Enhanced Synthesis of Poly(3-hydroxybutyrate) in Recombinant Escherichia coli by Means of Error-Prone PCR Mutagenesis, Saturation Mutagenesis, and In Vitro Recombination of the Type II Polyhydroxyalkanoate Synthase Gene

Type II synthase (PhaC1(Ps)) for polyhydroxyalkanoate (PHA) from Pseudomonas sp. 61-3 was subjected to an in vitro evolution system including PCR-mediated mutagenesis in order to improve the function of PhaC1(Ps) in terms of its ability to produce poly(3-hydroxybutyrate) [P(3HB)] in recombinant Escherichia coli. Based on our established in vivo assay system, two positions (Ser325 and Gln481) where mutations provided remarkable increases in P(3HB) synthesis were identified. Saturation mutagenesis at these positions was carried out to explore whether there might be more beneficial sequences for P(3HB) synthesis than those identified in the point mutation library. As a result, five single mutants [S325C (T) and Q481M (K, R)] gave rise to highly enhanced P(3HB) synthesis. Drastically enhanced P(3HB) synthesis (up to 340- to 400-fold the amount of that of the wild type) was further achieved by generation of all five variants of the double mutants combining the codons for residues 325/481. It is feasible that the replacement of Ser (specific for type II synthase) by Thr (specific for type I synthase) at position 325 resulted in acquiring greater P(3HB) synthesis ability as exhibited by type I synthases. The other hot spot, 481, that positively contributes to enhanced P(3HB) synthesis is located adjacent to a His479, a residue that forms a putative catalytic diad that can be inferred by sequence alignment.