Microbial phytosterol degradation is accompanied by the formation of steroid pathway intermediates, which are potential precursors in the synthesis of bioactive steroids. Degradation of these steroid intermediates is initiated by ⌬ 1 -dehydrogenation of the steroid ring structure. Characterization of a 2.9-kb DNA fragment of Rhodococcus erythropolis SQ1 revealed an open reading frame (kstD) showing similarity with known 3-ketosteroid ⌬ 1 -dehydrogenase genes. Heterologous expression of kstD yielded 3-ketosteroid ⌬ 1 -dehydrogenase (KSTD) activity under the control of the lac promoter in Escherichia coli. Targeted disruption of the kstD gene in R. erythropolis SQ1 was achieved, resulting in loss of more than 99% of the KSTD activity. However, growth on the steroid substrate 4-androstene-3,17-dione or 9␣-hydroxy-4-androstene-3,17-dione was not abolished by the kstD gene disruption. Bioconversion of phytosterols was also not blocked at the level of ⌬ 1 -dehydrogenation in the kstD mutant strain, since no accumulation of steroid pathway intermediates was observed. Thus, inactivation of kstD is not sufficient for inactivation of the ⌬ 1 -dehydrogenase activity. Native polyacrylamide gel electrophoresis of cell extracts stained for KSTD activity showed that R. erythropolis SQ1 in fact harbors two activity bands, one of which is absent in the kstD mutant strain.Rhodococcus species are well known for their catabolic potential (5, 40). Several Rhodococcus species degrade natural phytosterols. Microbial phytosterol degradation proceeds via the formation of steroids as pathway intermediates (16,21,22), i.e., 4-androstene-3,17-dione, 1,4-androstadiene-3,17-dione, and 9␣-hydroxy-4-androstene-3,17-dione (Fig.

The well-known large catabolic potential of rhodococci is greatly facilitated by an impressive gene multiplicity. This study reports on the multiplicity of kshA, encoding the oxygenase component of 3-ketosteroid 9␣-hydroxylase, a key enzyme in steroid catabolism. Five kshA homologues (kshA1 to kshA5) were previously identified in Rhodococcus rhodochrous DSM43269. These KshA DSM43269 homologues are distributed over several phylogenetic groups. The involvement of these KshA homologues in the catabolism of different classes of steroids, i.e., sterols, pregnanes, androstenes, and bile acids, was investigated. Enzyme activity assays showed that all KSH enzymes with KshA DSM43269 homologues are C-9 ␣-hydroxylases acting on a wide range of 3-ketosteroids, but not on 3-hydroxysteroids. KshA5 appeared to be the most versatile enzyme, with the broadest substrate range but without a clear substrate preference. In contrast, KshA1 was found to be dedicated to cholic acid catabolism. Transcriptional analysis and functional complementation studies revealed that kshA5 supported growth on any of the different classes of steroids tested, consistent with its broad expression induction pattern. The presence of multiple kshA genes in the R. rhodochrous DSM43269 genome, each displaying unique steroid induction patterns and substrate ranges, appears to facilitate a dynamic and fine-tuned steroid catabolism, with C-9 ␣-hydroxylation occurring at different levels during microbial steroid degradation.Rhodoccoci are capable of degrading a wide range of organic compounds (15,32). This strong catabolic potential is encoded by an extremely large genome, of Ͼ9.7 Mb in the case of Rhodococcus jostii RHA1, which also carries numerous gene homologues for various enzyme classes (20). Multiple steroid catabolic gene clusters, for example, have been identified in R. jostii RHA1 (19,20,33). In particular, several homologous genes encoding key enzymes involved in steroid ring opening have been identified, i.e., 3-ketosteroid 9␣-hydroxylase (KSH), encoded by kshA and kshB, and 3-ketosteroid ⌬1-dehydrogenase (KSTD), encoded by kstD (13, 33). Hydroxylation of steroid substrates at the C-9 position, together with dehydrogenation of the A-ring performed by KSTD, leads to opening of the steroid polycyclic ring structure and the formation of 3-hydroxy-9,10-secoandrost-1,3,5(10)-triene-9,17-dione (3-HSA) (7, 31). Knowledge of steroid catabolic enzymes is limited, despite the fact that sterol-degrading rhodococci and mycobacteria are of great industrial and pharmaceutical interest (6,9,12,17,25,32). In recent years, interest in steroid catabolic enzymes has gained momentum, following the discovery of cholesterol catabolic gene clusters in R. jostii RHA1 and in the human pathogen Mycobacterium tuberculosis H37Rv (33). Interestingly, kshA and kshB have been identified as essential factors in the pathogenesis of M. tuberculosis H37Rv (10).KSH is a two-component enzyme system, consisting of a terminal oxygenase, KshA, and a ferredoxin reductase, KshB. The kshA an...

The Rhodococcus erythropolis SQ1 kstD3 gene was cloned, heterologously expressed and biochemically characterized as a KSTD3 (3-keto-5alpha-steroid Delta(1)-dehydrogenase). Upstream of kstD3, an ORF (open reading frame) with similarity to Delta(4) KSTD (3-keto-5alpha-steroid Delta(4)-dehydrogenase) was found, tentatively designated kst4D. Biochemical analysis revealed that the Delta(1) KSTD3 has a clear preference for 3-ketosteroids with a saturated A-ring, displaying highest activity on 5alpha-AD (5alpha-androstane-3,17-dione) and 5alpha-T (5alpha-testosterone; also known as 17beta-hydroxy-5alpha-androstane-3-one). The KSTD1 and KSTD2 enzymes, on the other hand, clearly prefer (9alpha-hydroxy-)4-androstene-3,17-dione as substrates. Phylogenetic analysis of known and putative KSTD amino acid sequences showed that the R. erythropolis KSTD proteins cluster into four distinct groups. Interestingly, Delta(1) KSTD3 from R. erythropolis SQ1 clustered with Rv3537, the only Delta(1) KSTD present in Mycobacterium tuberculosis H37Rv, a protein involved in cholesterol catabolism and pathogenicity. The substrate range of heterologously expressed Rv3537 enzyme was nearly identical with that of Delta(1) KSTD3, indicating that these are orthologous enzymes. The results imply that 5alpha-AD and 5alpha-T are newly identified intermediates in the cholesterol catabolic pathway, and important steroids with respect to pathogenicity.

Rhodococcus equi causes fatal pyogranulomatous pneumonia in foals and immunocompromised animals and humans. Despite its importance, there is currently no effective vaccine against the disease. The actinobacteria R. equi and the human pathogen Mycobacterium tuberculosis are related, and both cause pulmonary diseases. Recently, we have shown that essential steps in the cholesterol catabolic pathway are involved in the pathogenicity of M. tuberculosis. Bioinformatic analysis revealed the presence of a similar cholesterol catabolic gene cluster in R. equi. Orthologs of predicted M. tuberculosis virulence genes located within this cluster, i.e. ipdA (rv3551), ipdB (rv3552), fadA6 and fadE30, were identified in R. equi RE1 and inactivated. The ipdA and ipdB genes of R. equi RE1 appear to constitute the α-subunit and β-subunit, respectively, of a heterodimeric coenzyme A transferase. Mutant strains RE1ΔipdAB and RE1ΔfadE30, but not RE1ΔfadA6, were impaired in growth on the steroid catabolic pathway intermediates 4-androstene-3,17-dione (AD) and 3aα-H-4α(3′-propionic acid)-5α-hydroxy-7aβ-methylhexahydro-1-indanone (5α-hydroxy-methylhexahydro-1-indanone propionate; 5OH-HIP). Interestingly, RE1ΔipdAB and RE1ΔfadE30, but not RE1ΔfadA6, also displayed an attenuated phenotype in a macrophage infection assay. Gene products important for growth on 5OH-HIP, as part of the steroid catabolic pathway, thus appear to act as factors involved in the pathogenicity of R. equi. Challenge experiments showed that RE1ΔipdAB could be safely administered intratracheally to 2 to 5 week-old foals and oral immunization of foals even elicited a substantial protective immunity against a virulent R. equi strain. Our data show that genes involved in steroid catabolism are promising targets for the development of a live-attenuated vaccine against R. equi infections.

Previously we have characterized 3-ketosteroid 9␣-hydroxylase (KSH), a key enzyme in microbial steroid degradation in Rhodococcus erythropolis strain SQ1, as a two-component iron-sulfur monooxygenase, comprised of the terminal oxygenase component KshA1 and the oxygenase-reductase component KshB. Deletion of the kshA1 gene resulted in the loss of the ability of mutant strain RG2 to grow on the steroid substrate 4-androstene-3,17-dione (AD). Here we report characteristics of a close KshA1 homologue, KshA2 of strain SQ1, sharing 60% identity at the amino acid level. Expression of the kshA2 gene in mutant strain RG2 restored growth on AD and ADD, indicating that kshA2 also encodes KSH activity. The functional complementation was shown to be dependent on the presence of kshB. Transcriptional analysis showed that expression of kshA2 is induced in parent strain R. erythropolis SQ1 in the presence of AD. However, promoter activity studies, using ␤-lactamase of Escherichia coli as a convenient transcription reporter protein for Rhodococcus, revealed that the kshA2 promoter in fact is highly induced in the presence of 9␣-hydroxy-4-androstene-3,17-dione (9OHAD) or a metabolite thereof. Inactivation of kshA2 in parent strain SQ1 by unmarked gene deletion did not affect growth on 9OHAD, cholesterol, or cholic acid. We speculate that KshA2 plays a role in preventing accumulation of toxic intracellular concentrations of ADD during steroid catabolism. A third kshA homologue was additionally identified in a kshA1 kshA2 double gene deletion mutant strain of R. erythropolis SQ1. The developed degenerate PCR primers for kshA may be useful for isolation of kshA homologues from other (actino) bacteria.Rhodococcus species display a wide range of metabolic capabilities, degrading a variety of environmental pollutants and transforming or synthesizing compounds with possible useful applications (3,26,30). A well-known feature of rhodococci is their ability to degrade a range of naturally occurring steroids, including cholesterol and phytosterols, e.g., ␤-sitosterol (26).3-Ketosteroid 9␣-hydroxylase (KSH) is a key enzymatic step in the microbial steroid catabolic pathway, acting on 4-androstene-3,17-dione (AD) or 1,4-androstadiene-3,17-dione (ADD) (Fig. 1). Introduction of the 9␣-hydroxyl moiety into the steroid polyheterocyclic ring structure, combined with steroid ⌬ 1 -dehydrogenation by 3-ketosteroid ⌬ 1 -dehydrogenase (KSTD), initiates steroid B-ring opening due to the formation of chemically unstable 9␣-hydroxy-1,4-androstadiene-3,17-dione (9OHADD) (Fig. 1). KSH of Rhodococcus erythropolis SQ1 is a two-component class IA monooxygenase comprised of the terminal oxygenase KshA1 and the oxygenase reductase component KshB (23). Classification of nonheme oxygenases is based on the number of constituent components and the nature of their redox centers (2,6,(8)(9)(10)13) In previous work we have shown that gene inactivation of either kshA1 (mutant strain RG2) or kshB (mutant strain RG4) in parent strain R. erythropolis SQ1 results in loss o...

Summary9a-Hydroxylation of 4-androstene-3,17-dione (AD) and 1,4-androstadiene-3,17-dione (ADD) is catalysed by 3-ketosteroid 9a-hydroxylase (KSH), a key enzyme in microbial steroid catabolism. Very limited knowledge is presently available on the KSH enzyme. Here, we report for the first time the identification and molecular characterization of genes encoding KSH activity. The kshA and kshB genes, encoding KSH in Rhodococcus erythropolis strain SQ1, were cloned by functional complementation of mutant strains blocked in AD(D) 9a-hydroxylation. Analysis of the deduced amino acid sequences of kshA and kshB showed that they contain domains typically conserved in class IA terminal oxygenases and class IA oxygenase reductases respectively. By definition, class IA oxygenases are made up of two components, thus classifying the KSH enzyme system in R. erythropolis strain SQ1 as a two-component class IA monooxygenase composed of KshA and KshB. Unmarked in frame gene deletion mutants of parent strain R. erythropolis SQ1, designated strains RG2 (kshA mutant) and RG4 (kshB mutant), were unable to grow on steroid substrates AD(D), whereas growth on 9a-hydroxy-4-androstene-3,17-dione (9OHAD) was not affected. Incubation of these mutant strains with AD resulted in the accumulation of ADD (30-50% conversion), confirming the involvement of KshA and KshB in AD(D) 9a-hydroxylation. Strain RG4 was also impaired in sterol degradation, suggesting a dual role for KshB in both sterol and steroid degradation.

Previously, Rhodococcus erythropolis SQ1 kstD, encoding ketosteroid ∆ 1 -dehydrogenase (KSTD1) was characterized. Surprisingly, a kstD gene deletion mutant (strain RG1) grew normally on steroids. UV mutagenesis of strain RG1 allowed isolation of strains (e.g. strain RG1-UV29) unable to perform the ∆ 1 -dehydrogenation of 4-androstene-3,17-dione (AD) and 9α-hydroxy-4-androstene-3,17-dione (9OHAD). Functional complementation of strain RG1-UV29 with total genomic DNA of strain RG1 resulted in identification of a 1698 nt ORF (kstD2) showing clear similarity (35 % identity at amino acid sequence level) with KSTD1. Expression of kstD2 in Escherichia coli resulted in high KSTD2 activity levels. Single gene deletion mutants of either kstD (strain RG1) or kstD2 (strain RG7) appeared unaffected in growth on the steroid substrates AD, 1,4-androstadiene-3,17-dione and 9OHAD. Strain RG7, but not strain RG1, showed temporary accumulation of 9OHAD during AD conversion. A kstD kstD2 double deletion mutant (strain RG8) was constructed. Strain RG8 was unable to grow on steroid substrates, had undetectable steroid ∆ 1 -dehydrogenation activity and efficiently converted AD into 9OHAD. Strain SQ1 thus employs two KSTD isoenzymes in steroid catabolism. Analysis of two null mutants in KSTD2 showed that the semi-conserved Ser325 and the highly conserved Thr503 play a role in KSTD enzyme activity.

A novel method to efficiently generate unmarked in-frame gene deletions in Rhodococcus equi was developed, exploiting the cytotoxic effect of 5-fluorocytosine (5-FC) by the action of cytosine deaminase (CD) and uracil phosphoribosyltransferase (UPRT) enzymes. The opportunistic, intracellular pathogen R. equi is resistant to high concentrations of 5-FC. Introduction of Escherichia coli genes encoding CD and UPRT conferred conditional lethality to R. equi cells incubated with 5-FC. To exemplify the use of the codA::upp cassette as counter-selectable marker, an unmarked in-frame gene deletion mutant of R. equi was constructed. The supA and supB genes, part of a putative cholesterol catabolic gene cluster, were efficiently deleted from the R. equi wild-type genome. Phenotypic analysis of the generated ΔsupAB mutant confirmed that supAB are essential for growth of R. equi on cholesterol. Macrophage survival assays revealed that the ΔsupAB mutant is able to survive and proliferate in macrophages comparable to wild type. Thus, cholesterol metabolism does not appear to be essential for macrophage survival of R. equi. The CD-UPRT based 5-FC counter-selection may become a useful asset in the generation of unmarked in-frame gene deletions in other actinobacteria as well, as actinobacteria generally appear to be 5-FC resistant and 5-FU sensitive.