Functions of Enyolreductase (ER) Domains of PKS Cluster in Lipid Synthesis and Enhancement of PUFAs Accumulation in Schizochytrium limacinum SR21 Using Triclosan as a Regulator of ER

The polyketide synthase (PKS) cluster genes are supposed to synthesize polyunsaturated fatty acids (PUFAs) in S. limacinum. In this study, two enyolreductase (ER) genes located on PKS cluster were knocked out through homologous recombination to explore their functions. The knock-out of OrfB-ER (located on OrfB subunit) decreased lipid content and had obvious decrease on PUFAs content, indicating OrfB-ER domain played a vital role on PUFAs synthesis; the knock-out of OrfC-ER (located on OrfC subunit) decreased SFAs content and increased total lipid content, indicating OrfC-ER domain was likely to be related with SFAs synthesis, and lipid production could be improved by down-regulating OrfC-ER domain expression. Therefore, the addition of triclosan as a reported regulator of ER domain induced the increase of PUFAs production by 51.74% and lipids yield by 47.63%. Metabolic analysis indicated triclosan played its role through inhibiting the expression of OrfC-ER to reduce the feedback inhibition of SFAs and further to enhance NADPH synthesis for lipid production, and by weakening mevalonate pathway and tricarboxylic acid (TCA) cycle to shift precursors for lipid and PUFAs synthesis. This research illuminates functions of two ER domains in S. limacinum and provides a potential targets for improving lipid production.