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The 7α‐hydroxysteroid dehydratase Hsh2 is essential for anaerobic degradation of the steroid skeleton of 7α‐hydroxyl bile salts in the novel denitrifying bacterium Azoarcus sp. strain Aa7
Author(s) -
Yücel Onur,
Borgert Sebastian Roman,
Poehlein Anja,
Niermann Karin,
Philipp Bodo
Publication year - 2019
Publication title -
environmental microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.954
H-Index - 188
eISSN - 1462-2920
pISSN - 1462-2912
DOI - 10.1111/1462-2920.14508
Subject(s) - biology , denitrifying bacteria , biochemistry , anoxic waters , bacteria , steroid , microbiology and biotechnology , hydroxylation , cholic acid , bile acid , denitrification , chemistry , enzyme , organic chemistry , ecology , genetics , hormone , nitrogen
Summary Bile salts are steroid compounds from the digestive tract of vertebrates and enter the environment via defecation. Many aerobic bile‐salt degrading bacteria are known but no bacteria that completely degrade bile salts under anoxic conditions have been isolated so far. In this study, the facultatively anaerobic Betaproteobacterium Azoarcus sp. strain Aa7 was isolated that grew with bile salts as sole carbon source under anoxic conditions with nitrate as electron acceptor. Phenotypic and genomic characterization revealed that strain Aa7 used the 2,3‐ seco pathway for the degradation of bile salts as found in other denitrifying steroid‐degrading bacteria such as Sterolibacterium denitrificans . Under oxic conditions strain Aa7 used the 9,10‐ seco pathway as found in, for example, Pseudomonas stutzeri Chol1. Metabolite analysis during anaerobic growth indicated a reductive dehydroxylation of 7α‐hydroxyl bile salts. Deletion of the gene hsh2 Aa7 encoding a 7‐hydroxysteroid dehydratase led to strongly impaired growth with cholate and chenodeoxycholate but not with deoxycholate lacking a hydroxyl group at C7. The hsh2 Aa7 deletion mutant degraded cholate and chenodeoxycholate to the corresponding C 19 ‐androstadienediones only while no phenotype change was observed during aerobic degradation of cholate. These results showed that removal of the 7α‐hydroxyl group was essential for cleavage of the steroid skeleton under anoxic conditions.