PhaG-Mediated Synthesis of Poly(3-Hydroxyalkanoates) Consisting of Medium-Chain-Length Constituents from Nonrelated Carbon Sources in Recombinant Pseudomonas fragi

Abstract

ABSTRACT Recently, a new metabolic link between fatty acid de novo biosynthesis and biosynthesis of poly(3-hydroxy-alkanoate) consisting of medium-chain-length constituents (C 6 to C 14 ) (PHA MCL ), catalyzed by the 3-hydroxydecanoyl-[acyl-carrier-protein]:CoA transacylase (PhaG), has been identified in Pseudomonas putida (B. H. A. Rehm, N. Krüger, and A. Steinbüchel, J. Biol. Chem. 273:24044–24051, 1998). To establish this PHA-biosynthetic pathway in a non-PHA-accumulating bacterium, we functionally coexpressed phaC1 (encoding PHA synthase 1) from Pseudomonas aeruginosa and phaG (encoding the transacylase) from P. putida in Pseudomonas fragi . The recombinant strains of P. fragi were cultivated on gluconate as the sole carbon source, and PHA accumulation to about 14% of the total cellular dry weight was achieved. The respective polyester was isolated, and GPC analysis revealed a weight average molar mass of about 130,000 g mol −1 and a polydispersity of 2.2. The PHA was composed mainly (60 mol%) of 3-hydroxydecanoate. These data strongly suggested that functional expression of phaC1 and phaG established a new pathway for PHA MCL biosynthesis from nonrelated carbon sources in P. fragi . When fatty acids were used as the carbon source, no PHA accumulation was observed in PHA synthase-expressing P. fragi , whereas application of the β-oxidation inhibitor acrylic acid mediated PHA MCL accumulation. The substrate for the PHA synthase PhaC1 is therefore presumably directly provided through the enzymatic activity of the transacylase PhaG by the conversion of ( R )-3-hydroxydecanoyl-ACP to ( R )-3-hydroxydecanoyl-CoA when the organism is cultivated on gluconate. Here we demonstrate for the first time the establishment of PHA MCL synthesis from nonrelated carbon sources in a non-PHA-accumulating bacterium, employing fatty acid de novo biosynthesis and the enzymes PhaG (a transacylase) and PhaC1 (a PHA synthase).