Nitrile pathway involving acyl-CoA synthetase: overall metabolic gene organization and purification and characterization of the enzyme

Two open reading frames (nhpS and acsA) were identified immediately downstream of the previously described Pseudomonas chlororaphis B23 nitrile hydratase (NHase) gene cluster (encoding aldoxime dehydratase, amidase, the two NHase subunits, and an uncharacterized protein). The amino acid sequence deduced from acsA shows similarity to that of acyl-CoA synthetase (AcsA). The acsA gene product expressed in Escherichia coli showed acyl-CoA synthetase activity toward butyric acid and CoA as substrates, with butyryl-CoA being synthesized. From the E. coli transformant, AcsA was purified to homogeneity and characterized. The quality of the recombinant protein was verified by the NH2-terminal amino acid sequence and the results of matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The apparent Km values for butyric acid, CoA, and ATP were 0.32 +/- 0.04, 0.37 +/- 0.02, and 0.22 +/- 0.02 mm, respectively. AcsA was shown to be a short-chain acyl-CoA synthetase, according to the catalytic efficiencies (kcat/Km) for various acids. The substrate specificity of AcsA was similar to those of aldoxime dehydratase, NHase, and amidase, the genes of which coexist in the same orientation in the gene cluster. P. chlororaphis B23 grew when cultured in a medium containing butyraldoxime as the sole carbon and nitrogen source. The activities of aldoxime dehydratase, NHase, and amidase were detected together with that of acyl-CoA synthetase under the culture conditions used. Moreover, on culture in a medium containing butyric acid as the sole carbon source, acyl-CoA synthetase activity was also detected. Together with the adjacent locations of the aldoxime dehydratase, NHase, amidase, and acyl-CoA synthetase genes, these findings suggest that the four enzymes are sequentially correlated with one another in vivo to utilize butyraldoxime as a carbon and nitrogen source. This is the first report of an overall "nitrile pathway" (aldoxime-->nitrile-->amide-->acid-->acyl-CoA) comprising these enzymes.     

Cloning and characterization of genes responsible for metabolism of nitrile compounds from Pseudomonas chlororaphis B23.

The nitrile hydratase (NHase) of Pseudomonas chlororaphis B23, which is composed of two subunits, alpha and beta, catalyzes the hydration of nitrile compounds to the corresponding amides. The NHase gene of strain B23 was cloned into Escherichia coli by the DNA-probing method with the NHase gene of Rhodococcus sp. strain N-774 as the hybridization probe. Nucleotide sequencing revealed that an amidase showing significant similarity to the amidase of Rhodococcus sp. strain N-774 was also coded by the region just upstream of the subunit alpha-coding sequence. In addition to these three proteins, two open reading frames, P47K and OrfE, were found just downstream of the coding region of subunit beta. The direction and close locations to each other of these open reading frames encoding five proteins (amidase, subunits alpha and beta, P47K, and OrfE, in that order) suggested that these genes were cotranscribed by a single mRNA. Plasmid pPCN4, in which a 6.2-kb sequence covering the region coding for these proteins is placed under control of the lac promoter, directed overproduction of enzymatically active NHase and amidase in response to addition of isopropyl-beta-D-thiogalactopyranoside. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the cell extract showed that the amount of subunits alpha and beta of NHase was about 10% of the total cellular proteins and that an additional 38-kDa protein probably encoded by the region upstream of the amidase gene was also produced in a large amount. The 38-kDa protein, as well as P47K and OrfE, appeared to be important for efficient expression of NHase activity in E. coli cells, because plasmids containing the NHase and amidase genes but lacking the region coding for the 38-kDa protein or the region coding for P47K and OrfE failed to express efficient NHase activity.     

Cloning and expression of the nitrile hydratase and amidase genes from Bacillus sp. BR449 into Escherichia coli

A moderate thermophile, Bacillus sp. BR449 was previously shown to exhibit a high level of nitrile hydratase (NHase) activity when growing on high levels of acrylonitrile at 55 degrees C. In this report, we describe the cloning of a 6.1 kb SalI DNA fragment encoding the NHase gene cluster of BR449 into Escherichia coli. Nucleotide sequencing revealed six ORFs encoding (in order), two unidentified putative proteins, amidase, NHase beta- and alpha-subunits and a small putative protein of 101 amino acids designated P12K. Spacings and orientation of the coding regions as well as their gene expression in E. coli suggest that the beta-subunit, alpha-subunit, and P12K genes are co-transcribed. Analysis of deduced amino acid sequences indicate that the amidase (348 aa, MW 38.6 kDa) belongs to the nitrilase-related aliphatic amidase family, and that the NHase beta- (229 aa, MW 26.5 kDa) and alpha- (214 aa, MW 24.5 kDa) subunits comprise a cobalt-containing member of the NHase family, which includes Rhodococcus rhodochrous J1 and Pseudomonas putida 5B NHases. The amidase/NHase gene cluster differs both in arrangement and composition from those described for other NHase-producing strains. When expressed in Escherichia coli DH5alpha, the subcloned NHase genes produced significant levels of active NHase enzyme when cobalt ion was added either to the culture medium or cell extracts. Presence of the P12K gene and addition of amide compounds as inducers were not required for this expression.