A non-canonical \u03949-desaturase synthesizing palmitoleic acid identified in the thraustochytrid Aurantiochytrium sp. T66

E-Ming Rau,Helga Ertesvåg,Inga Marie Aasen

doi:10.1007/s00253-021-11425-5

E-Ming Rau, Helga Ertesvåg + Show 1 more

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https://doi.org/10.1007/s00253-021-11425-5

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Abstract

Thraustochytrids are oleaginous marine eukaryotic microbes currently used to produce the essential omega-3 fatty acid docosahexaenoic acid (DHA, C22:6 n-3). To improve the production of this essential fatty acid by strain engineering, it is important to deeply understand how thraustochytrids synthesize fatty acids. While DHA is synthesized by a dedicated enzyme complex, other fatty acids are probably synthesized by the fatty acid synthase, followed by desaturases and elongases. Which unsaturated fatty acids are produced differs between different thraustochytrid genera and species; for example, Aurantiochytrium sp. T66, but not Aurantiochytrium limacinum SR21, synthesizes palmitoleic acid (C16:1 n-7) and vaccenic acid (C18:1 n-7). How strain T66 can produce these fatty acids has not been known, because BLAST analyses suggest that strain T66 does not encode any Δ9-desaturase-like enzyme. However, it does encode one Δ12-desaturase-like enzyme. In this study, the latter enzyme was expressed in A. limacinum SR21, and both C16:1 n-7 and C18:1 n-7 could be detected in the transgenic cells. Our results show that this desaturase, annotated T66Des9, is a Δ9-desaturase accepting C16:0 as a substrate. Phylogenetic studies indicate that the corresponding gene probably has evolved from a Δ12-desaturase-encoding gene. This possibility has not been reported earlier and is important to consider when one tries to deduce the potential a given organism has for producing unsaturated fatty acids based on its genome sequence alone.Key points• In thraustochytrids, automatic gene annotation does not always explain the fatty acids produced.• T66Des9 is shown to synthesize palmitoleic acid (C16:1 n-7).• T66des9 has probably evolved from Δ12-desaturase-encoding genes.

Highlights

Thraustochytrids are heterotrophic marine eukaryotes known for the ability to accumulate high levels of the omega-3 fatty acid DHA (Aasen et al 2016; Morabito et al 2019)
Polyunsaturated fatty acids (PUFAs) are synthesized from C16:0, the main product of the fatty acid synthase (FAS), by the desaturase-elongase (DE) pathway (Sun et al 2020; Thelen and Ohlrogge 2002) (Fig. 1)
Homology modeling using Phyre2 (Kelley et al 2015) indicated that only the sequences of membrane-bound fatty acids (FAs) desaturases from mammals (Bai et al 2015; Wang et al 2015) were sufficiently similar to obtain a model

Summary

Introduction

Thraustochytrids are heterotrophic marine eukaryotes known for the ability to accumulate high levels of the omega-3 fatty acid DHA (Aasen et al 2016; Morabito et al 2019). They belong to the Stramenopiles group and are divided into several genera such as Schizochytrium, Aurantiochytrium, and Thraustochytrium (Marchan et al 2018). PUFAs are synthesized from C16:0, the main product of the fatty acid synthase (FAS), by the desaturase-elongase (DE) pathway (Sun et al 2020; Thelen and Ohlrogge 2002) (Fig. 1). The desaturases of the DE pathway are mixed function oxidases, and the reaction is coupled to an electron flow

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