Eubacterium sp. strain VPI 12708 is a human intestinal bacterium which contains an inducible bile acid 7-dehydroxylase. Two-dimensional polyacrylamide gel electrophoresis showed that at least four new polypeptides were synthesized after exposure of growing cells to sodium cholate. One of these, of molecular weight 27,000 (PP-27), was implicated in 7-dehydroxylase catalysis. PP-27 was purified to greater than 95% homogeneity by DEAE-cellulose chromatography, high-pressure liquid chromatographic gel filtration, high-pressure liquid chromatography-DEAE chromatography, and preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The first 33 amino acid residues of the N terminus of PP-27 were determined with a gas-phase sequencer, and a corresponding mixed oligonucleotide (17-mer) was synthesized. Southern blot analysis of EcoRI total digests of chromosomal DNA showed a 2.2-kilobase fragment which hybridized to the 32P-labeled 17-mer. This fragment was enriched for by size fractionation of an EcoRI total digest of genomic DNA, ligated into the bacterial plasmid pUC8, and used to transform Escherichia coli HB101. Transformants containing the putative 7-dehydroxylase gene were detected with the 32P-labeled 17-mer by colony hybridization techniques. The insert was 2.2 kilobases in length and contained the first 290 bases of the PP-27 gene. Preliminary nucleic acid sequence data correlate with the amino acid sequence. The entire gene was cloned on a 1,150-base-pair TaqI fragment. Western blot analysis of E. coli strains containing these plasmids indicated that PP-27 is expressed in E. coli but is not regulated by bile acids under the conditions used.