The fatty acid synthetase (FAS) system in plants and bacteria consists of a type II FAS complex assembled from different monofunctional polypeptides catalysing the individual reactions involved in each cycle of the de novo synthesis of fatty acids. Enoyl-ACP reductase (EC 22.214.171.124) plays the determinant role in completing cycles of de novo fatty acid biosynthesis, by removing a trans-unsaturated double bond to give a saturated acyl-ACP. Genetic analyses revealed that the NADH-specific enoyl-ACP reductase (ENR) is encoded by a single gene in different plant species like Petunia hybrida and Arabidopsis thaliana, whereas the allotetraploid Nicotiana tabacum contains two enr encoding genes. This implies that enhanced expression of enr genes, e.g. during seed development, is due to a modification of the expression level of a housekeeping gene. Besides a high degree of homology observed within the exon and intron sequences of the analysed enr genes, the positions of the introns and exons were also found to be conserved. Other similarities are the presence of a large intron in the 5' untranslated leader sequence of the genes as well as a conserved 5' GC intron splice site. Experiments have been performed to analyse the functional importance of this 5' GC splice site with regard to the regulation of enr expression. Homology between the 5' flanking region of both tobacco enr genes, enr-T1 and enr-T2, appears to be limited to short stretches of conserved sequences which are frequently interrupted by insertions of different (retro) transposon-like elements in the promoter of enr-T2. By contrast, the arabidopsis enr promoter region does not show any significant homology when compared with similar regions of the tobacco enr genes.