Discovery of a High 3-Hydroxyhexanoate Containing Poly-3-hydroxybutyrate-co-3-hydroxyhexanoate Producer-, Cupriavidus sp. Oh_1 with Enhanced Fatty Acid Metabolism

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P(3HB-<i>co</i>-3HHx)) is a representative PHA copolymer that can improve the mechanical limitations of polyhydroxybutyrate (P(3HB)). Although genetic engineering can facilitate 3HHx incorporation, it often compromises cell growth and reduces polymer molecular weight owing to metabolic disruptions caused by the deletion of acetoacetyl coenzyme A (acetyl-CoA) reductase (PhaB). To address this issue, native strains capable of producing high levels of 3HHx were identified via oil-based <i>Cupriavidus</i> screening. Eight PHA-producing strains were isolated from various samples and <i>Cupriavidus</i> sp. Oh_1 exhibited the highest polyhydroxybutyrate (PHB) production at 15.23 g/L from 17.2 g/L of biomass using soybean oil. Moreover, Oh_1/<i>phaC</i>_Ra<i>J</i>_Pa, containing enoyl-CoA hydratase (<i>phaJ</i>) and PHA synthetase (<i>phaC</i>), was identified as the most effective novel strain producing the highest 3HHx mole fraction, 48.93 g/L of PHA from 52.3 g/L of biomass and achieving a maximum 3HHx accumulation of 27.2 mol%. The resulting P(3HB-<i>co</i>-3HHx) showed a higher Mw (12.3 × 10⁵) compared with P(3HB-<i>co</i>-3HHx) produced by the <i>phaB</i>-deleted strain (14.6 × 10⁴). Higher production of 3HHx was attributed to the higher expression of <i>phaC</i>_Ra and <i>phaJ</i>_Pa in Oh_1, with log2 fold changes of 2.94 and 8.2, respectively, as well as the upregulation of certain β-oxidation encoding operons. Collectively, these findings highlight a strain capable of synthesizing a substantial 3HHx fraction without requiring gene deletions or extensive genetic modifications.