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Identification and characterization of a bile acid 7alpha-dehydroxylation operon in Clostridium sp. strain TO-931, a highly active 7alpha-dehydroxylating strain isolated from human feces.

ABSTRACT: Clostridium sp. strain TO-931 can rapidly convert the primary bile acid cholic acid to a potentially toxic compound, deoxycholic acid. Mixed oligonucleotide probes were used to isolate a gene fragment encoding a putative bile acid transporter from Clostridium sp. strain TO-931. This DNA fragment had 60% nucleotide sequence identity to a known bile acid transporter gene from Eubacterium sp. strain VPI 12708, another bile acid-7alpha-dehydroxylating intestinal bacterium. The DNA (9.15 kb) surrounding the transporter gene was cloned from Clostridium sp. strain TO-931 and sequenced. Within this larger DNA fragment was a 7.9-kb region, containing six successive open reading frames (ORFs), that was encoded by a single 8.1-kb transcript, as determined by Northern blot analysis. The gene arrangement and DNA sequence of the Clostridium sp. strain TO-931 operon are similar to those of a Eubacterium sp. strain VPI 12708 bile acid-inducible operon containing nine ORFs. Several genes in the Eubacterium sp. strain VPI 12708 operon have been shown to encode products required for bile acid 7alpha-dehydroxylation. In Clostridium sp. strain TO-931, genes potentially encoding bile acid-coenzyme A (CoA) ligase, 3alpha-hydroxysteroid dehydrogenase, bile acid 7alpha-dehydratase, bile acid-CoA hydrolase, and a bile acid transporter were similar in size and exhibited amino acid homology to similar gene products from Eubacterium sp. strain VPI 12708 (encoded by baiB, baiA, baiE, baiF, and baiG, respectively). However, no genes similar to Eubacterium sp. strain VPI 12708 biaH or baiI were found in the Clostridium sp. strain TO-931 bai operon, and the two putative Eubacterium sp. strain VPI 12708 genes, baiC and baiD, were arranged in one continuous ORF in Clostridium sp. strain TO-931. Intergene regions showed no significant DNA sequence similarity, but primer extension analysis identified a region 115 bp upstream from the first ORF that exhibited 58% identity to a bai operator/promoter region identified in Eubacterium sp. strain VPI 12708. These results indicate that the gene organization, gene product amino acid sequences, and promoters of the bile acid-inducible operons of Clostridium sp. strain TO-931 and Eubacterium sp. strain VPI 12708 are highly conserved.

SUBMITTER: Wells JE

PROVIDER: S-EPMC91949 | biostudies-literature | 2000 Mar

REPOSITORIES: biostudies-literature

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Identification and characterization of a bile acid 7alpha-dehydroxylation operon in Clostridium sp. strain TO-931, a highly active 7alpha-dehydroxylating strain isolated from human feces.

Wells J E JE Hylemon P B PB

Applied and environmental microbiology 20000301 3

Clostridium sp. strain TO-931 can rapidly convert the primary bile acid cholic acid to a potentially toxic compound, deoxycholic acid. Mixed oligonucleotide probes were used to isolate a gene fragment encoding a putative bile acid transporter from Clostridium sp. strain TO-931. This DNA fragment had 60% nucleotide sequence identity to a known bile acid transporter gene from Eubacterium sp. strain VPI 12708, another bile acid-7alpha-dehydroxylating intestinal bacterium. The DNA (9.15 kb) surround ...[more]

PMID: 10698778

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Project description:Bile acids (BAs) are steroid-derived molecules with important roles in digestion, the maintenance of host metabolism, and immunomodulation. Primary BAs are synthesized by the host, while secondary BAs are produced by the gut microbiome through transformation of the former. The regulation of microbial production of secondary BAs is not well understood, particularly the production of 7-dehydroxylated BAs, which are the most potent agonists for host BA receptors. The 7-dehydroxylation of cholic acid (CA) is well established and is linked to the expression of a bile acid-inducible (bai) operon responsible for this process. However, little to no 7-dehydroxylation has been reported for other host-derived BAs (e.g., chenodeoxycholic acid, CDCA or ursodeoxycholic acid, UDCA). Here, we demonstrate that the 7-dehydroxylation of CDCA and UDCA by the human isolate Clostridium scindens is induced when CA is present, suggesting that CA-dependent transcriptional regulation is required for substantial 7-dehydroxylation of these primary BAs. This is supported by the finding that UDCA alone does not promote expression of bai genes. CDCA upregulates expression of the bai genes but the expression is greater when CA is present. In contrast, the murine isolate Extibacter muris exhibits a distinct response; CA did not induce significant 7-dehydroxylation of primary BAs, whereas BA 7-dehydroxylation was promoted upon addition of germ-free mouse cecal content in vitro. However, E. muris was found to 7-dehydroxylate in vivo. The distinct expression responses amongst strains indicate that bai genes are regulated differently. CA promoted bai operon gene expression and the 7-dehydroxylating activity in C. scindens strains. Conversely, the in vitro activity of E. muris was promoted only after the addition of cecal content and the isolate did not alter bai gene expression in response to CA. The accessory gene baiJ was only upregulated in the C. scindens ATCC 35704 strain, implying mechanistic differences amongst isolates. Interestingly, the human-derived C. scindens strains were also capable of 7-dehydroxylating murine bile acids (muricholic acids) to a limited extent. This study shows novel 7-dehydroxylation activity in vitro resulting from the presence of CA and suggests distinct bai gene expression across bacterial species.

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Dataset Information

Identification and characterization of a bile acid 7alpha-dehydroxylation operon in Clostridium sp. strain TO-931, a highly active 7alpha-dehydroxylating strain isolated from human feces.

Publications

Identification and characterization of a bile acid 7alpha-dehydroxylation operon in Clostridium sp. strain TO-931, a highly active 7alpha-dehydroxylating strain isolated from human feces.

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