Gene cloning and functional analysis of a second Δ6-fatty acid desaturase from an arachidonic acid-producing Mortierella fungus

We demonstrated that Mortierella alpina 1S-4 has two Delta6-desaturases, which are involved in the desaturation of linoleic acid to gamma-linolenic acid. For one of the two Delta6-desaturases, designated as Delta6I, gene cloning and its heterologous expression in a fungus, Aspergillus oryzae, has previously been reported. In addition, we indicated in this paper that there is an isozyme of the two Delta6-desaturases, designated as AMI, in M. alpina 1S-4. The predicted amino acid sequences of the Mortierella Delta6-desaturases were similar to those of ones from other organisms, i.e. borage and Caenorhabditis elegans, and had a cytochrome b(S)-like domain at the N-terminus, being different from the yeast Delta9-desaturase, which has the corresponding domain at the C-terminus. The full-length Delta6II cDNA was expressed in A. oryzae, resulting in the accumulation of gamma-linolenic acid (which was not detected in the control Aspergillus) up to 37% of the total fatty acids. The analysis of real-time quantitative PCR (RTQ-PCR) showed that the quantity of Delta6I RNA was 2.4-, 9-, and 17-fold higher than that of Delta6II RNA on 2, 3, and 4 days in M. alpina 1S-4, respectively. M. alpina 1S-4 is the first fungus to be confirmed to have two functional Delta6-desaturase genes.