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Abstract
Thraustochytrium, a unicellular marine protist, has been used as a commercial source of very long chain PUFAs (VLCPUFAs) such as DHA (22:6n-3). Our recent work indicates coexistence of a Δ4-desaturation-dependent pathway (aerobic) and a polyketide synthase-like PUFA synthase pathway (anaerobic) to synthesize the fatty acids in Thraustochytrium sp. 26185. Heterologous expression of the Thraustochytrium PUFA synthase along with a phosphopantetheinyl transferase in Escherichia coli showed the anaerobic pathway was highly active in the biosynthesis of VLCPUFAs. The amount of Δ4 desaturated VLCPUFAs produced reached about 18% of the total fatty acids in the transformant cells at day 6 in a time course of the induced expression. In Thraustochytrium, the expression level of the PUFA synthase gene was much higher than that of the Δ4 desaturase gene, and also highly correlated with the production of VLCPUFAs. On the other hand, Δ9 and Δ12 desaturations in the aerobic pathway were either ineffective or absent in the species, as evidenced by the genomic survey, heterologous expression of candidate genes, and in vivo feeding experiments. These results indicate that the anaerobic pathway is solely responsible for the biosynthesis for VLCPUFAs in Thraustochytrium.
Highlights
Thraustochytrium, a unicellular marine protist, has been used as a commercial source of very long chain PUFAs (VLCPUFAs) such as DHA (22:6n-3)
To further study the biosynthesis and assembly of VLCPUFAs in Thraustochytrium sp. 26185, we recently sequenced the whole genome of this species
Incomplete aerobic pathway for the biosynthesis of VLCPUFAs in Thraustochytrium. To interrogate whether both pathways were functional for the VLCPUFA biosynthesis, we attempted to clone all putative genes in the two pathways that were not biochemically characterized previously, and expressed them in yeast or E. coli for the functional analysis
Summary
Thraustochytrium, a unicellular marine protist, has been used as a commercial source of very long chain PUFAs (VLCPUFAs) such as DHA (22:6n-3). VLCPUFAs occurs only in certain types of oceanic microorganisms, while animals and plants lack the PUFA synthase system and possess just part of the entire aerobic biosynthetic pathway, and are unable to completely synthesize these fatty acids. For those marine microbes that can de novo synthesize VLCPUFAs, the biosynthetic process goes through either an aerobic pathway employing desaturases and elongases to introduce double bonds and extend carbon chains of preexisting fatty acids for producing the final products or an anaerobic pathway employing a PUFA synthase to carry out all reactions required for conversion of initial acetyl-CoA to VLCPUFAs
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