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The gene encoding phosphopantetheinyl transferase (PPTase),  pfaE , a component of the polyketide synthase (PKS) pathway, is crucial for the production of docosahexaenoic acid (DHA, 22:6ω3), along with the other  pfa  cluster members  pfaA ,  pfaB ,  pfaC  and  pfaD . DHA was produced in  Escherichia coli  by co-expressing  pfaABCD  from DHA-producing  Colwellia psychrerythraea  34H with one of four  pfaE  genes from bacteria producing arachidonic acid (ARA, 20:4ω6), eicosapentaenoic acid (EPA, 20:5ω3) or DHA, respectively. Substitution of the  pfaE  gene from different strain source in  E .  coli  did not influence the function of the PKS pathway producing DHA, although they led to different DHA yields and fatty acid profiles. This result suggested that the  pfaE  gene could be switchable between these strains for the production of DHA. The DHA production by expressing the reconstituted PKS pathway was also investigated in different  E .  coli  strains, at different temperatures, or with the treatment of cerulenin. The highest DHA production, 2.2 mg of DHA per gram of dry cell weight or 4.1% of total fatty acids, was obtained by co-expressing  pfaE (EPA) from the EPA-producing strain  Shewanella baltica  with  pfaABCD  in DH5α. Incubation at low temperature (10–15°C) resulted in higher accumulation of DHA compared to higher temperatures. The addition of cerulenin to the medium increased the proportion of DHA and saturated fatty acids, including C12:0, C14:0 and C16:0, at the expense of monounsaturated fatty acids, including C16:1 and C18:1. Supplementation with 1 mg/L cerulenin resulted in the highest DHA yield of 2.4 mg/L upon co-expression of  pfaE (DHA) from  C .  psychrerythraea .

DHA Production in Escherichia coli by Expressing Reconstituted Key Genes of Polyketide Synthase Pathway from Marine Bacteria