Identification and mutagenesis by allelic exchange of choE, encoding a cholesterol oxidase from the intracellular pathogen Rhodococcus equi

The role of cholesterol and its oxidation products in tuberculosis pathogenesis

Mycobacterium tuberculosis causes tuberculosis (TB), one of the world's most deadly infections. Lipids play an important role in M. tuberculosis pathogenesis. M. tuberculosis grows intracellularly within lipid-laden macrophages and extracellularly within the cholesterol-rich caseum of necrotic granulomas and pulmonary cavities. Evolved from soil saprophytes that are able to metabolize cholesterol from organic matter in the environment, M. tuberculosis inherited an extensive and highly conserved machinery to metabolize cholesterol. M. tuberculosis uses this machinery to degrade host cholesterol; the products of cholesterol degradation are incorporated into central carbon metabolism and used to generate cell envelope lipids, which play important roles in virulence. The host also modifies cholesterol by enzymatically oxidizing it to a variety of derivatives, collectively called oxysterols, which modulate cholesterol homeostasis and the immune response. Recently, we found that M. tuberculosis converts host cholesterol to an oxidized metabolite, cholestenone, that accumulates in the lungs of individuals with TB. M. tuberculosis encodes cholesterol-modifying enzymes, including a hydroxysteroid dehydrogenase, a putative cholesterol oxidase, and numerous cytochrome P450 monooxygenases. Here, we review what is known about cholesterol and its oxidation products in the pathogenesis of TB. We consider the possibility that the biological function of cholesterol metabolism by M. tuberculosis extends beyond a nutritional role.

Expression and characterization of cholesterol oxidase with high thermal and pH stability from Janthinobacterium agaricidamnosum

Cholesterol oxidases (COXases) have a diverse array of applications including analysis of blood cholesterol levels, synthesis of steroids, and utilization as an insecticidal protein. The COXase gene from Janthinobacterium agaricidamnosum was cloned and expressed in Escherichia coli. The purified COXase showed an optimal temperature of 60 °C and maintained about 96 and 72% of its initial activity after 30 min at 60 and 70 °C, respectively. In addition, the purified COXase exhibited a pH optimum at 7.0 and high pH stability over the broad pH range of 3.0-12.0. The pH stability of the COXase at pH 12.0 was higher than that of highly stable COXase from Chromobacterium sp. DS-1. The COXase oxidized cholesterol and β-cholestanol at higher rates than other 3β-hydroxysteroids. The Km, Vmax, and kcat values for cholesterol were 156 μM, 13.7 μmol/min/mg protein, and 14.4 s^-1, respectively. These results showed that this enzyme could be very useful in the clinical determination of cholesterol in serum and the production of steroidal compounds. This is the first report to characterize a COXase from the genus Janthinobacterium.

Recombinant Extracellular Cholesterol Oxidase from Nocardioides simplex

Cholesterol oxidase is a highly demanded enzyme used in medicine, pharmacy, agriculture, chemistry, and biotechnology. It catalyzes oxidation of 3β-hydroxy-5-ene- to 3-keto-4-ene- steroids with the formation of hydrogen peroxide. Here, we expressed 6xHis-tagged mature form of the extracellular cholesterol oxidase (ChO) from the actinobacterium Nocardioides simplex VKM Ac-2033D (55.6 kDa) in Escherichia coli cells. The recombinant enzyme (ChO_Ns) was purified using affinity chromatography. ChO_Ns proved to be functional towards cholesterol, cholestanol, phytosterol, pregnenolone, and dehydroepiandrosterone. Its activity depended on the structure and length of the aliphatic side chain at C17 atom of the steroid nucleus and was lower with pregnenolone and dehydroepiandrosterone. The enzyme was active in a pH range of 5.25÷6.5 with the pH optimum at 6.0. Kinetic assays and storage stability tests demonstrated that the characteristics of ChO_Ns were generally comparable with or superior to those of commercial ChO from Streptomyces hygroscopicus (ChO_Sh). The results contribute to the knowledge on microbial ChOs and evidence that ChO from N. simplex VKM Ac-2033D is a promising agent for further applications.

Fatal Infection in an Alpaca (Vicugna pacos) Caused by Pathogenic Rhodococcus equi

Simple Summary

Serious consequences of septicemic bacterial infections include the formation of purulent and pyogranulomatous inflammation resulting in abscesses in inner organs. Different bacteria are known to cause these infections in livestock. In this study, we report in detail on a case of a fatal Rhodococcus (R.) equi infection in an alpaca (Vicugna pacos), to our knowledge, for the first time. R. equi is a member of the actinomycetes, a bacterial group known to contain several pathogenic bacteria. R. equi primarily affects equine foals and other domestic animals, but also humans, which renders this bacterium a zoonotic agent. The rhodococcal infection of the alpaca reported herein caused septicemia, resulting in emaciation and severe lesions in the lungs and heart. The onset of infection was presumably caused by aspiration pneumonia, resulting in abscesses exclusively in the lungs. The R. equi isolate proved to be pathogenic, based on the virulence gene vapA encoding the virulence-associated protein A. Antibiotic susceptibility testing revealed a susceptibility to doxycycline, erythromycin, gentamycin, neomycin, rifampicin, trimethoprim/sulfamethoxazole, tetracycline and vancomycin. This report of an R. equi infection in an alpaca makes clear that we still have knowledge gaps about bacterial infectious diseases in alpacas and potential zoonotic impacts. Therefore, the determination of pathogenic, zoonotic bacteria in alpacas is essential for treatment and preventive measures with respect to sustaining the health, welfare and productivity of this camelid species.

Abstract

Rhodococcus (R.) equi is a pathogen primarily known for infections in equine foals, but is also present in numerous livestock species including New World camelids. Moreover, R. equi is considered an emerging zoonotic pathogen. In this report, we describe in detail a fatal rhodococcal infection in an alpaca (Vicugna pacos), to our best knowledge, for the first time. The alpaca died due to a septicemic course of an R. equi infection resulting in emaciation and severe lesions including pyogranulomas in the lungs and pericardial effusion. The onset of the infection was presumably caused by aspiration pneumonia. R. equi could be isolated from the pyogranulomas in the lung and unequivocally identified by MALDI-TOF MS analysis and partial sequencing of the 16S rRNA gene, the 16S-23S internal transcribed spacer (ITS) region and the rpoB gene. The isolate proved to possess the vapA gene in accordance with tested isolates originating from the lungs of infected horses. The R. equi isolates revealed low minimal inhibitory concentrations (MIC values) for doxycycline, erythromycin, gentamycin, neomycin, rifampicin, trimethoprim/sulfamethoxazole, tetracycline and vancomycin in antibiotic susceptibility testing. Investigations on the cause of bacterial, especially fatal, septicemic infections in alpacas are essential for adequately addressing the requirements for health and welfare issues of this New World camelid species. Furthermore, the zoonotic potential of R. equi has to be considered with regard to the One Health approach.

Reductive Power Generated by Mycobacterium leprae Through Cholesterol Oxidation Contributes to Lipid and ATP Synthesis

Upon infection, Mycobacterium leprae, an obligate intracellular bacillus, induces accumulation of cholesterol-enriched lipid droplets (LDs) in Schwann cells (SCs). LDs are promptly recruited to M. leprae-containing phagosomes, and inhibition of this process decreases bacterial survival, suggesting that LD recruitment constitutes a mechanism by which host-derived lipids are delivered to intracellular M. leprae. We previously demonstrated that M. leprae has preserved only the capacity to oxidize cholesterol to cholestenone, the first step of the normal cholesterol catabolic pathway. In this study we investigated the biochemical relevance of cholesterol oxidation on bacterial pathogenesis in SCs. Firstly, we showed that M. leprae increases the uptake of LDL-cholesterol by infected SCs. Moreover, fluorescence microscopy analysis revealed a close association between M. leprae and the internalized LDL-cholesterol within the host cell. By using Mycobacterium smegmatis mutant strains complemented with M. leprae genes, we demonstrated that ml1942 coding for 3β-hydroxysteroid dehydrogenase (3β-HSD), but not ml0389 originally annotated as cholesterol oxidase (ChoD), was responsible for the cholesterol oxidation activity detected in M. leprae. The 3β-HSD activity generates the electron donors NADH and NADPH that, respectively, fuel the M. leprae respiratory chain and provide reductive power for the biosynthesis of the dominant bacterial cell wall lipids phthiocerol dimycocerosate (PDIM) and phenolic glycolipid (PGL)-I. Inhibition of M. leprae 3β-HSD activity with the 17β-[N-(2,5-di-t-butylphenyl)carbamoyl]-6-azaandrost-4-en-3one (compound 1), decreased bacterial intracellular survival in SCs. In conclusion, our findings confirm the accumulation of cholesterol in infected SCs and its potential delivery to the intracellular bacterium. Furthermore, we provide strong evidence that cholesterol oxidation is an essential catabolic pathway for M. leprae pathogenicity and point to 3β-HSD as a prime drug target that may be used in combination with current multidrug regimens to shorten leprosy treatment and ameliorate nerve damage.

Antimicrobial resistance spectrum conferred by pRErm46 of emerging macrolide (multidrug)-resistant Rhodococcus equi

Clonal multidrug resistance recently emerged in Rhodococcus equi, complicating the therapeutic management of this difficult-to-treat animal and human pathogenic actinomycete. The currently spreading multidrug-resistant (MDR) "2287" clone arose in equine farms upon acquisition, and co-selection by mass macrolide-rifampin therapy, of the pRErm46 plasmid carrying the erm(46) macrolides-lincosamides-streptogramins resistance determinant, and an rpoB^S531F mutation. Here, we screened a collection of susceptible and macrolide-rifampin-resistant R. equi from equine clinical cases using a panel of 15 antimicrobials against rapidly growing mycobacteria (RGM), nocardiae and other aerobic actinomycetes (NAA). R. equi -including MDR isolates- was generally susceptible to linezolid, minocycline, tigecycline, amikacin and tobramycin according to Staphylococcus aureus interpretive criteria, plus imipenem, cefoxitin and ceftriaxone based on Clinical & Laboratory Standards Institute (CLSI) guidelines for RGM/NAA. Ciprofloxacin and moxifloxacin were in the borderline category according to European Committee on Antimicrobial Susceptibility Testing (EUCAST) criteria. Molecular analyses linked pRErm46 to significantly increased MICs for trimethoprim-sulfamethoxazole and doxycycline in addition to clarithromycin within the RGM/NAA panel, and to streptomycin, spectinomycin and tetracycline resistance. pRErm46 variants with spontaneous deletions in the class 1 integron (C1I) region, observed in ≈30% of erm(46)-positive isolates, indicated that the newly identified resistances were attributable to C1I's sulfonamide (sul1) and aminoglycoside (aaA9) resistance cassettes and adjacent tetRA(33) determinant. Most MDR isolates carried the rpoB^S531F mutation of the 2287 clone, while different rpoB mutations (S531L, S531Y) detected in two cases suggest the emergence of novel MDR R. equi strains.

Identification of cholesterol-assimilating actinomycetes strain and application of statistical modeling approaches for improvement of cholesterol oxidase production by Streptomyces anulatus strain NEAE-94

Background

Cholesterol oxidase biosensors have been used to determine the level of cholesterol in different serum and food samples. Due to a wide range of industrial and clinical applications of microbial cholesterol oxidase, isolation and identification of a new microbial source (s) of cholesterol oxidase are very important.

Results

The local isolate Streptomyces sp. strain NEAE-94 is a promising source of cholesterol oxidase. It was identified based on cultural, morphological and physiological characteristics; in addition to the 16S rRNA sequence. The sequencing product had been deposited in the GenBank database under the accession number KC354803. Cholesterol oxidase production by Streptomyces anulatus strain NEAE-94 in shake flasks was optimized using surface response methodology. The different process parameters were first screened using a Plackett-Burman design and the parameters with significant effects on the production of cholesterol oxidase were identified. Out of the 15 factors screened, agitation speed, cholesterol and yeast extract concentrations had the most significant positive effects on the production of cholesterol oxidase. The optimal levels of these variables and the effects of their mutual interactions on cholesterol oxidase production were determined using Box-Behnken design. Cholesterol oxidase production by Streptomyces anulatus strain NEAE-94 was 11.03, 27.31 U/mL after Plackett-Burman Design and Box-Behnken design; respectively, with a fold of increase of 6.06 times compared to the production before applying the Plackett-Burman design (4.51 U/mL).

Conclusions

Maximum cholesterol oxidase activity was obtained at the following fermentation conditions: g/L (cholesterol 4, yeast extract 5, NaCl 0.5, K₂HPO₄ 1, FeSO₄.7H₂O 0.01, MgSO₄.7H₂O 0.5), pH 7, inoculum size 4% (v/v), temperature 37°C, agitation speed of 150 rpm, medium volume 50 mL and incubation time 5 days.

Immobilization of Cholesterol Oxidase: An Overview

Background:

Cholesterol oxidases are bacterial oxidases widely used commercially for their application in the detection of cholesterol in blood serum, clinical or food samples. Additionally, these enzymes find potential applications as an insecticide, synthesis of anti-fungal antibiotics and a biocatalyst to transform a number of sterol and non-sterol compounds. However, the soluble form of cholesterol oxidases are found to be less stable when applied at higher temperatures, broader pH range, and incur higher costs. These disadvantages can be overcome by immobilization on carrier matrices.

Methods:

This review focuses on the immobilization of cholesterol oxidases on various macro/micro matrices as well as nanoparticles and their potential applications. Selection of appropriate support matrix in enzyme immobilization is of extreme importance. Recently, nanomaterials have been used as a matrix for immobilization of enzyme due to their large surface area and small size. The bio-compatible length scales and surface chemistry of nanoparticles provide reusability, stability and enhanced performance characteristics for the enzyme-nanoconjugates.

Conclusion:

In this review, immobilization of cholesterol oxidase on nanomaterials and other matrices are discussed. Immobilization on nanomatrices has been observed to increase the stability and activity of enzymes. This enhances the applicability of cholesterol oxidases for various industrial and clinical applications such as in biosensors.

Current taxonomy of Rhodococcus species and their role in infections

Rhodococcus is a genus of obligate aerobic, Gram-positive, partially acid-fast, catalase-positive, non-motile, and none-endospore bacteria. The genus Rhodococcus was first introduced by Zopf. This bacterium can be isolated from various sources of the environment and can grow well in non-selective medium. A large number of phenotypic characterizations are used to compare different species of the genus Rhodococcus, and these tests are not suitable for accurate identification at the genus and species level. Among nucleic acid-based methods, the most powerful target gene for revealing reliable phylogenetic relationships is 16S ribosomal RNA gene (16S rRNA gene) sequence analysis, but this gene is unable to differentiation some of Rhodococcus species. To date, whole genome sequencing analysis has solved taxonomic complexities in this genus. Rhodococcus equi is the major cause of foal pneumonia, and its implication in human health is related to cases in immunocompromised patients. Macrolide family together with rifampicin is one of the most effective antibiotic agents for treatment rhodococcal infections.

A case report on disseminated Rhodococcus equi infection in a Japanese black heifer

Rhodococcus equi was isolated from the granulomatous lesions of the lung, kidney, liver, and hepatic, mesenteric, and abomasum lymph nodes of a Japanese black heifer. R. equi isolates were analyzed by polymerase chain reaction for virulence-associated protein genes. The vapN gene was detected in all the isolates examined. This is the first report in which vapN-positive R. equi was isolated from cattle in Japan.

High-Efficiency, Two-Step Scarless-Markerless Genome Genetic Modification in Salmonella enterica

We present a two-step method for scarless–markerless genome genetic modification in Salmonella enterica based on the improved suicide plasmid pGMB152. The whole LacZYA gene can provide a lacZ-based blue/white screening strategy for fast selection of double-crossover mutants by allelic exchange. The high efficiency of this genetic engineering strategy permits the study of gene function by gene knockin, site-directed mutagenesis, and gene knockout to construct live attenuated vaccines.

The activity of CouR, a MarR family transcriptional regulator, is modulated through a novel molecular mechanism

CouR, a MarR-type transcriptional repressor, regulates the cou genes, encoding p-hydroxycinnamate catabolism in the soil bacterium Rhodococcus jostii RHA1. The CouR dimer bound two molecules of the catabolite p-coumaroyl-CoA (Kd = 11 ± 1 μM). The presence of p-coumaroyl-CoA, but neither p-coumarate nor CoASH, abrogated CouR's binding to its operator DNA in vitro. The crystal structures of ligand-free CouR and its p-coumaroyl-CoA-bound form showed no significant conformational differences, in contrast to other MarR regulators. The CouR-p-coumaroyl-CoA structure revealed two ligand molecules bound to the CouR dimer with their phenolic moieties occupying equivalent hydrophobic pockets in each protomer and their CoA moieties adopting non-equivalent positions to mask the regulator's predicted DNA-binding surface. More specifically, the CoA phosphates formed salt bridges with predicted DNA-binding residues Arg36 and Arg38, changing the overall charge of the DNA-binding surface. The substitution of either arginine with alanine completely abrogated the ability of CouR to bind DNA. By contrast, the R36A/R38A double variant retained a relatively high affinity for p-coumaroyl-CoA (Kd = 89 ± 6 μM). Together, our data point to a novel mechanism of action in which the ligand abrogates the repressor's ability to bind DNA by steric occlusion of key DNA-binding residues and charge repulsion of the DNA backbone.

An Invertron-Like Linear Plasmid Mediates Intracellular Survival and Virulence in Bovine Isolates of Rhodococcus equi

We report a novel host-associated virulence plasmid in Rhodococcus equi, pVAPN, carried by bovine isolates of this facultative intracellular pathogenic actinomycete. Surprisingly, pVAPN is a 120-kb invertron-like linear replicon unrelated to the circular virulence plasmids associated with equine (pVAPA) and porcine (pVAPB variant) R. equi isolates. pVAPN is similar to the linear plasmid pNSL1 from Rhodococcus sp. NS1 and harbors six new vap multigene family members (vapN to vapS) in a vap pathogenicity locus presumably acquired via en bloc mobilization from a direct predecessor of equine pVAPA. Loss of pVAPN rendered R. equi avirulent in macrophages and mice. Mating experiments using an in vivo transconjugant selection strategy demonstrated that pVAPN transfer is sufficient to confer virulence to a plasmid-cured R. equi recipient. Phylogenetic analyses assigned the vap multigene family complement from pVAPN, pVAPA, and pVAPB to seven monophyletic clades, each containing plasmid type-specific allelic variants of a precursor vap gene carried by the nearest vap island ancestor. Deletion of vapN, the predicted "bovine-type" allelic counterpart of vapA, essential for virulence in pVAPA, abrogated pVAPN-mediated intramacrophage proliferation and virulence in mice. Our findings support a model in which R. equi virulence is conferred by host-adapted plasmids. Their central role is mediating intracellular proliferation in macrophages, promoted by a key vap determinant present in the common ancestor of the plasmid-specific vap islands, with host tropism as a secondary trait selected during coevolution with specific animal species.

Cholesterol oxidase: ultrahigh-resolution crystal structure and multipolar atom model-based analysis

Examination of protein structure at the subatomic level is required to improve the understanding of enzymatic function. For this purpose, X-ray diffraction data have been collected at 100 K from cholesterol oxidase crystals using synchrotron radiation to an optical resolution of 0.94 Å. After refinement using the spherical atom model, nonmodelled bonding peaks were detected in the Fourier residual electron density on some of the individual bonds. Well defined bond density was observed in the peptide plane after averaging maps on the residues with the lowest thermal motion. The multipolar electron density of the protein-cofactor complex was modelled by transfer of the ELMAM2 charge-density database, and the topology of the intermolecular interactions between the protein and the flavin adenine dinucleotide (FAD) cofactor was subsequently investigated. Taking advantage of the high resolution of the structure, the stereochemistry of main-chain bond lengths and of C=O···H-N hydrogen bonds was analyzed with respect to the different secondary-structure elements.

Cholesterol oxidase with high catalytic activity from Pseudomonas aeruginosa: Screening, molecular genetic analysis, expression and characterization

An extracellular cholesterol oxidase producer, Pseudomonas aeruginosa strain PA157, was isolated by a screening method to detect 6β-hydroperoxycholest-4-en-3-one-forming cholesterol oxidase. On the basis of a putative cholesterol oxidase gene sequence in the genome sequence data of P. aeruginosa strain PAO1, the cholesterol oxidase gene from strain PA157 was cloned. The mature form of the enzyme was overexpressed in Escherichia coli cells. The overexpressed enzyme formed inclusion bodies in recombinant E. coli cells grown at 20 °C and 30 °C. A soluble and active PA157 enzyme was obtained when the recombinant cells were grown at 10 °C. The purified enzyme was stable at pH 5.5 to 10 and was most active at pH 7.5-8.0, showing optimal activity at pH 7.0 and 70 °C. The enzyme retained about 90% of its activity after incubation for 30 min at 70 °C. The enzyme oxidized 3β-hydroxysteroids such as cholesterol, β-cholestanol, and β-sitosterol at high rates. The Km value and Vmax value for the cholesterol were 92.6 μM and 15.9 μmol/min/mg of protein, respectively. The Vmax value of the enzyme was higher than those of commercially available cholesterol oxidases. This is the first report to characterize a cholesterol oxidase from P. aeruginosa.

Characterization of p-hydroxycinnamate catabolism in a soil Actinobacterium

p-Hydroxycinnamates, such as ferulate and p-coumarate, are components of plant cell walls and have a number of commercial applications. Rhodococcus jostii RHA1 (RHA1) catabolizes ferulate via vanillate and the β-ketoadipate pathway. Here, we used transcriptomics to identify genes in RHA1 that are upregulated during growth on ferulate versus benzoate. The upregulated genes included three transcriptional units predicted to encode the uptake and β-oxidative deacetylation of p-hydroxycinnamates: couHTL, couNOM, and couR. Neither ΔcouL mutants nor ΔcouO mutants grew on p-hydroxycinnamates, but they did grow on vanillate. Among several p-hydroxycinnamates, CouL catalyzed the thioesterification of p-coumarate and caffeate most efficiently (k(cat)/K(m) = ∼ 400 mM(-1) s(-1)). p-Coumarate was also RHA1's preferred growth substrate, suggesting that CouL is a determinant of the pathway's specificity. CouL did not catalyze the activation of sinapate, in similarity to two p-coumaric acid:coenzyme A (CoA) ligases from plants, and contains the same bulged loop that helps determine substrate specificity in the plant homologues. The couO mutant accumulated 4-hydroxy-3-methoxyphenyl-β-ketopropionate in the culture supernatant when incubated with ferulate, supporting β-oxidative deacetylation. This phenotype was not complemented with a D257N variant of CouO, consistent with the predicted role of Asp257 as a metal ligand in this amidohydrolase superfamily member. These data suggest that CouO functionally replaces the β-ketothiolase and acyl-CoA thioesterase that occur in canonical β-oxidative pathways. Finally, the transcriptomics data suggest the involvement of two distinct formaldehyde detoxification pathways in vanillate catabolism and identify a eugenol catabolic pathway. The results of this study augment our understanding of the bacterial catabolism of aromatics from renewable feedstocks.

Mouse lung infection model to assess Rhodococcus equi virulence and vaccine protection

The pathogenic actinomycete Rhodococcus equi causes severe purulent lung infections in foals and immunocompromised people. Although relatively unsusceptible to R. equi, mice are widely used for in vivo studies with this pathogen. The most commonly employed mouse model is based on systemic (intravenous) infection and determination of R. equi burdens in spleen and liver. Here, we investigated the murine lung for experimental infection studies with R. equi. Using a 10(7)CFU intranasal challenge in BALB/c mice, virulent R. equi consistently survived in quantifiable numbers up to 10 days in the lungs whereas virulence-deficient R. equi bacteria were rapidly cleared. An internally controlled virulence assay was developed in which the test R. equi strains are co-inoculated and monitored in the same mouse. Isogenic R. equi bacteria lacking either the plasmid vapA gene or the entire virulence plasmid were compared using this competitive assay. Both strains showed no significant differences in in vivo fitness in the lung, indicating that the single loss of the virulence factor VapA was sufficient to account for the full attenuation seen in the absence of the virulence plasmid. To test the adequacy of the lung infection model for monitoring R. equi vaccine efficacy, BALB/c mice were immunized with live R. equi and challenged intranasally. Vaccination conferred protection against acute pulmonary challenge with virulent R. equi. Our data indicate that the murine lung infection model provides a useful tool for both R. equi virulence and vaccine studies.

Cholesterol to cholestenone oxidation by ChoG, the main extracellular cholesterol oxidase of Rhodococcus ruber strain Chol-4

The choG ORF of Rhodococcus ruber strain Chol-4 (referred from now as Chol-4) encodes a putative extracellular cholesterol oxidase. In the Chol-4 genome this ORF is located in a gene cluster that includes kstD3 and hsd4B, showing the same genomic context as that found in other Rhodococcus species. The putative ChoG protein is grouped into the class II of cholesterol oxidases, close to the Rhodococcus sp. CECT3014 ChoG homolog. The Chol-4 choG was cloned and expressed in a CECT3014 ΔchoG host strain in order to assess its ability to convert cholesterol into cholestenone. The RT-PCR analysis showed that choG gene was constitutively expressed in all the conditions assayed, but a higher induction could be inferred when cells were growing in the presence of cholesterol. A Chol-4 ΔchoG mutant strain was still able to grow in minimal medium supplemented with cholesterol, although at a slower rate. A comparative study of the removal of both cholesterol and cholestenone from the culture medium of either the wild type Chol-4 or its choG deletion mutant revealed a major role of ChoG in the extracellular production of cholestenone from cholesterol and, therefore, this enzyme may be related with the maintenance of a convenient supply of cholestenone for the succeeding steps of the catabolic pathway.

Comparative analysis of genes encoding key steroid core oxidation enzymes in fast-growing Mycobacterium spp. strains

A comparative genome analysis of Mycobacterium spp. VKM Ac-1815D, 1816D and 1817D strains used for efficient production of key steroid intermediates (androst-4-ene-3,17-dione, AD, androsta-1,4-diene-3,17-dione, ADD, 9α-hydroxy androst-4-ene-3,17-dione, 9-OH-AD) from phytosterol has been carried out by deep sequencing. The assembled contig sequences were analyzed for the presence putative genes of steroid catabolism pathways. Since 3-ketosteroid-9α-hydroxylases (KSH) and 3-ketosteroid-Δ(1)-dehydrogenase (Δ(1) KSTD) play key role in steroid core oxidation, special attention was paid to the genes encoding these enzymes. At least three genes of Δ(1) KSTD (kstD), five genes of KSH subunit A (kshA), and one gene of KSH subunit B of 3-ketosteroid-9α-hydroxylases (kshB) have been found in Mycobacterium sp. VKM Ac-1817D. Strains of Mycobacterium spp. VKM Ac-1815D and 1816D were found to possess at least one kstD, one kshB and two kshA genes. The assembled genome sequence of Mycobacterium sp. VKM Ac-1817D differs from those of 1815D and 1816D strains, whereas these last two are nearly identical, differing by 13 single nucleotide substitutions (SNPs). One of these SNPs is located in the coding region of a kstD gene and corresponds to an amino acid substitution Lys (135) in 1816D for Ser (135) in 1815D. The findings may be useful for targeted genetic engineering of the biocatalysts for biotechnological application.

Rhodococcus equi: the many facets of a pathogenic actinomycete

Rhodococcus equi is a soil-dwelling pathogenic actinomycete that causes pulmonary and extrapulmonary pyogranulomatous infections in a variety of animal species and people. Young foals are particularly susceptible and develop a life-threatening pneumonic disease that is endemic at many horse-breeding farms worldwide. R. equi is a facultative intracellular parasite of macrophages that replicates within a modified phagocytic vacuole. Its pathogenicity depends on a virulence plasmid that promotes intracellular survival by preventing phagosome-lysosome fusion. Species-specific tropism of R. equi for horses, pigs and cattle appears to be determined by host-adapted virulence plasmid types. Molecular epidemiological studies of these plasmids suggest that human R. equi infection is zoonotic. Analysis of the recently determined R. equi genome sequence has identified additional virulence determinants on the bacterial chromosome. This review summarizes our current understanding of the clinical aspects, biology, pathogenesis and immunity of this fascinating microbe with plasmid-governed infectivity.

Synergistic and additive effects of chromosomal and plasmid-encoded hemolysins contribute to hemolysis and virulence in Photobacterium damselae subsp. damselae

Photobacterium damselae subsp. damselae causes infections and fatal disease in marine animals and in humans. Highly hemolytic strains produce damselysin (Dly) and plasmid-encoded HlyA (HlyA(pl)). These hemolysins are encoded by plasmid pPHDD1 and contribute to hemolysis and virulence for fish and mice. In this study, we report that all the hemolytic strains produce a hitherto uncharacterized chromosome-encoded HlyA (HlyAch). Hemolysis was completely abolished in a single hlyAch mutant of a plasmidless strain and in a dly hlyApl hlyAch triple mutant. We found that Dly, HlyA(pl), and HlyAch are needed for full hemolytic values in strains harboring pPHDD1, and these values are the result of the additive effects between HlyApl and HlyAch, on the one hand, and of the synergistic effect of Dly with HlyApl and HlyAch, on the other hand. Interestingly, Dly-producing strains produced synergistic effects with strains lacking Dly production but secreting HlyA, constituting a case of the CAMP (Christie, Atkins, and Munch-Petersen) reaction. Environmental factors such as iron starvation and salt concentration were found to regulate the expression of the three hemolysins. We found that the contributions, in terms of the individual and combined effects, of the three hemolysins to hemolysis and virulence varied depending on the animal species tested. While Dly and HlyApl were found to be main contributors in the virulence for mice, we observed that the contribution of hemolysins to virulence for fish was mainly based on the synergistic effects between Dly and either of the two HlyA hemolysins rather than on their individual effects.

ChoD and HsdD can be dispensable for cholesterol degradation in mycobacteria

Cholesterol degradation is achieved through a complex metabolic pathway that starts with the oxidation of the 17-alkyl side chain and the steroid ring system. In bacteria, the oxidation of the 3β-hydroxyl group and isomerization of the resulting cholest-5-en-3-one to cholest-4-en-3-one is catalyzed by hydroxysteroid dehydrogenase (HsdD) or cholesterol oxidase (ChoD). Genes encoding both enzymes were annotated in both fast and slow growing mycobacteria, however the enzymatic activity was confirmed for HsdD, exclusively. Here, we used homologous recombination to engineer multiple mutants, and directly show that both ChoD and HsdD are dispensable for cholesterol degradation in fast-growing Mycobacterium smegmatis mc(2)155 and slow-growing Mycobacterium tuberculosis H37Rv strains. The mutants deffective in the synthesis of ChoD, HsdD or both enzymes were able to grow in minimal media supplemented with cholesterol as a sole source of carbon and energy. Multiple mutants, defective in synthesis of ChoD, HsdD and ketosteroid dehydrogenase (KstD), showed attenuated growth in minimal medium supplemented with cholesterol and accumulated cholesterol degradation intermediates: androstendion (AD) and 9-hydroxy androstendion (9OHAD).

Phenotypic and genotypic characterization of Rhodococcus equi isolated from sputum

INTRODUCTION

Rhodococcus equi is an opportunistic pathogen, causing rhodococcosis, a condition that can be confused with tuberculosis. Often, without identifying M. tuberculosis, physicians initiate empiric treatment for tuberculosis. R. equi and M. tuberculosis have different susceptibility to drugs. Identification of R. equi is based on a variety of phenotypic, chromatographic, and genotypic characteristics.

OBJECTIVE

This study aimed to characterize bacterial isolates from sputum samples suggestive of R. equi.

METHODS

The phenotypic identification included biochemical assays; thin-layer chromatography (TLC) and polymerase chain reaction (PCR) were used for genotypic identification.

RESULTS

Among 78 Gram-positive and partially acid-fast bacilli isolated from the sputum of tuberculosis-suspected patients, 51 were phenotypically and genotypically characterized as R. equi based on literature data. Mycolic acid analysis showed that all suspected R. equi had compounds with a retention factor (R(f)) between 0.4-0.5. Genotypic characterization indicated the presence of the choE gene 959bp fragments in 51 isolates CAMP test positive. Twenty-two CAMP test negative isolates were negative for the choE gene. Five isolates presumptively identified as R. equi, CAMP test positive, were choE gene negative, and probably belonged to other bacterial species.

CONCLUSIONS

The phenotypic and molecular techniques used constitute a good methodological tool to identify R. equi.

Rhodococcus equi's extreme resistance to hydrogen peroxide is mainly conferred by one of its four catalase genes

Rhodococcus equi is one of the most widespread causes of disease in foals aged from 1 to 6 months. R. equi possesses antioxidant defense mechanisms to protect it from reactive oxygen metabolites such as hydrogen peroxide (H(2)O(2)) generated during the respiratory burst of phagocytic cells. These defense mechanisms include enzymes such as catalase, which detoxify hydrogen peroxide. Recently, an analysis of the R. equi 103 genome sequence revealed the presence of four potential catalase genes. We first constructed ΔkatA-, ΔkatB-, ΔkatC-and ΔkatD-deficient mutants to study the ability of R. equi to survive exposure to H(2)O(2)in vitro and within mouse peritoneal macrophages. Results showed that ΔkatA and, to a lesser extent ΔkatC, were affected by 80 mM H(2)O(2). Moreover, katA deletion seems to significantly affect the ability of R. equi to survive within murine macrophages. We finally investigated the expression of the four catalases in response to H(2)O(2) assays with a real time PCR technique. Results showed that katA is overexpressed 367.9 times (± 122.6) in response to exposure to 50 mM of H(2)O(2) added in the stationary phase, and 3.11 times (± 0.59) when treatment was administered in the exponential phase. In untreated bacteria, katB, katC and katD were overexpressed from 4.3 to 17.5 times in the stationary compared to the exponential phase. Taken together, our results show that KatA is the major catalase involved in the extreme H(2)O(2) resistance capability of R. equi.

Microbial steroid transformations: current state and prospects

Studies of steroid modifications catalyzed by microbial whole cells represent a well-established research area in white biotechnology. Still, advances over the last decade in genetic and metabolic engineering, whole-cell biocatalysis in non-conventional media, and process monitoring raised research in this field to a new level. This review summarizes the data on microbial steroid conversion obtained since 2003. The key reactions of structural steroid functionalization by microorganisms are highlighted including sterol side-chain degradation, hydroxylation at various positions of the steroid core, and redox reactions. We also describe methods for enhancement of bioprocess productivity, selectivity of target reactions, and application of microbial transformations for production of valuable pharmaceutical ingredients and precursors. Challenges and prospects of whole-cell biocatalysis applications in steroid industry are discussed.

Cholesterol metabolism in Mycobacterium smegmatis

The metabolism of cholesterol in Mycobacterium smegmatis mc(2) 155 has been investigated by using a microarray approach. The transcriptome of M. smegmatis growing in cholesterol was compared with that of cells growing in glycerol as the sole carbon and energy sources during the middle exponential phase. Microarray analyses revealed that only 89 genes were upregulated at least threefold during growth on cholesterol compared with growth on glycerol. The upregulated genes are scattered throughout the 7 Mb M. smegmatis genome and likely reflect a general physiological adaptation of the bacterium to grow on this highly hydrophobic polycyclic compound. Nevertheless, 39 of the catabolic genes are organized in three specific clusters. These results not only supported the role of KstR and KstR2 as auto-regulated repressors of cholesterol catabolism, and revealed some metabolic similarities and differences on actinobacteria, but more important, they have facilitated the identification of new catabolic genes, opening a research scenario that might provide important clues on the role of cholesterol in tuberculosis infection.

Cholesterol oxidase ChoD is not a critical enzyme accounting for oxidation of sterols to 3-keto-4-ene steroids in fast-growing Mycobacterium sp. VKM Ac-1815D

Fast-growing strain of Mycobacterium sp. VKM Ac-1815D is capable of effective oxidizing of sterols (phytosterol, cholesterol, ergosterol) to androstenedione and other valuable 3-oxo-steroids. To elucidate the role of cholesterol oxidase in sterol catabolism by the strain, the choD gene has been cloned and sequenced. The deduced gene product (M(r) 63.5kDa) showed homologies over its entire length to a large number of proteins belonging to the InterPro-family EPR006076, which includes various FAD dependent oxidoreductases. The expression of choD in Escherichia coli was shown to result in the synthesis of membrane associated cholesterol oxidase. In addition to cholesterol, the enzyme oxidized β-sitosterol, dehydroepiandrosterone, ergosterol, pregnenolone, and lithocholic acid. Knock-out of choD in Mycobacterium sp. VKM Ac-1815D strain was obtained by the gene replacement technique. The mutant strain transformed sitosterol forming exclusively 3-keto-4-ene steroids with androstenedione as a major product, thus evidencing that choD knock out did not abrogate sterol A-ring oxidation. The results indicated that ChoD is not a critical enzyme responsible for modification of 3β-hydroxy-5-ene- to 3-keto-4-ene steroids in Mycobacterium sp. VKM Ac-1815D. Article from a special issue on steroids and microorganisms.

Catabolism and biotechnological applications of cholesterol degrading bacteria

Cholesterol is a steroid commonly found in nature with a great relevance in biology, medicine and chemistry, playing an essential role as a structural component of animal cell membranes. The ubiquity of cholesterol in the environment has made it a reference biomarker for environmental pollution analysis and a common carbon source for different microorganisms, some of them being important pathogens such as Mycobacterium tuberculosis. This work revises the accumulated biochemical and genetic knowledge on the bacterial pathways that degrade or transform this molecule, given that the characterization of cholesterol metabolism would contribute not only to understand its role in tuberculosis but also to develop new biotechnological processes that use this and other related molecules as starting or target materials.

Development of a live, attenuated, potential vaccine strain of R. equi expressing vapA and the virR operon, and virulence assessment in the mouse

Pneumonia caused by Rhodococcus equi remains a significant problem in foals. The objective of this study was to develop a safe and efficacious attenuated strain of R. equi for eventual use in oral immunization of foals. The approach involved expression of vapA in a live, virulence plasmid-negative, strain of R. equi (strain 103-). PCR-amplified fragments of the vapA gene, with and without the upstream genes virR, orf5, vapH, orf7 and orf8 (orf4-8), were cloned into a shuttle vector pNBV1. These plasmids, named pAW48A and pAWVapA respectively, were electroporated into strain 103-. The presence of the recombinant vectors in the attenuated strain (103-) and the integrity of the inserted genes were confirmed, and both constructs expressed VapA. The virulence of the two strains was compared to that of wild type R. equi 103+ and negative controls by their intravenous inoculation into mice, followed by examination of liver clearance 4 days later. Mice inoculated with R. equi 103-, 103-/pAWVapA and 103-/pNBV1 completely cleared infection, whereas strain 103-/pAW48A persisted in 47% of mice.

Insight from the draft genome of Dietzia cinnamea P4 reveals mechanisms of survival in complex tropical soil habitats and biotechnology potential

The draft genome of Dietzia cinnamea strain P4 was determined using pyrosequencing. In total, 428 supercontigs were obtained and analyzed. We here describe and interpret the main features of the draft genome. The genome contained a total of 3,555,295 bp, arranged in a single replicon with an average G+C percentage of 70.9%. It revealed the presence of complete pathways for basically all central metabolic routes. Also present were complete sets of genes for the glyoxalate and reductive carboxylate cycles. Autotrophic growth was suggested to occur by the presence of genes for aerobic CO oxidation, formate/formaldehyde oxidation, the reverse tricarboxylic acid cycle and the 3-hydropropionate cycle for CO₂ fixation. Secondary metabolism was evidenced by the presence of genes for the biosynthesis of terpene compounds, frenolicin, nanaomycin and avilamycin A antibiotics. Furthermore, a probable role in azinomycin B synthesis, an important product with antitumor activity, was indicated. The complete alk operon for the degradation of n-alkanes was found to be present, as were clusters of genes for biphenyl ring dihydroxylation. This study brings new insights in the genetics and physiology of D. cinnamea P4, which is useful in biotechnology and bioremediation.

The steroid catabolic pathway of the intracellular pathogen Rhodococcus equi is important for pathogenesis and a target for vaccine development

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.

Rhodococcus equi causes fatal pyogranulomatous bronchopneumonia in young foals and is an emerging opportunistic pathogen of immunocompromised humans. Despite its importance, there is currently no safe and effective vaccine against R. equi infections. Like Mycobacterium tuberculosis, the causative agent of human tuberculosis, R. equi is able to infect, survive and multiply inside alveolar macrophages. Recently we have shown that essential steps in the cholesterol catabolic pathway (encoded by the rv3551, rv3552, fadE30 genes) are involved in the pathogenicity of M. tuberculosis. We hypothesized that the orthologous genes in the cholesterol catabolic gene cluster of R. equi also are essential for its virulence mechanism. Analysis of the respective R. equi strain RE1 mutants revealed that they were impaired in growth on intermediates of the steroid catabolic pathway and had attenuated phenotypes in a macrophage infection assay. Mutant RE1ΔipdAB, carrying a deletion of the orthologs of rv3551 and rv3552, could be safely administered to 2–5 week-old foals intratracheally and oral immunization provided a substantial protection against infection by a virulent R. equi strain. Our data show that genes important for methylhexahydroindanone propionate degradation, part of the steroid catabolic pathway, are promising targets for the development of a live-attenuated vaccine against R. equi infections.

The genome of a pathogenic rhodococcus: cooptive virulence underpinned by key gene acquisitions

We report the genome of the facultative intracellular parasite Rhodococcus equi, the only animal pathogen within the biotechnologically important actinobacterial genus Rhodococcus. The 5.0-Mb R. equi 103S genome is significantly smaller than those of environmental rhodococci. This is due to genome expansion in nonpathogenic species, via a linear gain of paralogous genes and an accelerated genetic flux, rather than reductive evolution in R. equi. The 103S genome lacks the extensive catabolic and secondary metabolic complement of environmental rhodococci, and it displays unique adaptations for host colonization and competition in the short-chain fatty acid–rich intestine and manure of herbivores—two main R. equi reservoirs. Except for a few horizontally acquired (HGT) pathogenicity loci, including a cytoadhesive pilus determinant (rpl) and the virulence plasmid vap pathogenicity island (PAI) required for intramacrophage survival, most of the potential virulence-associated genes identified in R. equi are conserved in environmental rhodococci or have homologs in nonpathogenic Actinobacteria. This suggests a mechanism of virulence evolution based on the cooption of existing core actinobacterial traits, triggered by key host niche–adaptive HGT events. We tested this hypothesis by investigating R. equi virulence plasmid-chromosome crosstalk, by global transcription profiling and expression network analysis. Two chromosomal genes conserved in environmental rhodococci, encoding putative chorismate mutase and anthranilate synthase enzymes involved in aromatic amino acid biosynthesis, were strongly coregulated with vap PAI virulence genes and required for optimal proliferation in macrophages. The regulatory integration of chromosomal metabolic genes under the control of the HGT–acquired plasmid PAI is thus an important element in the cooptive virulence of R. equi.

Rhodococcus is a prototypic genus within the Actinobacteria, one of the largest microbial groups on Earth. Many of the ubiquitous rhodococcal species are biotechnologically useful due to their metabolic versatility and biodegradative properties. We have deciphered the genome of a facultatively parasitic Rhodococcus, the animal and human pathogen R. equi. Comparative genomic analyses of related species provide a unique opportunity to increase our understanding of niche-adaptive genome evolution and specialization. The environmental rhodococci have much larger genomes, richer in metabolic and degradative pathways, due to gene duplication and acquisition, not genome contraction in R. equi. This probably reflects that the host-associated R. equi habitat is more stable and favorable than the chemically diverse but nutrient-poor environmental niches of nonpathogenic rhodococci, necessitating metabolically more complex, expanded genomes. Our work also highlights that the recruitment or cooption of core microbial traits, following the horizontal acquistion of a few critical genes that provide access to the host niche, is an important mechanism in actinobacterial virulence evolution. Gene cooption is a key evolutionary mechanism allowing rapid adaptive change and novel trait acquisition. Recognizing the contribution of cooption to virulence provides a rational framework for understanding and interpreting the emergence and evolution of microbial pathogenicity.

Cholesterol oxidase: structure and function

Cholesterol oxidase is a bacterial-specific flavoenzyme that catalyzes the oxidation and isomerisation of steroids containing a 3beta hydroxyl group and a double bond at the Delta5-6 of the steroid ring system. The enzyme is a member of a large family of flavin-specific oxidoreductases and is found in two different forms: one where the flavin adenine dinucleotide (FAD) cofactor is covalently linked to the protein and one where the cofactor is non-covalently bound to the protein. These two enzyme forms have been extensively studied in order to gain insight into the mechanism of flavin-mediated oxidation and the relationship between protein structure and enzyme redox potential. More recently the enzyme has been found to play an important role in bacterial pathogenesis and hence further studies are focused on its potential use for future development of novel antibacterial therapeutic agents. In this review the biochemical, structural, kinetic and mechanistic features of the enzyme are discussed.

Identification of atypical Rhodococcus-like clinical isolates as Dietzia spp. by 16S rRNA gene sequencing

Rhodococcus equi and Dietzia spp. are closely related actinomycetes that show similar phenotypic properties. In humans, R. equi is an opportunistic pathogen associated with severe immunodeficiency. Dietzia spp. are environmental bacteria that have been isolated recently from clinical material and are presumptively associated with human infections. During the last 5 years, 15 bacterial isolates from human clinical samples collected at the Hospital Marqués de Valdecilla, Santander, Spain, were identified as R. equi by the API Coryne test. 16S rRNA gene sequencing confirmed seven isolates to be true R. equi strains, whereas the other eight were identified as members of the genus Dietzia, including Dietzia maris (four isolates), Dietzia natronolimnaea (two isolates), and Dietzia timorensis and Dietzia sp. (one isolate each). The eight Dietzia isolates were highly sensitive to 12 antimicrobial compounds.

Extracellular cholesterol oxidase from Rhodococcus sp.: isolation and molecular characterization

BACKGROUND

Cholesterol oxidase (CHO) has various clinical and industrial applications. Recently, microbial CHO have received a great attention for their wide usage in medicine. Here, taxonomic characterizations of isolated strain from soil, optimization of the conditions for CHO production and biochemical characterizations of produced CHO enzyme were described. Finally, CHO gene was cloned into a cloning vector.

METHODS

Various samples were collected and cultivated in a screening medium consisting of cholesterol. For isolation of CHO-producing bacteria, well-grown colonies were inoculated into an optimized medium. Different biochemical and microbiological tests were performed on isolated bacteria to identify their properties. For phylogenic analysis, a partial sequence of l6s rRNA was amplified by PCR using universally conserved primers. A modified method was applied for determination of CHO activity. Then, extracellular CHO activity was assessed under different temperature, pH and cholesterol concentration conditions. Finally, CHO gene was amplified by PCR and cloned into STV28.

RESULTS

According to the morphological, cultural and biochemical tests, the isolated bacterium was identified as Rhodococcus sp. strain 501 and deposited in GenBank with accession number FN298676. Results showed that optimum temperature and pH for CHO activity were 35 degrees C and 7.5, respectively. Alignment of nucleotide sequence of CHO gene showed 99% homology with other bacterial CHO genes.

CONCLUSION

Rhodococcus sp. strain 501 produced significant levels of extracellular CHO in an optimized medium for a short period. CHO gene was cloned into cloning vector that can be a valuable tool for better identification and further studies on gene expression.

A hydrogen-bonding network is important for oxidation and isomerization in the reaction catalyzed by cholesterol oxidase

Cholesterol oxidase is a flavoenzyme that catalyzes the oxidation and isomerization of 3beta-hydroxysteroids. Structural and mutagenesis studies have shown that Asn485 plays a key role in substrate oxidation. The side chain makes an NH...pi interaction with the reduced form of the flavin cofactor. A N485D mutant was constructed to further test the role of the amide group in catalysis. The mutation resulted in a 1800-fold drop in the overall k(cat). Atomic resolution structures were determined for both the N485L and N485D mutants. The structure of the N485D mutant enzyme (at 1.0 A resolution) reveals significant perturbations in the active site. As predicted, Asp485 is oriented away from the flavin moiety, such that any stabilizing interaction with the reduced flavin is abolished. Met122 and Glu361 form unusual hydrogen bonds to the functional group of Asp485 and are displaced from the positions they occupy in the wild-type active site. The overall effect is to disrupt the stabilization of the reduced FAD cofactor during catalysis. Furthermore, a narrow transient channel that is shown to form when the wild-type Asn485 forms the NH...pi interaction with FAD and that has been proposed to function as an access route of molecular oxygen, is not observed in either of the mutant structures, suggesting that the dynamics of the active site are altered.

Cholesterol oxidase: physiological functions

An important aspect of catalysis performed by cholesterol oxidase (3beta-hydroxysteroid oxidase) concerns the nature of its association with the lipid bilayer that contains the sterol substrate. Efficient catalytic turnover is affected by the association of the protein with the membrane as well as the solubility of the substrate in the lipid bilayer. In this review, the binding of cholesterol oxidase to the lipid bilayer, its turnover of substrates presented in different physical environments, and how these conditions affect substrate specificity, are discussed. The physiological functions of the enzyme in bacterial metabolism, pathogenesis and macrolide biosynthesis are reviewed in this context.

Cholesterol oxidase: biotechnological applications

Cholesterol oxidase is a bacterial FAD-containing flavooxidase that catalyzes the first reaction in cholesterol catabolism. Indeed, this enzyme catalyzes two reactions: the oxidation of the C(3)-OH group of cholesterol (and other sterols) to give cholest-5-en-3-one; and its isomerization to cholest-4-en-3-one. In the past several years, the structural and functional characterization of cholesterol oxidase has been developed together with its application as a biological tool. Cholesterol oxidase has been used in biocatalysis for the production of a number of steroids, as an insecticidal protein against boll weevil larvae and, in particular, as a diagnostic enzyme for determining serum levels of cholesterol. These applications prompted various laboratories worldwide to isolate this flavooxidase from different sources and to improve its properties by protein engineering, further increasing our knowledge on its structure-function relationships. These studies also discovered new physiological roles for cholesterol oxidase (e.g. in virulence and as an antifungal sensor). We assume that the investigations of cholesterol oxidase and its applications will continue to grow quickly in the near future, in particular to uncover unexpected, new areas of application.

Cholesterol oxidase ChoL is a critical enzyme that catalyzes the conversion of diosgenin to 4-ene-3-keto steroids in Streptomyces virginiae IBL-14

Diosgenin transformation was studied in Streptomyces virginiae IBL-14, a soil-dwelling bacterium with diosgenin-degrading capacity. All of the derivatives isolated were identified as 4-ene-3-keto steroids. We cloned ChoL, a fragment of a cholesterol oxidase from S. virginiae IBL-14, and used gene-disruption techniques to determine its function in the oxidation of diosgenin to 4-ene-3-keto steroids. Subsequently, the entire open reading frame of ChoL was cloned by chromosome walking, and the His(6)-tagged recombinant protein was overproduced, purified, and characterized. ChoL consisted of 1,629 nucleotides that encoded a protein of 542 amino acids, including a 34-residue putative signal peptide at the N-terminal. ChoL showed 85% amino acid similarity to ChoA from Streptomyces sp. SA-COO. This enzyme can also oxidize other steroids such as cholesterol, sitosterol, and dehydroepiandrosterone, which showed higher affinity (K(m) = 0.195 mM) to diosgenin. The catalytic properties of this enzyme indicate that it may be useful in diosgenin transformation, degradation, and assay.

Characteristics and biotechnological applications of microbial cholesterol oxidases

Microbial cholesterol oxidase is an enzyme of great commercial value, widely employed by laboratories routinely devoted to the determination of cholesterol concentrations in serum, other clinical samples, and food. In addition, the enzyme has potential applications as a biocatalyst which can be used as an insecticide and for the bioconversion of a number of sterols and non-steroidal alcohols. The enzyme has several biological roles, which are implicated in the cholesterol metabolism, the bacterial pathogenesis, and the biosynthesis of macrolide antifungal antibiotics. Cholesterol oxidase has been reported from a variety of microorganisms, mostly from actinomycetes. We recently reported cholesterol oxidases from gram-negative bacteria such as Burkholderia and Chromobacterium. These enzymes possess thermal, detergent, and organic solvent tolerance. There are two forms of cholesterol oxidase, one containing a flavin adenine dinucleotide cofactor non-covalently bound to the enzyme (class I) and the other containing the cofactor covalently linked to the enzyme (class II). These two enzymes have no significant sequence homology. The phylogenetic tree analyses show that both class I and class II enzymes can be further divided into at least two groups.

Molecular and infection biology of the horse pathogen Rhodococcus equi

The soil actinomycete Rhodococcus equi is a pulmonary pathogen of young horses and AIDS patients. As a facultative intracellular bacterium, R. equi survives and multiplies in macrophages and establishes its specific niche inside the host cell. Recent research into chromosomal virulence factors and into the role of virulence plasmids in infection and host tropism has presented novel aspects of R. equi infection biology and pathogenicity. This review will focus on new findings in R. equi biology, the trafficking of R. equi-containing vacuoles inside host cells, factors involved in virulence and host resistance and on host-pathogen interaction on organismal and cellular levels.