Abstract
Novel biodegradable bacterial plastics, made up of units of 3-hydroxy-n-phenylalkanoic acids, are accumulated intracellularly by Pseudomonas putida U due to the existence in this bacterium of (i) an acyl-CoA synthetase (encoded by the fadD gene) that activates the aryl-precursors; (ii) a beta-oxidation pathway that affords 3-OH-aryl-CoAs, and (iii) a polymerization-depolymerization system (encoded in the pha locus) integrated by two polymerases (PhaC1 and PhaC2) and a depolymerase (PhaZ). The complete assimilation of these compounds requires two additional routes that specifically catabolize the phenylacetyl-CoA or the benzoyl-CoA generated from these polyesters through beta-oxidation. Genetic studies have allowed the cloning, sequencing, and 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 the catabolism of their derivatives (acuA, fadA, and paa genes). Additional experiments showed that the blockade of either fadD or phaC1 hindered the synthesis and accumulation of plastic polymers. Disruption of phaC2 reduced the quantity of stored polymers by two-thirds. The blockade of phaZ hampered the mobilization of the polymer and decreased its production. Mutations in the paa genes, encoding the phenylacetic acid catabolic enzymes, did not affect the synthesis or catabolism of polymers containing either 3-hydroxyaliphatic acids or 3-hydroxy-n-phenylalkanoic acids with an odd number of carbon atoms as monomers, whereas the production of polyesters containing units of 3-hydroxy-n-phenylalkanoic acids with an even number of carbon atoms was greatly reduced in these bacteria. Yield-improving studies revealed that mutants defective in the glyoxylic acid cycle (isocitrate lyase(-)) or in the beta-oxidation pathway (fadA), stored a higher amount of plastic polymers (1.4- and 2-fold, respectively), suggesting that genetic manipulation of these pathways could be useful for isolating overproducer strains. The analysis of the organization and function of the pha locus and its relationship with the core of the phenylacetyl-CoA catabolon is reported and discussed.
Highlights
Novel biodegradable bacterial plastics, made up of units of 3-hydroxy-n-phenylalkanoic acids, are accumulated intracellularly by Pseudomonas putida U due to the existence in this bacterium of (i) an acyl-CoA synthetase that activates the aryl-precursors; (ii) a -oxidation pathway that affords 3-OH-aryl-CoAs, and (iii) a polymerization-depolymerization system integrated by two polymerases (PhaC1 and PhaC2) and a depolymerase (PhaZ)
From the present findings the following conclusions can be drawn. (i) P. putida U synthesizes and accumulates novel biodegradable plastic polymers containing as monomers 3-hydroxy-n-phenyl derivatives (10 Յ n Ն 5). (ii) A single acylCoA synthetase seems to be involved in the activation of phenylalkanoics and alkanoic acids to their CoA thioesters, and a similar transport system seems to be required for the uptake of alkanoic and phenylalkanoic acids
The disruption of the fadD gene handicapped the uptake of precursors, their activation, and, the synthesis of plastics. (iii) Polymerization of the monomers is carried out by two polymerases (PhaC1 and PhaC2), which are involved in the synthesis of polyhydroxyalkanoates
Summary
The restricted capacity of plastic-producing bacteria to store polyesters bearing a phenyl group suggests that the synthesis of these unusual compounds requires (i) a specific uptake system for the transport of the aromatic precursors, (ii) a specific acyl-CoA synthetase (ACS), (iii) a specific polymerase or a mutated enzyme with broader substrate specificity [14], and/or (iv) the existence of an additional catabolic route, linked to the -oxidation pathway, to ensure complete assimilation of the -oxidation products (benzoyl-CoA or phenylacetyl-CoA) generated from the monomers (3-hydroxyphenylalkanoyl-CoA derivatives) once released from the stored polymer (Fig. 1) [9] This complete assimilation could reduce the possibility that intermediate metabolites might inhibit the biosynthetic process. The inclusion of this pathway in the phenylacetyl-CoA catabolon [53] and its influence in the evolution of the catabolic potential of P. putida U are discussed
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
