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Pseudomonas putidaDOC21, a soil-dwelling proteobacterium, catabolizes a variety of steroids and bile acids. Transposon mutagenesis and bioinformatics analyses identified four clusters of steroid degradation (std) genes encoding a single catabolic pathway. The latter includes three predicted acyl-CoA synthetases encoded bystdA1,stdA2andstdA3respectively. The ΔstdA1and ΔstdA2deletion mutants were unable to assimilate cholate or other bile acids but grew well on testosterone or 4-androstene-3,17-dione (AD). In contrast, a ΔstdA3mutant grew poorly in media containing either testosterone or AD. When cells were grown with succinate in the presence of cholate, ΔstdA1accumulated Δ1/4-3-ketocholate and Δ1,4-3-ketocholate, whereas ΔstdA2only accumulated 7α,12α-dihydroxy-3-oxopregna-1,4-diene-20-carboxylate (DHOPDC). When incubated with testosterone or bile acids, ΔstdA3accumulated 3aα-H-4α(3′propanoate)-7aβ-methylhexahydro-1,5-indanedione (HIP) or the corresponding hydroxylated derivative. Biochemical analyses revealed that StdA1 converted cholate, 3-ketocholate, Δ1/4-3-ketocholate, and Δ1,4-3-ketocholate to their CoA thioesters, while StdA2 transformed DHOPDC to DHOPDC-CoA. In contrast, purified StdA3 catalysed the CoA thioesterification of HIP and its hydroxylated derivatives. Overall, StdA1, StdA2 and StdA3 are acyl-CoA synthetases required for the complete degradation of bile acids: StdA1 and StdA2 are involved in degrading the C-17 acyl chain, whereas StdA3 initiates degradation of the last two steroid rings. The study highlights differences in steroid catabolism betweenProteobacteriaandActinobacteria.