Identification of a thiolase gene essential for β-oxidation of the acyl side chain of the steroid compound cholate in Pseudomonas sp. strain Chol1.

Bile salts such as cholate are steroid compounds occurring ubiquitously in the environment through excretion by animals. Cholate degradation by Pseudomonas sp. strain Chol1 is initiated by A-ring oxidation and β-oxidation of the acyl side chain. A transposon mutant of strain Chol1 was isolated that could not grow with cholate, but transformed it into several steroid compounds accumulating in culture supernatants. The main product was identified as (22E)-7α,12α-dihydroxy-3-oxochola-1,4,22-triene-24-oate (DHOCTO). A further compound was identified as 7α,12α,22-trihydroxy-3-oxochola-1,4-diene-24-oate (THOCDO). The structures of DHOCTO and THOCDO indicate that they are intermediates of the β-oxidation of the acyl side chain. The interrupted gene was named skt and had similarities to the 3-ketoacyl-CoA thiolase domain of the eukaryotic sterol carrier protein SCP-x. An skt mutant grew with intermediates of cholate degradation, from which the acyl side chain had been partly or completely removed. Growth with cholate was restored by an intact skt copy on a plasmid. These results strongly suggest that skt encodes a β-ketothiolase responsible for the cleavage of acetyl-CoA from the acyl side chain of cholate. Sequence comparisons revealed that other steroid-degrading bacteria such as Comamonas testosteroni contain genes encoding proteins very similar to Skt, suggesting a widespread role of this enzyme in bacterial steroid degradation.