Rationally engineered biotransformation of p-nitrophenol

An operon encoding enzymes responsible for degradation of the EPA priority contaminant para-nitrophenol (PNP) from Pseudomonas sp. ENV2030 contains more genes than would appear to be necessary to mineralize PNP. To determine some necessary genes for PNP degradation, the genes encoding the proposed enzymes in the degradation pathway (pnpADEC) were assembled into a broad-host-range, BioBricks-compatible vector under the control of a constitutive promoter. These were introduced into Escherichia coli DH10b and two Pseudomonas putida strains, one with a knockout of the aromatic transport TtgB and the parent with the native transporter. The engineered strains were assayed for PNP removal. E. coli DH10b harboring several versions of the refactored pathway was able to remove PNP from the medium up to a concentration of 0.2 mM; above which PNP was toxic to E. coli. A strain of P. putida harboring the PNP pathway genes was capable of removing PNP from the medium up to 0.5 mM. When P. putida harboring the native PNP degradation cluster was exposed to PNP, pnpADEC were induced, and the resulting production of beta-ketoadipate from PNP induced expression of its chromosomal degradation pathway (pcaIJF). In contrast, pnpADEC were expressed constitutively from the refactored constructs because none of the regulatory genes found in the native PNP degradation cluster were included. Although P. putida harboring the refactored construct was incapable of growing exclusively on PNP as a carbon source, evidence that the engineered pathway was functional was demonstrated by the induced expression of chromosomal pcaIJF.