Nitrile hydratase gene from Rhodococcus sp. N-774 requirement for its downstream region for efficient expression

For improvement of the production of nitrile hydratase (NHase) from Rhodococcus sp. N-774 by recombinant DNA techniques, several plasmids, each of which had a deletion of the upstream or downstream region of the genes encoding the alpha and beta subunits of NHase, were constructed. Enzyme assays of recombinant R. rhodochrous and Escherichia coli cells showed that a downstream region of the NHase genes was indispensable for the production of active NHase in both cells, but for the production of the active amidase, no genes other than the amidase structural gene were required. The nucleotide sequence of the downstream region contained a single open reading frame (Orf1188) with 396 amino acids. Orf1188 showed similarity in amino acid sequence to P47K, an open reading frame found downstream of the NHase genes from Pseudomonas chlororaphis B23, and also to the cobW gene product, which may be involved in cobalamin biosynthesis in Pseudomonas denitrificans. Because the distance between the TGA stop codon for the NHase beta-subunit and the ATG codon for Orf1188 is only 98 bp, and because production of both Orf1188 and NHase is dependent on a promoter upstream of the amidase gene, these genes appear to be co-transcribed in a polycistronic manner, forming an operon. By optimization of the culture conditions of R. rhodochrous carrying pKRNH2, which contained the amidase, NHase, and Orf1188 genes, the transformant showed the NHase activity 6-fold higher than that of the original strain, Rhodococcus sp. N-774.