CLONING, SEQUENCING, AND EXPRESSION OF THE GENE CODING FOR BILE-ACID 7-ALPHA-HYDROXYSTEROID DEHYDROGENASE FROM EUBACTERIUM SP STRAIN VPI-12708

Southern blot analysis indicated that the gene encoding the constitutive, NADP-linked bile acid 7-alpha-hydroxysteroid dehydrogenase of Eubacterium sp. strain VPI 12708 was located on a 6.5-kb EcoRI fragment of the chromosomal DNA. This fragment was cloned into bacteriophage lambda gt11, and a 2.9-kb piece of this insert was subcloned into pUC19, yielding the recombinant plasmid pBH51. DNA sequence analysis of the 7-alpha-hydroxysteroid dehydrogenase gene in pBH51 revealed a 798-bp open reading frame, coding for a protein with a calculated molecular weight of 28,500. A putative promoter sequence and ribosome binding site were identified. The 7-alpha-hydroxysteroid dehydrogenase mRNA transcript in Eubacterium sp. strain VPI 12708 was about 0.94 kb in length, suggesting that it is monocistronic. An Escherichia coli DH5-alpha-transformant harboring pBH51 had approximately 30-fold greater levels of 7-alpha-hydroxysteroid dehydrogenase mRNA, immunoreactive protein, and specific activity than Eubacterium sp. strain VPI 12708. The 7-alpha-hydroxysteroid dehydrogenase purified from the pBH51 transformant was similar in subunit molecular weight, specific activity, and kinetic properties to that from Eubacterium sp. strain VPI 12708, and it reacted with antiserum raised against the authentic enzyme on Western immunoblots. Alignment of the amino acid sequence of the 7-alpha-hydroxysteroid dehydrogenase with those of 10 other pyridine nucleotide-linked alcohol/polyol dehydrogenases revealed six conserved amino acid residues in the N-terminal regions thought to function in coenzyme binding.