A recently cloned cDNA from sunflower codes for a fusion protein composed of an N-terminal cytochrome b5 and a domain similar to membrane-bound acyl lipid desaturases. For a functional identification, homologous cDNAs from Brassica napus and Arabidopsis thaliana were expressed in Saccharomyces cerevisiae, and sphingolipid long chain bases were analyzed. The expression of the heterologous enzyme results in significant proportions of new Delta8, 9-cis/trans-phytosphingenines that accompany the residual C18-phytosphinganine predominating in wild-type yeast cells. These results represent the first identification of a gene coding for a sphingolipid desaturase and for a stereounselective desaturase showing trans-activity from any organism. Furthermore, this fusion protein is a new member of the cytochrome b5 superfamily. The formation of the two regioisomeric phytosphingenines in the transformed yeast sheds new light on the factors controlling regioselectivity.