Cloning of a human cDNA encoding a novel enzyme involved in the elongation of long-chain polyunsaturated fatty acids.

The Saccharomyces cerevisiae protein ELO2p is involved in the elongation of saturated and monounsaturated fatty acids. Among several sequences with limited identity with the S. cerevisiae ELO2 gene, a consensus cDNA sequence was identified from the LifeSeq(R) database of Incyte Pharmaceuticals, Inc. Human liver cDNA was amplified by PCR using oligonucleotides complementary to the 5' and 3' ends of the putative human cDNA sequence. The resulting full-length sequence, termed HELO1, consisted of 897 bp, which encoded 299 amino acids. However, in contrast with the ELO2 gene, expression of this open reading frame in S. cerevisiae demonstrated that the encoded protein was involved in the elongation of long-chain polyunsaturated fatty acids, as determined by the conversion of gamma-linolenic acid (C(18:3, n-6)) into dihomo-gamma-linolenic acid (C(20:3, n-6)), arachidonic acid (C(20:4, n-6)) into adrenic acid (C(22:4, n-6)), stearidonic acid (C(18:4, n-3)) into eicosatetraenoic acid (C(20:4, n-3)), eicosapentaenoic acid (C(20:5, n-3)) into omega3-docosapentaenoic acid (C(22:5, n-3)) and alpha-linolenic acid (C(18:3, n-3)) into omega3-eicosatrienoic acid (C(20:3, n-3)). The predicted amino acid sequence of the open reading frame had only 29% identity with the yeast ELO2 sequence, contained a single histidine-rich domain and had six transmembrane-spanning regions, as suggested by hydropathy analysis. The tissue expression profile revealed that the HELO1 gene is highly expressed in the adrenal gland and testis. Furthermore, the HELO1 gene is located on chromosome 6, best known for encoding the major histocompatibility complex, which is essential to the human immune response.

cDNA cloning and characterization of human Delta5-desaturase involved in the biosynthesis of arachidonic acid.

Two human expressed sequence tag (EST) cDNA sequences with identity with Delta(5)- and Delta(6)-desaturases from a filamentous fungus, Mortierella alpina, were identified from the LifeSeq(R) database of Incyte Pharmaceuticals, Inc. (Palo Alto, CA, U.S.A.). An oligonucleotide complementary to the 3' EST cDNA sequences was used to screen human liver cDNA using rapid amplification of cDNA ends (RACE)-PCR. The amplified DNA fragment had 98% identity with a putative open reading frame (ORF) predicted from a human genomic sequence, and encoded 444 amino acids. Expression of this ORF in mouse fibroblast cells demonstrated that the encoded protein was a Delta(5)-desaturase, as determined by the conversion of dihomo-gamma-linolenic acid (C(20:3,n-6)) into arachidonic acid (C(20:4,n-6)). The human Delta(5)-desaturase contained a predicted N-terminal cytochrome b(5)-like domain, as well as three histidine-rich domains. A tissue expression profile revealed that this gene is highly expressed in fetal liver, fetal brain, adult brain and adrenal gland. A search of the existing databases led to localization of this ORF within a 14 kb interval flanked by the flap endonuclease-1 (FEN1) and vitelliform macular dystrophy (Best's disease; VMD2) loci of chromosome 11q12.

Cloning of delta12- and delta6-desaturases from Mortierella alpina and recombinant production of gamma-linolenic acid in Saccharomyces cerevisiae.

Two cDNA clones with homology to known desaturase genes were isolated from the fungus Mortierella alpina. The open reading frame in one clone encoded 399 amino acids and exhibited delta12-desaturase activity when expressed in Saccharomyces cerevisiae in the presence of endogenous fatty acid substrate oleic acid. The insert in another clone contained an open reading frame encoding 457 amino acids and exhibited delta6-desaturase activity in S. cerevisiae in the presence of exogenous fatty acid substrate linoleic acid. Expression of the delta12-desaturase gene under appropriate media and temperature conditions led to the production of linoleic acid at levels up to 25% of the total fatty acids in yeast. When linoleic acid was provided as an exogenous substrate to the yeast cultures expressing the delta6-desaturase activity, the level of gamma-linolenic acid reached 10% of the total yeast fatty acids. Co-expression of both the delta6- and delta12-desaturase cDNA resulted in the endogenous production of gamma-linolenic acid. The yields of gamma-linolenic acid reached as high as 8% of total fatty acids in yeast.

Identification of Delta5-desaturase from Mortierella alpina by heterologous expression in Bakers' yeast and canola.

A DNA fragment with homology to Delta6-desaturases from borage and cyanobacteria was isolated after polymerase chain reaction amplification of Mortierella alpina cDNA with oligonucleotide primers corresponding to the conserved regions of known Delta6-desaturase genes. This fragment was used as a probe to isolate a cDNA clone with an open reading frame encoding 446 amino acids from a M. alpina library. Expression of this open reading frame from an inducible promoter in Saccharomyces cerevisiae in the presence of various substrates revealed that the recombinant product had Delta5-desaturase activity. The effects of growth and induction conditions as well as host strain on activity of the recombinant Delta5-desaturase in S. cerevisiae were evaluated. Expression of the M. alpina Delta5-desaturase cDNA in transgenic canola seeds resulted in the production of taxoleic acid (Delta5,9-18:2) and pinolenic acid (Delta5,9,12-18:3), which are the Delta5-desaturation products of oleic and linoleic acids, respectively.