Isolation and characterization of a bile acid inducible 7α-dehydroxylating operon in Clostridium hylemonae TN271

Primary bile acids are synthesized from cholesterol in the liver, conjugated to either glycine or taurine and secreted into bile. Bile salts undergo enterohepatic circulation several times each day. During this process, they are biotransformed into a variety of metabolites by gut bacteria. The major biotransformation is the 7α-dehydroxylation of cholic acid and chenodeoxycholic acid yielding deoxycholic acid and lithocholic acid, respectively. 7α-Dehydroxylation is a multi-step pathway. The genes encoding enzymes in this pathway have been identified in two species of “high” activity strains of clostridia. Here, we report the isolation and characterization of a bile acid inducible (bai) operon in Clostridium hylemonae, a “low” activity 7α-dehydroxylating strain. The gene organization and sequence of the baiBCDEFGHI operon was highly conserved between C. hylemonae and “high” activity strains. Surprisingly, the baiA gene was missing from the bai operon of C. hylemonae. The baiA gene was isolated using PCR and degenerate oligonucleotide primers. The mRNA start site for the large bai operon was determined and shown to be only 11 bp from the initiation codon of the first gene. It was also discovered that allocholic acid (5α) induced the bai operon and stimulated the conversion of [24-14C] cholic acid to [24-14C] allodeoxycholic acid in cultures of C. scindens and C. hylemonae allodeoxycholic acid. Finally, it was discovered that the addition of testosterone to the growth medium markedly increased 7α-dehydroxylation of cholic acid in Clostridium scindens and C. hylemonae. We hypothesize that testosterone may be a gratuitous inducer of genes involved in the reductive arm of the bile acid 7α-dehydroxylation pathway.