Abstract:
A cloning vector contains a DNA sequence derived from a Nocardioform microorganism, and is capable of expressing the DNA sequence in a host microorganism. Recombinant cholesterol oxidase is produced by a host microbial cell transformed with a cloning vector containing cholesterol oxidase encoding DNA derived from a Nocardioform microorganism. A method for determining cholesterol oxidase in a sample of fluid from a human includes contacting the sample with the Nocardioform microorganism derived recombinant cholesterol oxidase.

Description:
This is a continuation of application Ser. No. 07/914,048, filed Jul. 13, 1992, now abandoned, which is a continuation of application Ser. No. 07/436,234, filed Nov. 14, 1989, now abandoned, which is application is a Continuation-in-Part of application Ser. No. 07/269,669, filed Nov. 14, 1988, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to gene expression. 
     The organisms grouped as Nocardioforms in Bergey&#39;s Manual of Systematic Bacteriology (Williams and Wilkins, 1986, pp. 1458-1481) include nine genera which are close phylogenetically to Corynebacterium, Athrobacter and Mycobacterium. Members of the Nocardioforms including the genera Nocardia and Rhodococcus exhibit a wide range of useful metabolic activities including the assimilation of unusual compounds such as alkanes and aromatic hydrocarbons; the degradation of lignin, detergents and pesticides; the production of enzymes useful in xenobiotic transformations; and the biosynthesis of antibiotics, amino acids and biosurfactants. In particular, this group of bacteria have been exploited for their production of important steroid modifying enzymes, including cholesterol esterase and cholesterol oxidase, which are used in diagnostic assays to determine cholesterol levels in blood and serum. 
     The capacity to degrade cholesterol is widespread among microorganisms other than the Nocardioforms. For example, cholesterol oxidases have also been isolated from species of Mycobacterium, Pseudomonas, and Streptomyces. It is well known from scientific and patent literature and from commercial practice that the Nocardioform-type cholesterol oxidase is distinct from the other microbial cholesterol oxidases. 
     The Nocardioform enzyme is very stable and active over a wide pH range (pH 6.0-8.0), the Km for cholesterol is 1.4×10 -5  mol/liter at 25°, and the enzyme is highly specific for Δ 4  - or Δ 5  -3β-sterols. As produced in the Nocardioforms, cholesterol oxidase is membrane associated and requires detergent extraction and lipid removal for purification. 
     Singer, et al., J. Bacteriol., 1988, Vol. 170, pp. 638-645, describes expression of genes in Rhodococcus sp. from both E. coli and Streptomyces sp. and the development of a shuttle plasmid based on an E. coli plasmid and containing a Rhodococcus plasmid-derived origin of replication. The plasmid is capable of replicating in E. coli and in several but not all of the Rhodococcus species tested. 
     SUMMARY OF THE INVENTION 
     We have discovered that recombinant DNA sequences derived from Nocardioform microorganisms can be expressed efficiently in a host microorganism. 
     The invention features, in one aspect, a cloning vector which expresses DNA sequences, derived from a Nocardioform organism, in a host microorganism. In preferred embodiments, the DNA sequence is derived from Rhodococcus sp. NCIB 10554 (National Collection of Industrial Bacteria, Aberdeen, Scotland); the DNA sequence encodes cholesterol oxidase; the organism in which the cloning vector can express the Nocardioform DNA is a gram-positive microorganism, more preferably a species of Streptomyces, and most preferably S. lividans; and the cloning vector is the plasmid pSL81. 
     Cholesterol oxidase is produced according to the invention by providing a cloning vector containing a DNA sequence encoding cholesterol oxidase, which is derived from a Nocardioform microorganism, transforming a host cell with the cloning vector to obtain a recombinant host cell, culturing the recombinant host cell under conditions permitting expression of the DNA sequence, and recovering the cholesterol oxidase. In preferred embodiments, the Nocardioform organism is Rhodococcus sp. NCIB 10554; the host cell is a gram-positive microorganism, preferably a species of Streptomyces, and more preferably S. lividans; and the cloning vector is the plasmid pSL81. 
     Cholesterol oxidase produced according to the invention can be used for determining cholesterol in a sample, by contacting the sample with the recombinant cholesterol oxidase and determining the extent of oxidation of cholesterol; specifically, by measuring oxygen consumption with an oxygen electrode; or by measuring the production of hydrogen peroxide; or by spectrophotometrically determining cholesterol conversion to Δ 4  -cholestenone. 
     The invention provides for the expression of DNA sequences derived from a Nocardioform microorganism in a host microorganism that is easier to manipulate under conditions of commercial production than is the parent organism itself, and for which cloning techniques are known. Recombinant cholesterol oxidase derived from Nocardioform microorganisms is expressed as an extracellular protein, an expression condition which can lead to substantially higher production of enzyme. Moreover, the expression does not require addition of an inducer, and the expressed enzyme is free of the lipids of the Nocardioform membrane. 
     Other features and advantages of the invention will be apparent from the following description of the preferred embodiment thereof, and from the claims. 
    
    
     BRIEF OF THE DRAWINGS 
     FIG. 1 is a restriction endonuclease map of plasmid pSL81. 
     FIGS. 2A-2F are a sequence description of the Rhodococcus 10554 cholesterol oxidase gene. 
     FIG. 3 is a restriction endonuclease map of plasmid pSL81, showing the position of the pSL81Δ15 fragment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Gene Isolation and Expression 
     The expression of DNA sequences derived from a Nocardioform microorganism can be accomplished, in general, by first constructing a genomic bank or library by incorporating the DNA sequences into a suitable cloning vector containing a suitable origin of replication compatible with the desired host organism. Next, a host cell from any microorganism, advantageously any gram-positive microorganism, such as, for example, species of Streptomyces or Bacillus, is transformed with the vector containing the DNA sequences. Then, transformants are screened for the activity of the desired gene product, and active clones are isolated and characterized. 
     Construction of Library 
     The following protocol, presented for illustrative purposes, describes construction of a genomic library of Rhodococcus sp. NCIB 10554 (Rhodococcus 10554) DNA in a host such as S. lividans which is capable of expressing the DNA from the parent microorganism. In the example described below, a population of transformed cells prepared using this library is screened for the expression of the cholesterol oxidase gene of Rhodococcus 10554. 
     Generally, a genomic library of Rhodococcus 10554 DNA was constructed from high molecular weight DNA which had been isolated and fragmented with the restriction enzyme Sau3A to yield fragments in the size range 4-8 kilobase pairs. Size fractionated DNA from Rhodococcus 10554 was then ligated to the cloning plasmid pIJ702 which had been digested with BglII. The pIJ702 plasmid is commercially available through the American Type Culture Collection (Rockville, Md., USA). Hybrid plasmids of the library were transformed into S. lividans cells to generate the genomic library of Rhodococcus 10554 DNA in the host microorganism. 
     Details of the procedure were as follows: A single colony of Rhodococcus 10554 was used to inoculate 50 ml of NBYE medium which contained per liter: Difco nutrient broth, 8 g, and Difco yeast extract, 5 g. Other appropriate media described for growing Rhodococcus sp. could also be used. The culture was grown for 48 hours at 30° C. and the cells collected by centrifugation at 10,000 rpm for 15 minutes in a Sorvall RC-5B centrifuge using an SS-34 rotor. The cells were resuspended in 18 ml of Tris-glucose (10 mM Tris-HCl, pH 8.0, 10 mM EDTA, 25% w/v glucose), and 2 ml of lysozyme solution (10 mg/ml lysozyme (Sigma) in Tris-glucose solution) was added. This mixture was incubated at 37° C. for 60 minutes. Next, 2.4 ml of 10% w/v sodium dodecyl sulfate (Sigma) was added and the mixture stirred vigorously to form a homogenous solution of lysed cells. The lysed cells were incubated at 55° C. for 2 hours, and then 2.7 ml of 5M NaClO 4  (Sigma) were added. Denatured protein and cell debris were removed by extracting the lysed cell solution twice with 20 ml chloroform:isoamyl alcohol (24:1) and resolving the phases by centrifugation at 12,000 rpm for 10 minutes in a Sorvall SS-34 rotor. The aqueous phase was removed and two volumes of cold ethanol added. The solution was mixed gently, and the high molecular weight DNA was collected using a glass rod as described by Marmur et al., 1961, J. Mol. Biol., Vol. 3, pp. 203-218. The collected DNA was washed twice in 70% ethanol and dissolved in 5 ml TE (10 mM Tris, 1 mM EDTA, pH 8.0). DNAse free RNAse (Sigma) was added at 100 mg/ml to the DNA solution and incubated at 42° C. for 30 minutes. Next, 0.6 ml of 10% w/v of sodium dodecyl sulfate and 0.6 ml of 5M NaCl were added, and the mixture was extracted once with phenol:chloroform (1:1) and once with chloroform:isoamyl alcohol (24:1). DNA was precipitated with 2 volumes of ethanol and collected as before using a glass rod. The DNA was washed once with 70% ethanol and air dried prior to resuspension in TE to a final concentration of approximately 1 mg/ml. 
     In order to establish the condition for isolating partially digested high molecular weight Rhodococcus 10554 DNA, test reactions were set up to digest DNA with different concentrations of Sau3A restriction enzyme. A reaction mixture was prepared with 10 μl DNA, 15 μl 10× Sau3A buffer (200 mM Tris HCl, pH 7.4, 50 mM MgCl 2 , 500 mM KCl) and 125 μl water. Aliquots of 15μl were dispensed into tubes labeled 1-9 and chilled on ice. Exactly 4 units of Sau3A enzyme (10 units/μl BRL) were added to tube 1 and mixed well. Two-fold serial dilutions of the reaction mixtures from tube 1 sequentially through to tube 8 were performed to yield reaction mixtures containing two-fold decreasing concentrations of Sau3A enzyme. Tube 9 which contained no enzyme represented the undigested control reaction. The tubes were incubated at 37° C. for 1 hour and stopped by returning the tubes to ice and adding EDTA (20 mM). The extent of digestion in each tube was monitored by agarose gel electrophoresis, and the sample which yielded DNA fragments in the approximate range of 2-20 kb was scaled up 200 fold and the reaction repeated. The digested Rhodococcus 10554 DNA was separated by agarose gel electrophoresis using low gelling temperature agarose (SeaPlaque, FMC). The region of the gel containing DNA fragments in the size range 4-8 kb was excised with a sterile scalpel and the DNA recovered by electro-elution. The DNA was precipitated by conventional methods, and the precipitate was resuspended in TE to a concentration of 0.025 mg/ml. 
     A single colony of S. lividans/pIJ702 was used to inoculate 50 ml of YEME which contained per liter: Difco yeast extract, 3 g; Difco peptone, 5 g; Oxoid malt extract, 3 g; glucose, 10 g; sucrose, 340 g; MgCl 2  at 5 mM; and glycine at 0.4% w/v; and was supplemented with the plasmid selective agent thiostrepton (CalBiochem), at 50 μg/ml. Other media described for Streptomyces could also be used. The culture was incubated at 30° C. for 3 days. The cells were harvested by centrifugation at 10,000 rpm for 30 minutes using a Sorvall centrifuge and SS-34 rotor. The pellet was resuspended in 5 ml TSE Buffer (25 mM Tris, pH 8.0; 0.3M sucrose; 25 mM EDTA) containing 10 mg lysozyme. The cells were incubated at 37° C. for 30 minutes. A 3 ml volume of alkaline SDS solution (0.3M NaOH, 2% w/v sodium dodecyl sulfate) was added, and the mixture was incubated at room temperature for 10 minutes and at 70° C. for 10 minutes. The lysed cell solution was cooled to room temperature and extracted with 1 volume of phenol: chloroform (1:1). The aqueous phase was removed to a clean tube and 0.3M sodium acetate, pH 4.8, and 1 volume of isopropanol added. The DNA was precipitated for 30 minutes at -20° C. and recovered by centrifugation, using a Sorvall SS-34 rotor at 10,000 rpm for 10 minutes. The pellet was resuspended in 2 ml TE and the plasmid DNA purified by ultracentrifugation to equilibrium in a cesium chloride density gradient according to known techniques. 
     Plasmid pIJ702 prepared as above was resuspended in TE to a concentration of 1 mg/ml. About 10 μl (10 μg) of DNA was added to 5 μl 10× BqlII buffer (500 mM Tris HCl, pH 7.4; 50 mM MgCl 2  ; 500 mM KCl), 32 μl distilled water and 3 μl of BqlII restriction enzyme (10 units/μl, BRL) and incubated for 2 hours at 37° C. The reaction was terminated by one extraction with phenol:chloroform (1:1) and one extraction with chloroform:isoamyl alcohol (24:1). The DNA was precipitated by the addition of one-half volume of 7.5M ammonium acetate and 2 volumes of cold ethanol. After incubation at -20° C. for at least 2 hours, the DNA was collected in an Eppendorf microfuge by centrifugation for 10 minutes. The DNA pellet was washed once with 70% ethanol, dried and resuspended in 50 μl 10 mM Tris HCl, pH 8.0. To dephosphorylate the D A, 1 μl of calf intestine alkaline phosphatase (2 U/μl, IBI) was added and the reaction incubated at 37° C. for 30 minutes. The reaction was stopped by adding 150 μl stop buffer (10 mM Tris HCl, pH 7.5; 1 mM EDTA, pH 7.5; 200 mM NaCl; 0.5% w/v sodium dodecyl sulfate) and then extracted once with phenol:chloroform (1:1) and once with chloroform:isoamyl alcohol (24:1). The DNA was precipitated with ethanol, dried and resuspended in TE to a final concentration of 0.05 mg/ml and stored at -20° C. 
     About 10 μl (0.5 μg) of the dephosphorylated BqIII digested pIJ702 DNA was added to about 20 μl (0.5 μg) of the Sau3A digested Rhodococcus DNA. A 20 μl aliquot of 5× ligase buffer (BRL), 0.5 μl ligase (NEB) and 50 μl water were added and the ligation reaction incubated at 14° C. for 20 hours. The reaction was terminated by heating the mixture to 65° C. for 10 minutes, and the sample was stored at -20° C. 
     A vegetative inoculum was prepared by growing S. lividans in 25 ml culture broth as described for the preparation of S. lividans/pIJ702, except the medium contained no thiostrepton. The cells were harvested in a benchtop centrifuge at 3,000 rpm for 10 minutes and washed twice with 10 ml of 10.3% w/v sucrose. The cell pellet was resuspended in 4 ml lysozyme medium (L-medium, Thompson et al., 1982, J. Bacteriol., Vol. 151, pp. 668-677) and incubated at 30° C. for 30 minutes. A 5 ml aliquot of protoplast medium (P-medium, Okanishi, et al., 1974, J. Gen. Microbiol., Vil. 80, pp. 389-400) was added and the solution mixed by pipetting up and down. Protoplasts were filtered through glass wool (which traps mycelia but allows protoplasts to pass through) harvested by centrifugation, and resuspended in 2 ml P-medium. Protoplasts were used fresh or stored frozen in P-medium at -70° C. 
     Up to 3 μl of ligated plasmid DNA in ligation buffer was mixed gently with 50 μl S. lividans protoplasts in a small sterile tube. To this mixture, 200 μl transformation buffer (T-buffer, Thompson, et al., 1982, J. Bacteriol., Vol. 151, pp. 668-667) was added and pipetted to mix. Immediately, the cells were spread gently onto R2YE medium (Thompson, et al., 1980, Nature (London), Vol. 286, pp. 525-527) and incubated overnight at 30° C. The regenerated protoplasts were overlaid with filter paper (Whatman 541, 9.0 cm) containing 100 μg/ml thiostrepton and incubated at 30° C. for 3 days. About 200-500 transformants per plate were obtained using these transformation conditions. Approximately 12,000 thiostrepton-resistant S. lividans transformants were obtained in this way. Over 70% of these transformants contained recombinant pIJ702 plasmids as estimated by miniprep DNA analysis. These recombinant plasmids represented the genomic library of Rhodococcus 10554 DNA. 
     Screening for Cholesterol Oxidase Activity 
     As an example, showing the expression of one DNA sequence derived from a Nocardioform microorganism, recombinant S. lividans cells containing the genomic library of Rhodococcus 10554 DNA were screened for the expression of cholesterol oxidase activity. Cholesterol oxidase was determined using a modified method of Allain et al., 1974, Physical Chem., Vol. 20, pp. 470-475. Filter paper discs were soaked in an assay buffer containing 4-aminoantipyrine, phenol, horse radish peroxidase and triton-X100, and laid down on colonies growing on agar medium containing cholesterol substrate. Alternatively, these colonies could be assayed by halo formation on agar medium containing cholesterol, in which halo formation depended on the conversion of cholesterol to cholestenone by soluble cholesterol oxidase. Recombinant S. lividans colonies which produced cholesterol oxidase were purified, and characterized. 
     Details of the screening of the genomic library for cholesterol oxidase activity were as follows: Filter papers used to apply thiostrepton drug to R2YE transformation plates for the selection of S. lividans transformants, were lifted and placed onto screening plates [medium containing per liter: agar, 15 g; glycerol, 20 g; yeast extract, 1 g; MgSO 4 , 0.66 g; buffer salts, 66 ml (per liter: Na 2  HPO 4 , 28 g; NaH 2  PO 4 , 30 g; K 2  HPO 4 , 20 g; (NH 4 ) 2  SO 4 , 127.5 g) and trace salts, 2 ml (per liter: concentrated HCl, 250 ml; CaCl 2 , 3.57 g; ZnSO 4 , 20 g; CuCl 2 , 0.85 g; NaMoO 4 , 4.8 g; MnCl 2 , 2.0 g; FeCl 3 , 5.4 g; boric acid, 0.3 g; CoCl 2 , 2.4 g)] with cholesterol (1.6 mM, Sigma) and thiostrepton (50 μg/ml). These replica plates were incubated at 30° C. for 3 days. The replica filters were removed and replaced with sterile filter paper discs soaked in cholesterol oxidase assay reagent (100 mM citric acid buffer, pH 6.0; 0.2 mM 4-aminoantipyrine (Aldrich); horse radish peroxidase I, 600 units per liter; 6.4 mM cholesterol; 10 mM phenol and 4% w/v Triton×100). The plates were incubated at 30° C., and the colonies were screened over a 4 hour period for the development of a red zone due to cholesterol oxidase activity which diffused onto the white filter paper. After incubation at 30° C. for 4 hours, the assay reagent filter papers were removed from the screening plates, and the plates were re-incubated at 30° C. overnight to allow for the formation of clear halos associated with cholesterol oxidase degradation of cholesterol. From the approximately 8,000 recombinant S. lividans colonies screened, two showed cholesterol oxidase activity. The plasmid DNA from one of these isolates was called pSL81 and was analyzed further. 
     Plasmid DNA from the S. lividans/pSL81 recombinant was isolated and transformed back into S. lividans using the same procedure described for isolating and transforming pIJ702. All the thiostrepton-resistant transformants obtained contained the same plasmid pIJ702 with approximately 6 kb insert DNA, and showed the cholesterol oxidase activity diagnostic of the original recombinant. Curing of plasmid pSL81 from S. lividans cells by growth in the absence of drug selective pressure resulted in the simultaneous loss of thiostrepton resistance and cholesterol oxidase activity. Southern hybridization demonstrated that the insert DNA was derived from the Rhodococcus 10554 genome. Referring to FIG. 1, the recombinant plasmid designated pSL81 includes cloning vector pIJ702 12 and insert DNA 14 from Rhodococcus 10554, and has restriction endonuclease sites as indicated. 
     Plasmid pSL81, prepared as above, was used to probe chromosomal digests from the cholesterol oxidase producing Nocardioforms Rhodococcus NCIB 10554, R. erythropolis NCIB 9158, Nocardia erythropolis ATCC 17895 and Nocardia erythropolis ATCC 4277. The strains all showed cross-hybridizing DNA bands suggesting that the cholesterol oxidase coding region in these Nocardioforms is the same. Plasmid pSL81 showed no specific cross hybridization with DNA isolated from a cholesterol oxidase producing Streptomyces, namely Streptomyces violascene NRRL B-2700. 
     DNA Sequencing 
     In order to sequence the cholesterol oxidase gene, subclones of pSL81 were constructed and a cholesterol oxidase coding region was localized to a 2.8 kb BglII fragment in the subclone pSL81Δ15 (FIG. 3). This fragment was then introduced into E. coli on plasmid pBR322, and a series of deletion derivatives were constructed using available restriction sites. Sequencing was performed on denatured double stranded plasmid DNA using the dideoxy chain termination method of Sanger et al., PNAS USA, Vol. 74, pp. 5463-67. Specifically, a Sequenase Version 2.0 kit (United States Biochemical Corp.) was used according to the manufacturer&#39;s specifications. Deoxyguanidine triphosphate or Deoxy-7-deazoguanidine triphosphate was used to terminate chain polymerization reactions depending on the degree of G+C content of a particular region of DNA being sequenced. The primer extended oligonucleotides were electrophoresed on denaturing 6% polyacrylamide and the DNA sequences of adjacent DNA fragments were pieced together to construct a complete DNA sequence of the cholesterol oxidase gene. The sequence is shown in FIG. 2. 
     Subcloning of the Rhodococcus cholesterol oxidase gene in pSL81Δ15 can result in an increase in expression of the cholesterol oxidase protein. Recombinant S. lividans cells were grown on RZYE agar containing 50 μg/ml thiostrepton. After 2-3 days incubation at 30° C., the mycelia and spores were scraped from the surface of the agar and used to inoculate 50 ml YEME+12.5 μg/ml thiostrepton seed cultures. These cultures were grown at 30° C. with vigorous shaking and after 48 h, 2 l fermenters containing YEME+12.5 μg/ml thiostrepton were inoculated with the seed. The fermentations were controlled at 28° C. and pH 7.0 with aeration at 0.5 l/min and agitation at 1000 rpm. Cholesterol oxidase activity peaked after 48 h and in some such cultures expression was substantially greater in the S. lividans/pSL81Δ15 fermentations than in the S. lividans/pSL81 fermentations. 
     Protein Characterization 
     Recombinant cholesterol oxidase was isolated from 3 day old cultures of S. lividans/pSL81 cells grown in YEME (25 ml) containing thiostrepton (50 μg/ml). Non-recombinant cholesterol oxidase was isolated from 48 hour Rhodococcus 10554 cultures grown in liquid production medium which was the same medium as that used for the screening plates not solidified with agar. Induction experiments were performed using the same growth conditions in the presence of sterols as an inducer. Intracellular and extracellular fractions were recovered by centrifugation and assayed for cholesterol oxidase activity using a cholesterol oxidase specific diagnostic assay. The specificity of the recombinant cholesterol oxidase from S. lividans and the purified cholesterol oxidase from Rhodococcus 10554 were compared using various sterols substrates. Both enzymes showed similar profiles with the order of activity being cholesterol, pregnenolone, dihydroxycholesterol, stigmasterol and ergosterol. 
     Recombinant cholesterol oxidase from S. lividans/pSL81 was functional in standard diagnostic assays for the determination of cholesterol. An aliquot of cell-free supernatant from a 3 day old culture of YEME grown recombinant cells was mixed with the cholesterol oxidase assay reagent (1.4 ml of 0.33 mg/ml 4-aminophenazone in 0.1M phosphate buffer, pH 7.0, 0.05% Triton X-100 (w/v); 1.4 ml of 1 mg/ml phenol in phosphate-Triton X buffer; 0.1 ml of 4 mM cholesterol; and 0.01 ml of 10 mg/ml peroxidase) and incubated at 30° C. Hydrogen peroxide formed during the oxidation of cholesterol and chelated with the aminoantipyrine in the assay mixture was determined by measuring the absorption of the red complex at 505 nm versus a reagent blank containing water. The rate of cholesterol degradation by the recombinant enzyme was also measured directly by following the increase in absorbance at 240 nm owing to Δ 4  -cholestenone production. To 2.9 ml of sodium phosphate buffer (0.1M, pH 7.0) in a cuvette, 100 μl of 3% Triton X-100 (w/v) and 50 μl of a 6 mM cholesterol solution were added. Diluted enzyme at approximately 0.2 U/ml in buffer was added and the absorbance at 240 nm monitored using a Beckman spectrophotometer. Cholesterol oxidase activity was calculated as: ##EQU1## where RV=reaction volume 
     DF=dilution factor 
     SV=sample volume 
     PC=protein concentration 
     Further characterization of the recombinant cholesterol oxidase from S. lividans/pSL81 demonstrated that the enzyme could also be used to determine cholesterol in sera using the peroxidase coupled reaction. 
     Recombinant cholesterol oxidase from S. livldans/pSL81 and purified Rhodococcus 10554 cholesterol oxidase were analyzed by polyacrylamide gel electrophoresis using a 10% polyacrylamide gel and the buffer system of Laemelli, 1970, Nature, Vol. 227, pp. 680 et seq. Both enzymes were determined to be approximately 55 kD in size when compared with Biorad molecular weight standards. 
     Peptide Sequencing 
     Recombinant cholesterol oxidase from S. lividans/pSL8115 and non-recombinant cholesterol oxidase from R. rhodococcus were isolated as single bands from a polyacrylamide gel, and the amino-terminal peptide sequences were determined using a protein sequenator. Two peptide sequences were recovered from the recombinant protein band: 
     NH 2  -Gly-Gly-Pro-Val-Ser-Thr-Leu-Thr-Pro-Pro-Pro-Ala-Phe-, and 
     NH 2  -Gly-Pro-Val-Ser-Thr-Leu-Thr-Pro-Pro-Pro-Ala-Phe-. 
     These peptide sequences are identical to those deduced directly from the DNA sequence between positions 280 and 320, and between positions 283 and 320, respectively. The elimination of a glycine residue at the N-terminus of one recombinant sequence is probably owing to a difference in post-translational processing by the host cell. 
     Amino terminal sequencing of cholesterol oxidase from R. rhodococcus identified the sequence NH 2  -Thr-Pro-Pro-Pro-Ala-Phe-Pro-Glu-Gly-Ile-Ala-Leu-Tyr-Gln-Gln-, which corresponds to the peptide sequence deduced from the DNA sequence between positions 300 and 346. Therefore, the enzyme found in the S. lividans/pSL81Δ15 culture medium is either 6 or 7 amino acids longer that the membrane bound enzyme found in R. rhodococcus. 
     Processing of gram-positive secreted proteins often involves signal sequence cleavage followed by hydrolysis at a secondary processing site. In the case of native cholesterol oxidase this secondary site is at the threonine. Apparently such a secondary cleavage does not occur when the protein is synthesized and secreted extracellularly by S. lividans. 
     OTHER EMBODIMENTS 
     Other embodiments are within the following claims. For example, other species of any genus of the Nocardioforms including Rhodococcus NCIB 10554, R. erythropolis NCIB 9158, Nocardia erythropolis ATCC 17895 and Nocardia erythropolis ATCC 4277 could be used to prepare the genomic library; or the libraries could be screened for other useful metabolic activities such as the assimilation of alkanes and aromatic hydrocarbons; the degradation of lignin, detergents, and pesticides; the production of enzymes useful in xenobiotic transformation; the biosynthesis of antibiotics, amino acids and biosurfactants; and the production of other steroid modifying enzymes such as cholesterol esterase. 
     As other Streptomyces and also Bacillus species share the advantages of S. lividans for the expression of heterologous genes, they could also serve as suitable hosts for the expression of DNA sequences derived from Nocardioform microorganisms. Other bacteria belonging to the high G+C subdivision of the gram-positive bacteria including any of the Nocardioforms, Mycobacterium, Corynebacterium, and Athrobacter would be suitable hosts for cloning DNA sequences derived from Nocardioform microorganisms. 
     S. lividans/pSL81 can undergo rearrangement in one or more regions outside the cholesterol oxidase coding region and can retain cholesterol oxidase transforming activity. One such S. lividans/pSL81 rearranged plasmid, has at least 3.6 kb of the 6 kb Rhodococcus DNA unchanged as determined by restriction mapping. In this rearranged S. lividans/pSL81 plasmid a DNA insertion of approximately 2.5 kb has occurred in the region to the right of the PvuII site. Cholesterol oxidase was recoverable from cultures of S. lividans transformed with this rearranged plasmid, in lower quantities than from S. lividans/pSL81 cultures. 
     In still other embodiments expression of cholesterol oxidase may be increased by further manipulating the pSL81Δ15 clone, as will be apparent to one skilled in the art. For example, a higher copy number vector could be used, such as, for example, pIJ303, described in Hopwood et al., 1985, Genetic Manipulation of Streptomyces: A Laboratory Manual. Gene expression could also be enhanced by replacing the cholesterol oxidase promoter with a stronger exogenous promoters, such as ErmE promoter, described in Bibb et al., 1985, Gene, Vol. 38, pp. 215-226; aph promoter, described in Hopwood et al., 1985; or a tac promoter, described in Amman et al., 1984, Gene, Vol. 25, pp. 167 et seq. 
     DEPOSIT 
     Under the terms of the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purpose of Patent Procedure, deposit of plasmid pSL81 has been made with the American Type Culture Collection (ATCC) of Rockville, Md., USA, where the deposit was given Accession Number 67853, deposited Nov. 9, 1988. 
     Applicants&#39; assignee, Genzyme Corporation, represents that the ATCC is a depository affording permanence of the deposit and ready accessibility thereto by the public if a patent is granted. All restrictions on the availability to the public of the material so deposited will be irrevocably removed upon the granting of a patent. The material will be available during the pendency of the patent application to one determined by the Commissioner to be entitled thereto under 37 CFR 1.14 and 35 U.S.C. § 122. The deposited material will be maintained with all the care necessary to keep it viable and uncontaminated for a period of at least five years after the most recent request for the furnishing of a sample of the deposited plasmid, and in any case, for a period of at least thirty (30) years after the date of deposit or for the enforceable life of the patent, whichever period is longer. Applicants&#39; assignee acknowledges its duty to replace the deposit should the depository be unable to furnish a sample when requested due to the condition of the deposit.