Characterization of a novel thioesterase (PtTE) from Phaeodactylum tricornutum.

The Phaeodactylum tricornutum (P. tricornutum) thioesterase PtTE was encoded by a 648 bp open reading frame. The deduced 216 amino acids showed no similarity with plant acyl-acyl carrier protein (ACP) thioesterases and bacterial thioesterases. Southern blot analysis revealed that one copy of PtTE was present in the P. tricornutum genome, and Real-time quantitative PCR showed that PtTE was up-regulated upon nitrogen deprivation. Thioesterase activity of PtTE was established by heterologous expression of PtTE cDNA in Escherichia coli (E. coli) XL1-Blue and K27fadD88, a mutant strain of fatty acid β-oxidation pathway. The substrate specificity of PtTE was determined by fatty acid profile analyses of the culture supernatant and membrane lipid of recombinant strains. Recombinant PtTE in E.coli enhanced total fatty acid content of XL1-Blue by 21%, and also changed the fatty acid compositions of membrane lipid and culture supernatant. These changes were directed predominantly towards C18:0 and C18:1 fatty acids. Overexpression of PtTE alone in P. tricornutum did not alter the fatty acid composition of P. tricornutum, but enhanced total fatty acid content by 72%. This novel thioesterase gene shows its potential in metabolic engineering for enhancing lipid yield of microalgae. This is so far the first report of thioesterase from eukaryotic microalgae.