Evolution of the membrane-bound fatty acid desaturases

The deduced amino acid sequences of the membrane-bound desaturase genes have been compared in order to infer their phylogenetic relationships. All the deduced proteins share three highly conserved histidine rich motifs suggesting a common origin. The phylogenetic analysis revealed three distinct clusters within the membrane desaturases. One cluster consisted of Delta9 desaturase sequences, the second group included the Delta12/omega3 desaturases, and the third cluster comprised the so called 'front-end' desaturases, namely Delta5, Delta6 and Delta8 desaturases. Based on functional data the Delta9 desaturase gene is assumed to be the ancestor of the remaining membrane desaturase genes. The arrangement of the second cluster suggest that omega3 desaturases originated in a prokaryotic lineage from a Delta12 desaturase gene. The first two clusters were essentially consistent with conventional species trees, as the arrangement within the different desaturase classes reflected the evolutionary relationships of the organisms concerned. Conversely, within the 'front-end' desaturase cluster, phylogenetic and functional data indicate that Delta5 desaturase genes originated independently in different evolutionary lineages from an ancestral Delta6 desaturase. In addition, Delta8 desaturases seem to have evolved once in plants from a Delta6 desaturase gene. Available genomic sequences of front-end desaturase genes from higher plants are intronless, while those from lower plants and animals are split. Evolutionary implications of these findings are discussed. (C) 2003 Elsevier Science Ltd. All rights reserved.