Poly[(R)-3-hydroxybutyrate] formation in Escherichia coli from glucose through an enoyl-CoA hydratase-mediated pathway

In this study, a new metabolic pathway for the synthesis of poly[(R)-3-hydroxybutyrate] [P(3HB)] was constructed in a recombinant Escherichia coli strain that utilized forward and reverse reactions catalyzed by two substrate-specific enoyl-CoA hydratases, R-hydratase (PhaJ) and S-hydratase (FadB), to epimerize (S)-3HB-CoA to (R)-3HB-CoA via a crotonyl-CoA intermediate. The R-hydratase gene (phaJAc) from Aeromonas caviae was coexpressed with the PHA synthase gene (phaCRe) and 3-ketothiolase gene (phaARe) from Ralstonia eutropha in fadR mutant E. coli strains (CAG18497 and LS5218), which had constitutive levels of the β-oxidation multienzyme FadBEc. When grown on glucose as the sole carbon source, the cells accumulated P(3HB) up to an amount 6.5 wt% of the dry cell weight, whereas the control cells without phaJAc or fadR mutation accumulated significantly smaller amounts of P(3HB). These results suggest that PhaJAc and FadBEc played an important role in supplying monomers for P(3HB) synthesis in the pathway. Furthermore, by using this pathway, a P(3HB)-concentration-dependent fluorescent staining screening technique was developed to rapidly identify cells that possess active R-hydratase.