Abstract
BackgroundSchizochytrium species are known for their abundant production of docosahexaenoic acid (DHA). Low temperatures can promote the biosynthesis of polyunsaturated fatty acids (PUFAs) in many species. This study investigates low-temperature effects on DHA biosynthesis in Schizochytrium sp. TIO01 and its underlying mechanism.ResultsThe Schizochytrium fatty acid biosynthesis pathway was evaluated based on de novo genome assembly (contig N50 = 2.86 Mb) and iTRAQ-based protein identification. Our findings revealed that desaturases, involved in DHA synthesis via the fatty acid synthase (FAS) pathway, were completely absent. The polyketide synthase (PKS) pathway and the FAS pathway are, respectively, responsible for DHA and saturated fatty acid synthesis in Schizochytrium. Analysis of fatty acid composition profiles indicates that low temperature has a significant impact on the production of DHA in Schizochytrium, increasing the DHA content from 43 to 65% of total fatty acids. However, the expression levels of PKS pathway genes were not significantly regulated as the DHA content increased. Further, gene expression analysis showed that pathways related to the production of substrates (acetyl-CoA and NADPH) for fatty acid synthesis (the branched-chain amino acid degradation pathway and the pentose phosphate pathway) and genes related to saturated fatty acid biosynthesis (the FAS pathway genes and malic enzyme) were, respectively, upregulated and downregulated. These results indicate that low temperatures increase the DHA content by likely promoting the entry of relatively large amounts of substrates into the PKS pathway.ConclusionsIn this study, we provide genomic, proteomic, and transcriptomic evidence for the fatty acid synthesis pathway in Schizochytrium and propose a mechanism by which low temperatures promote the accumulation of DHA in Schizochytrium. The high-quality and nearly complete genome sequence of Schizochytrium provides a valuable reference for investigating the regulation of polyunsaturated fatty acid biosynthesis and the evolutionary characteristics of Thraustochytriidae species.
Highlights
Schizochytrium species are known for their abundant production of docosahexaenoic acid (DHA)
The intermediates of polyunsaturated fatty acid (PUFA) compounds involved in the fatty acid synthase (FAS) pathway for DHA synthesis, such as C18:1, C18:2, C18:3, and C20:3, were not detected by GC. These results indicate that the FAS pathway for DHA synthesis is incomplete in Schizochytrium and that the polyketide synthase (PKS) pathway is responsible for DHA synthesis
Based on the high-quality genome sequence of Schizochytrium sp TIO01, we found that the desaturase involved in the FAS pathway for PUFA synthesis was absent in Schizochytrium sp
Summary
Schizochytrium species are known for their abundant production of docosahexaenoic acid (DHA). In 2002, Metz et al reported that the PKS pathway is responsible for PUFA synthesis and does not rely on elongation and desaturation of the FAS pathway in Schizochytrium through the use of labeling experiments [7] This result was subsequently validated by Lippmeier et al who showed that knock-outs of the PKS gene led to PUFA auxotrophic behavior [8]. These biochemical and genetic studies on Schizochytrium have suggested that the PKS pathway is responsible for DHA biosynthesis [7,8,9], recent studies revealed that enhancement of the FAS pathway contributes significantly to increased DHA productivity in Schizochytrium [10, 11]. Genome sequencing is an ideal approach for investigating this issue, information on high-quality whole genomes of Schizochytrium currently available in public databases is limited [12], preventing identification of the Schizochytrium PUFA biosynthesis pathway
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