Two fatty acid Δ9-desaturase genes, ole1 and ole2, from Mortierella alpina complement the yeast ole1 mutation

Genes encoding two distinct fatty acid Delta 9-desaturases were isolated from strains of the oleaginous fungus Mortierella alpina. Two genomic sequences, Delta 9-1 and Delta 9-2, each containing a single intron, were cloned from strain CBS 528.72 while one cDNA clone, LM9, was isolated from strain CBS 210.32. The Delta 9-1 gene encoded a protein of 445 aa which shared 99 % identity with the LM9 gene product. These proteins also showed 40-60 % identity to the Delta 9-desaturases (Ole1p) of other fungi and contained the three conserved histidine boxes, C-terminal cytochrome b(5) fusion and transmembrane domains characteristic of endoplasmic reticulum membrane-bound Delta 9-desaturases. LM9 and Delta 9-1 are therefore considered to represent the same gene (ole1). The ole1 gene was transcriptionally active in all M. alpina strains tested and its function was confirmed by complementation of the Saccharomyces cerevisiae ole1 mutation. Fatty acid analysis of yeast transformants expressing the CBS 210.32 ole1 gene showed an elevated level of oleic acid (18:1) compared to palmitoleic acid (16:1), the major fatty acid component of wild-type S. cerevisiae. This indicated that the M. alpina Delta 9-desaturase had a substrate preference for stearic acid (18:0) rather than palmitic acid (16:0). Genomic clone Delta 9-2 (ole2) also encoded a protein of 445 aa which had 86 % identity to the Delta 9-1 and LM9 proteins and whose ORF also complemented the yeast ole1 mutation. The transcript from this gene could only be detected in one of the six M, alpina strains tested, suggesting that its expression may be strain-specific or induced under certain physiological conditions.