Abstract
Arbuscular mycorrhizal (AM) fungi are oleaginous organisms, and the most abundant fatty acyl moiety usually found in their lipids is palmitvaccenic acid (16:1Δ11cis). However, it is not known how this uncommon fatty acid species is made. Here, we have cloned two homologues of lepidopteran fatty acyl‐coenzyme A Δ11 desaturases from the AM fungus Rhizophagus irregularis. Both enzymes, DES1 and DES2, are expressed in intraradical mycelium and can complement the unsaturated fatty acid‐requiring auxotrophic growth phenotype of the Saccharomyces cerevisiae ole1Δ mutant. DES1 expression leads almost exclusively to oleic acid (18:1Δ9cis) production, whereas DES2 expression results in the production of 16:1Δ11cis and vaccenic acid (18:1Δ11cis). DES2 therefore encodes a Δ11 desaturase that is likely to be responsible for the synthesis of 16:1Δ11cis in R. irregularis.
Highlights
Arbuscular mycorrhizal (AM) fungi are oleaginous organisms, and the most abundant fatty acyl moiety usually found in their lipids is palmitvaccenic acid (16:1D11cis)
The lipids in many AM fungi are dominated by a single molecular species of monounsaturated fatty acid called 11-cis-palmitvaccenic acid (16:1D11cis), which can account for over 70 mol% of the fatty acyl moieties in their spores and is present mainly in the form of triacylglycerols [4,10,11,12]. 16:1D11cis is unusual in that it contains a double bond at the x5 position, and it has been used as a biomarker for arbuscular mycorrhization because it is not found in plants and it is rarely present in other soil microorganisms [10]. 16:1D11cis has been used in chemotaxonomy, because it is abundant in many AM fungi (Glomeromycota) but is lacking in certain species of the families Glomeraceae and Gigasporaceae [11]
transmembrane helices (TMHs) predicted by the Kyte–Doolittle hydropathy scale [18] and TMHMM [19] were in qualitative agreement (Fig. S1), and both algorithms placed the N and C termini of DES1 and DES2 in the cytosol, consistent with the topology of mammalian stearoyl (18:0)-coenzyme A (CoA) desaturases (SCDs) [30]
Summary
Arbuscular mycorrhizal (AM) fungi are oleaginous organisms, and the most abundant fatty acyl moiety usually found in their lipids is palmitvaccenic acid (16:1D11cis). It is not known how this uncommon fatty acid species is made. Genomic analysis has suggested that AM fungi are fatty acid auxotrophs [4] and subsequent studies have shown that they rely on their host plant to supply them with long-chain fatty acyl moieties so that they can make fungal lipids [5,6,7,8]. Abbreviations 16:1D11cis, 11-cis-palmitvaccenic acid; 18:1D11cis, 11-cis-vaccenic acid; AM, arbuscular mycorrhizal; CoA, coenzyme A; SCD, stearoyl-CoA desaturase; TEF1, translational elongation factor EF-1a
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