Isolation and sequencing of the gene encoding delta 5-3-ketosteroid isomerase of Pseudomonas testosteroni: overexpression of the protein.

We describe the cloning, sequencing, and overexpression of the steroid isomerase (3-oxosteroid delta 5-delta 4-isomerase, EC 5.3.3.1) gene of Pseudomonas testosteroni. A genomic library of P. testosteroni total DNA constructed from partial EcoRI digests ligated to a lambda gtWES vector was probed with a 23-base oligonucleotide mixture [ATGAAC(T)ACC(A,T)CCG(C,A)GAG(A)CAC(T)ATGAC] corresponding to the NH2-terminal sequence of steroid isomerase. Subclones derived from a recombinant phage containing a 5400-base-pair insert were sequenced and found to contain the expected 375-nucleotide open reading frame flanked at both ends by in-frame TGA termination codons. The DNA sequence agreed with the above 125-amino acid sequence except for codons 22, 24, 33, and 38, all of which encoded Asp rather than Asn, and codon 77, which encoded Glu rather than Gln. A 1370-base-pair fragment was inserted into pUC 19 plasmid vector and used to construct a strain of Escherichia coli JM 101 that overexpressed the isomerase gene in the presence of isopropyl beta-D-thiogalactopyranoside. Cytosolic extracts of this strain contained a major soluble protein that migrated with the steroid delta-isomerase subunit on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. These cytosolic extracts had 10-50% of the specific activity of crystalline isomerase, depending on the method of preparation. The recombinant enzyme was crystallized in both monoclinic (flat plates) and hexagonal (bipyramids) crystal forms, described previously for the enzyme isolated from P. testosteroni. The kinetic properties of the crystalline recombinant enzyme, including specific activity, Km for 5-androstene-3,17-dione (340 microM), and Ki for the competitive inhibitor 19-nortestosterone (11.9 microM), agreed closely with the values reported for the isolated enzyme.