Novel biodegradable aromatic plastics from a bacterial source - Genetic and biochemical studies on a route of the phenylacetyl-CoA catabolon

Novel biodegradable bacterial plastics, made up of units of 3-hydroxy-n-phe nylalkanoic acids, are accumulated intracellularly by Pseudomonas putida U due to the existence in this bacterium of (i) an acyl-CoA synthetase (encod ed by the fadD gene) that activates the aryl-precursors; (ii) a beta-oxidat ion pathway that affords 3-OH-aryl-CoAs, and (iii) a polymerization-depolym erization system (encoded in the pha locus) integrated by two polymerases ( PhaC1 and PhaC2) and a depolymerase (PhaZ). The complete assimilation of th ese compounds requires two additional routes that specifically catabolize t he phenylacetyl-CoA or the benzoyl-CoA generated from these polyesters thro ugh beta-oxidation. Genetic studies have allowed the cloning sequencing, an d disruption of the genes included in the pha locus (phaC1, phaC2, and phaZ ) as well as those related to the biosynthesis of precursors (fadD) or to t he catabolism of their derivatives (acuA, fadA, and paa genes). Additional experiments showed that the blockade of either fadD or phaC1 hindered the s ynthesis and accumulation of plastic polymers, Disruption of phaC2 reduced the quantity of stored polymers by two-thirds. The blockade of phaZ hampere d the mobilization of the polymer and decreased its production. Mutations i n the paa genes, encoding the phenylacetic acid catabolic enzymes, did not affect the synthesis or catabolism of polymers containing either 3-hydroxya liphatic acids or 3-hydroxy-n-phenylalkanoic acids with an odd number of ca rbon atoms as monomers, whereas the production of polyesters containing uni ts of 3-hydroxy-n-phenylalkanoic acids with an even number of carbon atoms was greatly reduced in these bacteria. Yield-improving studies revealed tha t mutants defective in the glyoxylic acid cycle (isocitrate lyase(-)) or in the beta-oxidation pathway (fadA), stored a higher amount of plastic polym ers (1.4- and 2-fold, respectively), suggesting that genetic manipulation o f these pathways could be useful for isolating overproducer strains. The an alysis of the organization and function of the pha locus and its relationsh ip with the core of the phenylacetyl-CoA catabolon is reported and discusse d.