III, pfaE is integrated into pfaC/E, and the cluster is considered to consist of four genes. In this review, the remnant sequence downstream of the pfaC/E gene was regarded as a partial sequence of pfaD. The original nomenclature (Dai and Zhang, unpublished) (GenBank accession no. ABF00130) is shown in parentheses. The third type of cluster has been reported for Pseudoalteromonas sp. strain DS-12 only.