Two different pathways are involved in the β-oxidation of n-alkanoic and n-phenylalkanoic acids in Pseudomonas putida U: Genetic studies and biotechnological applications

In Pseudomonas putida U, the degradation of n-alkanoic and n-phenylalkanoic acids is carried out by two sets of β-oxidation enzymes (β_I and β_II). Whereas the first one (called β_I) is constitutive and catalyses the degradation of n-alkanoic and n-phenylalkanoic acids very efficiently, the other one (β_II), which is only expressed when some of the genes encoding β_I enzymes are mutated, catabolizes n-phenylalkanoates (n > 4) much more slowly. Genetic studies revealed that disruption or deletion of some of the β_I genes handicaps the growth of P. putida U in media containing n-alkanoic or n-phenylalkanoic acids with an acyl moiety longer than C₄. However, all these mutants regained their ability to grow in media containing n-alkanoates as a result of the induction of β_II, but they were still unable to catabolize n-phenylalkanoates completely, as the β_I-FadBA enzymes are essential for the β-oxidation of certain n-phenylalkanoyl-CoA derivatives when they reach a critical size. Owing to the existence of the β_II system, mutants lacking β_I-fadB/A are able to synthesize new poly 3-OH-n-alkanoates (PHAs) and poly 3-OH-n-phenylalkanoates (PHPhAs) efficiently. However, they are unable to degrade these polymers, becoming bioplastic overproducer mutants. The genetic and biochemical importance of these results is reported and discussed.