Functional analysis of the dehydratase domains of a PUFA synthase from Thraustochytrium in Escherichia coli

Thraustochytrium sp. 26185, a unicellular marine protist, synthesizes docosahexaenoic acid, an omega-3 very long chain polyunsaturated fatty acid (VLC-PUFAs), by a polyunsaturated fatty acid (PUFA) synthase comprising three large subunits with multiple catalytic dehydratase (DH) domains critical for introducing double bonds at the specific position of fatty acids. To investigate functions of these DH domains, one DH domain from subunit-A and two DH domains from subunit-C of the PUFA synthase were dissected and expressed as stand-alone enzymes in Escherichia coli. The results showed that all these DH domains could complement the defective phenotype of a E. coli FabA temperature sensitive mutant, despite they have only modest sequence similarity with FabA, indicating they can function as 3-hydroxyacyl-ACP dehydratase for the biosynthesis of unsaturated fatty acids in E. coli. Site-directed mutagenesis analysis confirmed the authenticity of active site residues in these domains. In addition, overexpression of the three domains in a wild type E. coli strain resulted in the substantial alteration of fatty acid profiles including productions and ratio of unsaturated to saturated fatty acids. A combination of evidences from sequence comparison, functional expression, and mutagenesis analysis suggest that the DH domain from subunit-A is similar to DH domains from polyketide synthases, while the DH domains from subunit-C are more comparable to E. coli FabA in catalytic functions. Successful complementation and functional expression of the embedded DH domains from the PUFA synthase in E. coli is an important step towards for elucidating the molecular mechanism in the biosynthesis of VLC-PUFAs in Thraustochytrium.