Functional Identification of a Δ8-Sphingolipid Desaturase from Borago officinalis

The similarities between Δ12- and Δ15-fatty acyl desaturase sequences were used to construct degenerate primers for PCR experiments with cDNA transcribed from mRNA of developing borage seeds. Screening of a borage seed cDNA library with an amplified DNA fragment resulted in the isolation of a full-length cDNA corresponding to a deduced open-reading frame of 446 amino acids. The protein showed high similarity to plant Δ8-sphingolipid desaturases as well as to the Δ6-fatty acyl desaturase from Borago officinalis. The sequence is characterized by the presence of a N-terminal cytochrome b5 domain. Expression of this open-reading frame in Saccharomyces cerevisiae resulted in the formation of Δ8-trans/cis-phytosphingenines not present in wild-type cells, as shown by HPLC analysis of sphingoid bases as their dinitrophenyl derivatives. GLC-MS analysis of the methylated di-0-trimethylsilyl ether derivatives confirmed the presence of Δ8-stereoisomers of C18- and C20-phytosphingenine. Furthermore, Northern blotting showed that the gene encoding a stereo-unselective Δ8-sphingolipid desaturase is primarily expressed in young borage leaves.