Patent Grant
8222434
This invention relates to the field of antibiotic compound and compositions and synthesis thereof.
Bactobolin analogs (e.g. bactobolin, bactobolin B and bactobolin C) are known antibiotics.1 However, due to their complex chemical structures, it is difficult to produce bactobolin analogs in high volume. It is also challenging to synthesize novel analogs of bactobolin for more desirable properties. Therefore, a need exists to prepare novel bactobolin analogs and to produce bactobolin analogs in greater amount.
One aspect of the invention relates to a novel bactobolin analog bactobolin D.
Another aspect of the invention relates to a method of preparing a composition comprising a bactobolin analog (e.g. bactobolin, bactobolin B, bactobolin C and bactobolin D) using a bacteria strain comprising a bacterial cell comprising the biosynthetic locus of the bactobolin analog in Burkholderia thailandensis (bta cluster) or a homolog structure (at least 95% sequence identity) thereof. In certain embodiments, the bacterial strain is a wild type bacterial strain such as a Burkholderia strain comprising a bta cluster (e.g. Burkholderia thailandensis (e.g. E264, Bt4, and TXDOH) and Burkholderia pseudomallei (e.g. K96243, 1106a, and 1106b). In certain embodiments, the bacterial strain comprises a genetically-engineered bacterial cell comprising an inserted bta cluster.
In certain embodiments, the bactobolin analog(s) is (are) further isolated and purified.
Another aspect of the invention relates to a method of preparing a bactobolin analog by mutating the bta cluster in Burkholderia thailandensis.
Another aspect of the invention relates to a method of preparing a bactobolin analog using a bacteria strain comprising a bacterial cell comprising a mutant bta cluster or a homolog structure thereof.
One aspect of the invention relates to bactobolin D having a chemical structure of the following Structure I:
Another aspect of the invention relates to a method of preparing a composition comprising a bactobolin analog in high volume using a Burkholderia strain that comprises the biosynthetic locus of the bactobolin analog in Burkholderia thailandensis (bta cluster) or a homolog structure (at least 95% sequence identity) thereof, which comprises the following steps 1-3:
As used herein, the term “bactobolin analog” means a compound comprising a chemical structure of Structure I, wherein:
R1 is selected from the group consisting of H and OH; and
R2 is selected from the group consisting of H and L-Ala.
Examples of bactobolin analog include, without limitation, bactobolin (Structure I, wherein R1 is OH and R2 is H), bactobolin B (Structure I, wherein R1 is OH and R2 is L-Ala), bactobolin C (Structure I, wherein R1 and R2 are both H) and bactobolin D.
Examples of bacterial strains comprising a bta cluster include, without limitation, Burkholderia thailandensis (B. thailandensis, e.g. B. thailandensis E264, B. thailandensis Bt4, and B. thailandensis TXDOH) and Burkholderia pseudomallei (B. pseudomallei, e.g. B. pseudomallei K96243, B. pseudomallei 1106a, and B. pseudomallei 1106b).
In certain embodiments, a stationary phase culture of B. thailandensis was prepared by growing the bacteria in Luria-Bertani (LB) broth containing MOPS (50 mM, pH 7.0) at 37° C. until B. thailandensis reached late stationary phase (OD600 9-10).
Another aspect of the invention relates to a method of preparing a composition comprising a bactobolin analog using a bacteria strain comprising a bacterial cell comprising bta cluster or a homolog structure thereof, which comprises the following steps 1-4:
In certain embodiments, the bactobolin analog is selected from the group consisting of bactobolin, bactobolin B, bactobolin C, and bactobolin D.
Examples of bacterial cell that can be used in the method of the present disclosure include, without limitation, E. coli.
Another aspect of the invention relates to a method of preparing a bactobolin analog in high volume using a Burkholderia strain that comprises bta cluster or a homolog structure (at least 95% sequence identity) thereof, which comprises the following steps 1-5:
In certain embodiments, the bactobolin analog is selected from the group consisting of bactobolin, bactobolin B, bactobolin C, and bactobolin D.
In certain embodiments, the bacterial strain is a B. thailandensis strain (e.g. B. thailandensis E264, B. thailandensis Bt4, and B. thailandensis TXDOH) or a B. pseudomallei strain (e.g. B. pseudomallei K96243, B. pseudomallei 1106a, and B. pseudomallei 1106b).
Another aspect of the invention relates to a method of preparing a bactobolin analog using a bacteria strain comprising a bacterial cell comprising bta cluster or a homolog thereof, which comprises the following steps 1-4:
In certain embodiments, the bactobolin analog is selected from the group consisting of bactobolin, bactobolin B, bactobolin C, and bactobolin D.
Another aspect of the invention relates to a method of preparing a bactobolin analog by mutating the bta cluster in a Burkholderia strain, which comprises the following steps 1-5:
In certain embodiments, the bactobolin analog is selected from the group consisting of bactobolin, bactobolin B, bactobolin C, and bactobolin D.
In certain embodiments, a method comprises:
providing a mutant bta cluster;
inserting the mutant bta cluster into a bacterial cell;
providing a stationary phase culture comprising the bacterial cell; removing the cells from the cell culture; and
filtering the culture fluid to obtain a composition comprising at least one bactobolin analog.
Another aspect of the invention relates to a method of preparing a bactobolin analog by feeding a Burkholderia strain comprising the bta cluster alternative natural or unnatural amino acids, which comprises the following steps:
In certain embodiments, the bacterial strain is a B. thailandensis strain (e.g. B. thailandensis E264, B. thailandensis Bt4, and B. thailandensis TXDOH) or a B. pseudomallei strain (e.g. B. pseudomallei K96243, B. pseudomallei 1106a, and B. pseudomallei 1106b).
As used herein, the term “bma genes” means luxR and luxI homologs in Burkholderia mallei (B. mallei). The term “bps genes” means luxR and luxI homologs in Burkholderia pseudomallei (B. pseudomallei). The term “bta genes” means luxR and luxI homologs in Burkholderia thailandensis (B. thailandensis). The term “bta cluster” means a DNA sequence comprising the genes that are required for the biosynthetic locus of the composition comprising at least one bactobolin analog of B. thailandensis. In certain embodiments, a bta cluster comprises genes selected from the group consisting of btaK, btaN, btaO, btaM, btaL, btaI2, btaR2, BtaD, BtaC, BtaE, BtaA, BtaU and BtaH, and any combination thereof.
As used herein, btaK gene refers to 1233, btaT refers to 1241, btaL refers to 1234, btaP refers to 1238, btaB refers to 1223, and btaU refers to 1242 respectively on chromosome II
The following examples are provided to better illustrate the claimed invention and are not to be interpreted in any way as limiting the scope of the invention. All specific compositions, materials, and methods described below, in whole or in part, fall within the scope of the invention. These specific compositions, materials, and methods are not intended to limit the invention, but merely to illustrate specific embodiments falling within the scope of the invention. One skilled in the art may develop equivalent compositions, materials, and methods without the exercise of inventive capacity and without departing from the scope of the invention. It will be understood that many variations can be made in the procedures herein described while still remaining within the bounds of the invention. It is the intention of the inventors that such variations are included within the scope of the invention.
A) Bacterial Culture Conditions.
The bacterial strains used in this study are listed in Table 1-A.
Unless otherwise indicated all bacteria except S. pyogenes were grown in Luria-Bertani (LB) broth. S. pyogenes was grown on Todd Hewitt broth supplemented with 0.2% (wt/vol) yeast extract. The following antibiotics were used at the indicated concentrations for marker selection and for maintaining plasmids (per ml): 100 pg ampicillin, 15 pg gentamicin, 300 pg trimethoprim and 25 pg kanamycin for E. coli; 100 pg trimethoprim and 150 pg kanamycin for B. thailandensis. To induce arabinose promoter-controlled gene expression in E. coli, we used 0:2% (wt/vol) L-arabinose. We used 0.2% (wt/vol) L-rhamnose to activate the rhamnose responsive promoter in B. thailandensis. All bacteria were grown at 37° C. except P. fluorescens which was grown at 30° C.
B) Plasmids and Recombinant DNA Procedures.
The plasmids used in this study are listed in Table 1-B:
Oligonucleotides were purchased from Integrated DNA technologies (Coralville, Iowa). B. thailandensis genomic DNA was purified by using a Gentra Puregene Cell Plus isolation kit (Qiagen, Valencia, Calif.). PCR fragments were amplified with the Failsafe™ PCR System and buffer J or K premix solution (Epicentre Biotechnologies, Madison, Wis.) or with an Expand Long Template kit (Roche Diagnostics, Pleasanton, Calif.). B. thailandensis genomic DNA was used as the template for all PCR reactions. Plasmid DNA was isolated by using a Qiaprep mini-spin kit and PCR fragments used for cloning were purified with a Qiaquick PCR gel extraction kit (Qiagen).
To create the BtaR2 expression vector pJNR2, the open reading frame of btaR2 (BTH-111231) from bp −18 to +705 relative to the predicted translational start site was PCR amplified. This procedure introduced PsfI and SacI restriction sites at the ends of the btaR2 fragment. The btaR2 PCR product was ligated to PsfI-SacI-digested pJNIO5, which carries the L-arabinose inducible promoter (PBAD).3 To generate the lacZ transcriptional fusion constructs pI2P50 and pQF1233, B. thailandensis DNA upstream of btaI2 (BTH_II1227) and BTH_II1233 was PCR amplified by using primer pairs that incorporated NcoI and HindIII restriction sites at the ends of the PCR products. These products were cloned into NcoI-HindIII-digested pQF50.4 The btaI2::lacZ fusion extends from +6 to −238 in relation to the predicted translational start site. BTH_II1233::lacZ extends from +13 to −409 in reference to the predicted translational start site.
To regulate expression of BtaR2 in B. thailandensis it was necessary to place btaR2 under the control of an inducible promoter other than the L-arabinose promoter. This is because B. thailandensis is known to metabolize L-arabinose.5 We chose the rhamnose inducible promoter because of its utility in other Burkholderia species.6 We constructed the BtaR2 expression vector pSCR2 by incorporating a PCR-amplified btaR2-containing DNA fragment into NcoI-HindIII-digested pSCrhaB2.6 above
The suicide vector pJRC115 was used to deliver modified genes into the B. thailandensis genome by allelic replacement. The method used for mutant construction in B. thailandensis is similar to that described recently.7 To create pLARA1233, overlap extension PCR was used to generate DNA fragments of approximately 1-kb that were homologous to sequences flanking the intended site of deletion in BTH_II1233. These products were mixed and a second step PCR was done to anneal and amplify the fragments. Amplification created an internal XbaI in the product. The resulting PCR product was digested with XmaI and HindIII and ligated to XmaI-HindIII digested pJRC115, to yield pLARA1233. pLARA1233 was digested with XbaI and ligated to a 1.2-kb DNA fragment from XbaI digested pGEM-T Easy; amrRAB-oprA::FRT-Krn, which contains a kanamycin resistance cassette, creating the marked suicide vector pKM1233. pINF1233 was made by ligating a 2-kb DNA fragment generated by overlap extension PCR and this product was digested and ligated to XbaI-HindIII digested pJRC115. To construct the BTH_II1233 kanamycin-insertion mutant strain BD909 and the unmarked BTH_II1233 deletion strain BD20, pKM1233 and pINF1233 were mobilized from E. coli SM10 into B. thailandensis. Trimethoprim-resistant transconjugants were identified and then counter-selected by a method similar to that described elsewhere.7 PCR followed by DNA sequencing was used to confirm the mutations. The nucleic acid sequence coordinates of insertion and replacement during the construction of BD909 and BD20 can be found in Table 1-A.
C) LC/MS/MS Identification of acyl-HSLs.
Acyl-HSLs were isolated from 10-ml cultures (OD600 of 4.0) of wild-type B. thailandensis and the indicated mutant strains. Cells were removed from the culture fluid by centrifugation and the culture fluid was sterilized by using a 0.2 μM filter (Millipore, Billerica, Mass.). For LC/MS/MS, 0.4 nmol of deuterated C6-HSL (D3-C6-HSL) and deuterated C12-HSL (D3-C12-HSL) were added to the filtered culture fluid, which was then extracted with two equal volumes of acidified ethyl acetate (0.1 ml/L glacial acetic acid) and dried to completion under a constant stream of nitrogen gas. This was followed by a solid phase extraction, suspension of the material in 50 μL of 5% MeOH. Ten μL of each sample was analyzed by LC/MS/MS as described elsewhere.8 The relative abundance of acyl-HSLs in each sample was determined by integrating the area under the analyte peaks and comparing these values to those of D3-C6-HSL and D3-C12-HSL standard curves.8 Relative abundance is a measurement of the ratio of the areas of the analyte and internal standard peaks to the ratio of the amount of analyte and internal standard in each sample.
D) Acyl-HSL Bioassays.
To measure the BtaR2 response to acyl-HSLs we used recombinant E. coli with a btaI2-lacZ fusion or a BTH_II1233-lacZ fusion. Bioassays were performed as described elsewhere.9 Briefly, an overnight culture of E. coli MG410 containing pJNR2 and pI2P50 was used to inoculate fresh LB broth containing arabinose (starting OD600 0.05). When the OD600 reached 0.7, 0.5 ml of culture was added to tubes containing dried C8-HSL, C10-HSL, C12-HSL, 3OHC6-HSL, 3OHC8-HSL, or 3OHC10-HSL as indicated. After 3 h with shaking at 37° C., 50 μL of chloroform was added to each tube and β-galactosidase activity was measured using a Tropix Galacto-Light Plus kit according to the manufacturer's recommendations (Applied Biosystems, Foster City, Calif.). E. coli MG4 carrying pJNR2 and pQF1233 was used to measure the regulation of the BTH_II1233 promoter by BtaR2. The protocol was identical to that described for E. coli MG4 containing pJNR2 and pI2P50 except that only 3OHC8-HSL and 3OHC10-HSL were tested as signals.
E) Real-Time PCR.
Wild-type B. thailandensis and the btaR2 mutant JBT108 were grown in 25 ml of LB broth supplemented with 2% glycerol and 25 mM MOPS pH 7.0. At an OD600 of 2.7, 4 ml of culture was treated with RNA protect (Qiagen) and after centrifugation the resulting cell pellet was suspended in 200 μL of Tris-EDTA (10 mM Tris base and 1 mM EDTA, pH 8) containing 10 mg/ml of lysozyme. Total RNA was purified with an RNeasy spin purification kit (Qiagen). For quantitative real-time PCR (qRT-PCR), cDNA was generated from 2 μg of RNA by using al TaqMan kit (Applied Biosystems). Primers were designed to amplify 100-200-bp targets for use in qRT-PCR reactions. The qRT-PCR reactions used 2× SYBR green master mix (Applied Biosystems) with 40 cycles of 15 s at 95° C. followed by 60 s at 62° C. The qRT-PCR program ended with a dissociation curve that was used to verify that a single product was amplified in each reaction and that primer dimers did not form. Ct values were obtained with a manual threshold setting of 0.2. Values were generated by the calculation 2(35-C)t. Results are reported as the calculated transcript amount of a given gene per calculated rpoB transcript. Reported values show the averages and range of biological replicates assayed in triplicate.
F) Antimicrobial Susceptibility Testing.
To assess antimicrobial activity in B. thailandensis culture fluid we routinely used a diffusion disc assay.11 B. thailandensis cultures were grown in LB broth containing MOPS (50 mM, pH 7.0) at 37° C. Colonies grown overnight on LB-agar plates were used as the starting inoculum. Unless otherwise indicated, when B. thailandensis cultures reached late stationary phase (OD600 9-10) the cells were removed by centrifugation and the culture fluid was filtered thorough a 0.45 μm membrane. Antibiotic assay discs (13 mm, Whatman, Florham Park, N.J.) were saturated with sterile culture fluid and deposited onto LB agar plates overlayed with 100 μl of a 1:10 dilution of an overnight culture of B. subtilis 16812 or other bacterial species as indicated. The plates were incubated at 37° C. overnight.
To measure antimicrobial activity more precisely and to assess whether the activity was bacteriocidal or bacteriostatic we used the following assay: Fluid from 24 h B. thailandensis cultures grown in LB containing 50 mM MOPS (pH 7.0) at 37° C. was collected and filter-sterilized (0.45 μm-pore-size membrane). Early log phase B. subtilis 361013 cells were inoculated into the filtered B. thailandensis culture fluid diluted with fresh LB-MOPS broth (initial B. subtilis density was approximately 1×106˜cells per ml). After 24 h with shaking at 37° C. B. subtilis cell density was determined by plate counting on LB agar.
A) The btaI2 Gene Product Catalyzes the Synthesis of 3OHC8-HSL and 3OHC10-HSL.
Acyl-HSL detection often involves bioassays or bioassays coupled to thin-layer-chromatographic separation of acyl-HSLs.14,15 These methods rely on a specific LuxR homolog for acyl-HSL detection and they are biased for specific acyl-HSLs. It is cumbersome to conduct a comprehensive, quantitative analysis of acyl-HSLs using bioassays. Thus, we employed liquid chromatography-electrospray ionization-tandem mass spectrometry (LC/MS/MS), which detects acyl-HSLs in a complex mixture with high sensitivity irrespective of acyl side chain length or substitution and directly measures the relative abundance of acyl-HSLs,8 to analyze acyl-HSLs produced by B. thailandensis. We analyzed acyl-HSLs in ethyl acetate extracts of stationary phase (OD600 4) culture fluid from wild-type B. thailandensis and quorum sensing mutants lacking btaI2 (JBT102) or both btaI1 and btaI3 (JBT105). Wild-type B. thailandensis produced C8-HSL, 3OHC8-HSL, 3OHC10-HSL, and a small amount of N-3-hydroxy-dodecanoyl-HSL (3OHC12-HSL) (Table 2-A).
aValues are the average percentage of acy-HSLs from two independent cultures and the range is indicated in parentheses.
bNone detected.
The btaI2 mutant produced C8-HSL and 3OHC8-HSL but it did not produce detectable levels of 3OHC10-HSL (Table 2-A). This analysis suggests that BtaI2 is responsible for synthesis of 3OHC10-HSL. The btaI1-btaI3 double mutant produced both 3OHC8-HSL and 3OHC10-HSL. Taken together the data indicate that in B. thailandensis BtaI2 produced 3OHC8-HSL and 3OHC10-HSL. C8-HSL appears to be produced by either BtaI1 or BtaI3. BtaI1, BtaI3 or both are also capable of producing significant amounts of 3OHC8-HSL. Consistent with this interpretation the closely related B. mallei and B. pseudomallei|1 synthases produce C8-HSL,9,16 and the B. mallei BmaI3 synthesizes 3OHC8-HSL.17
B) BtaR2 Regulates btaI2 Transcription in Response to 3OHC8-HSL or 3OHC10-HSL.
It is common that luxI homologs are positively autoregulated by the acyl-HSL signal(s) produced by the proteins they encode and the transcription factor encoded by the adjacent luxR homolog,10 The btaR2 and btaI2 gene organization is unusual in that there is approximately 3.3-kb of DNA between these two genes and this intervening DNA contains three open reading frames. We hypothesized that btaR2 and btaI2 may represent a cognate pair of quorum sensing genes and that btaI2 transcription would be activated by BtaR2 in response to 3OHC8-HSL, 3OHC10-HSL, or both signals. To test our hypothesis we created a plasmid with a 244-bp DNA fragment containing the putative promoter region of btaI2 (from −238 to +6 of the predicted translational start site) fused to a promoterless lacZ. This plasmid was introduced into E. coli with or without an arabinose-inducible btaR2. Activation of btaI2-lacZ transcription required BtaR2 and either 3OHC8-HSL or 3OHC10-HSL (
To verify that btaI2 transcription is regulated by BtaR2 in B. thailandensis we measured the abundance of btaI2 mRNA in wild type and the btaR2 mutant strain JBTI08 by using quantitative real-time PCR (qRT-PCR). The btaI2 gene is the first of five genes in a putative operon residing on Chromosome II. The qRT-PCR analysis showed that transcript levels of btaI2 and a downstream gene (1224) were substantially higher in the wild type than they were in the btaR2 mutant (
C) The btaR2 and btaI2 Genes Reside Among a Cluster of Putative Antimicrobial Biosynthetic Genes Conserved in B. Pseudomallei but Absent in B. Mallei.
Three of the genes in the putative btaI2 operon are predicted to code for enzymes involved in the production of secondary metabolites (
The btaI2 and btaR2 genes reside amid a large cluster of genes, many of which we suspect are involved in production of secondary metabolites (
D) B. Thailandensis Produces an Antibiotic.
Because btaI2 and btaR2 are embedded in a chromosomal region with many genes annotated as coding for functions known to be involved in antibiotic synthesis, transport and immunity by other bacteria, we hypothesized that B. thailandensis produces an antibiotic and that antibiotic production may require BtaR2 and an acyl-HSL produced by BtaI2. To test the hypothesis we first screened fluid taken from B. thailandensis cultures at various cell densities (
We tested the specificity of the antibacterial component from stationary phase culture fluid (OD600 9-10) by using the diffusion disc assay to assess whether the component inhibited growth of other bacterial species. Growth of all of the Gram-positive bacteria tested was inhibited (S. auerus including MRSA, Listeria monocyfogenes, and Streptococcus pyogenes). Neither of the two Gram-negative bacteria we tested, E. coli and Pseudomonas fluorescens, showed growth inhibition by B. thailandensis culture fluid.
E) The btaR2-btaI2 Quorum Sensing System is Required for Antibiotic Production.
To determine whether stationary-phase antibiotic production required the btaR2-btaI2 quorum-sensing system we first asked whether synthesis of the factor required acyl-HSL signaling as follows: We tested cell-free stationary phase culture fluid of a B. thailandensis strain with mutations in all three of the acyl-HSL synthase genes, btaI1, I2 and I3 for anti-B. subtilis activity as described above (
To ask whether the acyl-HSL-dependent production of antibacterial activity required BtaR2 we tested the ability of the btaR2 mutant, JBT108, to produce an antibacterial factor. As shown in
F) A Biosynthetic Locus (bta Cluster) for Quorum Sensing Regulated Antibiotic Production in B. Thailandensis Includes Genes 1233-1241.
As discussed in a previous section the btaI2 gene cluster and a cluster of genes located upstream of btaR2 encode proteins annotated as NRPS and PKS enzymes, which we predict are responsible for the production of the BtaR2—BtaI2 controlled antibiotic. The cluster of genes distal to btaR2 (1233-1241) is predicted to constitute an operon. We first asked whether the promoter upstream of the first gene in the cluster, 1233, was quorum-sensing controlled by constructing a transcriptional fusion of −409 to +I3 (with respect to the predicted translational start site) of the putative 1233 promoter to lacZ. We introduced a 1233-promoter-lacZ fusion plasmid into E. coli containing a BtaR2 expression vector and tested whether either 3OHC8-HSL or 3OHC10-HSL activated the lacZ reporter (
To obtain further data about quorum sensing control of the 1233-1241 genes we compared 1233, 1236 and 1239 transcript levels in the wild type and the btaR2 JBT108 mutant by qRT-PCR. The wild type showed about 10-fold more transcript for these genes compared to the mutant (
To determine whether gene 1233 itself is required for antibiotic production we constructed a 1233 insertion mutant strain and a 1233 in-frame deletion strain of B. thailandensis and tested stationary phase culture fluid from both of these 1233 mutants for antibiotic activity. Neither strain made detectable levels of the antibiotic (
A) Isolation, Purification and Characterization of Bactobolin Analogs from Antibiotics Produced by B. Thailandensis.
To isolate the antibiotic, B. thailandensis was cultivated as disclosed supra. The antibiotic's highly polar, amphoteric nature and its optimal production in rich media (LB) necessitated unusual chromatographic steps. Using activity-guided fractionation against B. subtilis 3610, four active compounds were purified, and their structures elucidated by 1-D/2-D NMR (Tables 3-A1-3-A4), HR-MS and MS/MS (Table 3-A5) methods.
General Procedures.
1H and 2-D NMR spectra for bactobolins were recorded in the inversedetection probe of a Varian Inova spectrometer (600 MHz for 1 H, 150 MHz for 13C). 13C NMR spectra were recorded on the same instrument with a broad-band probe. Optical rotations were measured on an Autopol IV Automatic Polarimeter (Rudolph Research Analytical). HPLC purifications were carried out on an Agilent 1100 or 1200 Series HPLC system equipped with a photo diode array detector. LC-MS analysis was carried out on an Agilent 1200 Series HPLC system equipped with a diode array detector and a 6130 Series ESI mass spectrometer using an analytical Thermo Scientific Hypercarb column (5 μm, 4.6×100 mm) operating at 0.7 mL/min with a gradient of 10% MeCN in H2O to 100% MeCN over 25 min. High resolution mass spectrometry (HR-MS) and tandem MS (MS/MS) were performed at the University of Illinois Urbana-Champaign Mass. Spectrometry Facility. Activity assays for activity-guided fractionation were performed using the disc diffusion assay on LB-Agar plates and overnight cultures of B. subtilis 3610 or B. subtilis 168 in LB as the test organism.
Cultivation of B. Thailandensis.
B. thailandensis E264 was obtained from DSMZ. All cultivation was carried out on a shaker/incubator at 30° C. and 250 rpm, unless otherwise noted. A small 5 mL culture of B. thailandensis in LB was grown to saturation overnight. A 500 mL Erlenmeyer flask containing 125 mL of LB was inoculated with 1 mL of the overnight culture, and subsequently grown to saturation overnight. Each of three 4 L Erlenmeyer flasks containing LB+50 mM Mops, pH 7.0, were inoculated with 15 mL of the overnight culture and grown for three days at 30° C. and 225 rpm. The cells were pelleted by centrifugation (8000 g, 12 min, 4° C.) and bactobolins purified as described below.
Purification of Bactobolins.
1.5 L of the cell-free B. thailandensis supernatant from above was loaded directly onto an HP-20 column (˜120 mL, d=2.5 cm, I=25 cm), which had been equilibrated in water, at ˜5 mL/min. bactobolin eluted in the flow-through, which was loaded directly onto a DEAE column (˜80 mL, d=2.5 cm, I=17 cm), which had been equilibrated in water, at ˜5 mL/min. bactobolin eluted in the flow-through. These initial columns do not bind bactobolin, but provide a crude separation of bactobolin from non-polar and small negatively charged molecules. The flow-through was concentrated 10-fold in vacuo and stored at 4° C. overnight. A precipitate that forms after these steps was removed by centrifugation and the supernatant, which contains bactobolin, loaded onto a Hypercarb column (Thermo Scientific, ˜25 mL, d=1.25 cm, I=20 cm), which had been equilibrated in 20% MeCN in water. The column was washed with 5-7 column volumes (CV) of 20% MeCN, then 10 CV of 50% MeCN in water, and finally with 8-10 CV of 100% MeCN; bactobolin eluted in the 50% MeCN fraction, which was concentrated ˜12-fold in vacuo and then loaded with successive 2 mL injections onto an Agilent Eclipse C8 column (5 μm, 9.4×250 mm). bactobolin eluted at 25-30% MeCN in H2O. Fractions containing bactobolin were pooled, concentrated and separated to individual congeners on a Supelco Discovery HS C18 column (10 μm, 10×250 mm) with an isocratic gradient of 12% MeCN (+0.1% formic acid) in H2O (+0.1% formic acid) over 25 minutes, followed by a gradient from 12 to 45% of MeCN (+0.1% formic acid) over 15 minutes. This was repeated to obtain each analog in >95% purity (as judged by NMR). bactobolins A, B, C, and D eluted at 11, 14, 20, and 29 minutes with a yield of 9, 8, 14 and 1.5 mg per 1.5 L, respectively.
Characterization of Bactobolin Analogs Using NMR Spectra.
NMR data for bactobolin A are summarized in Tables 3-A1:
NMR data for bactobolin B are summarized in Tables 3-A2:
NMR data for bactobolin C are summarized in Tables 3-A3:
NMR data for bactobolin D are summarized in Tables 3-A4:
The stereochemistry of bactobolin A can be demonstrated using the data summarized supra. The large coupling between H5 and H6 (9.5-9.8 Hz) suggests an axial-axial relationship between these protons. H10 couples to H5 with a similar coupling constant resulting in a triplet H5 signal and indicating an axial-axial interaction between H5-10. H4 appears as a doublet with a 3.3 Hz coupling to H10 indicating an equatorial-axial interaction between H4-H10. H10 is also involved in homoallylic coupling interaction with H7ax with a constant of 2.4 Hz consistent with the axial position of H10. This orientation is consistent with the ROESY interactions (
The stereochemistry is based on [α]D comparisons (Table 3-A5):
aHR − MS and MS/MS data were acquired under positive ionization mode.
bMS/MS spectra were acquired under positive ionization mode. The observed (M + H) of the dominant peak is indicated, and corresponds to bactobolin minus the Ala or Ala-Ala fragment.
cThe specific rotation data have been scaled to c = 1 for comparison with published values. Optical rotation for bactobolins A, B, C, and D were obtained with c = 0.9, c = 0.8, c = 1 and c = 0.15, respectively.
dThe reported [α]D data are from reference 1.
enr, not reported.
J coupling and ROESY spectral analyses which confirm the axial-axial relationship of protons H5/H10 and H5/H6, the axial-equatorial relationship of H10/H4, and the homoallylic coupling between H10 and H7ax.1,26 The stereochemistry of the Ala residue(s) was confirmed by acid hydrolysis followed by the following experiment:27
To determine the absolute configuration of the Ala residues in bactobolin, 0.1-0.7 mg of bactobolin A-D was dissolved in ˜1 mL of 6 N HCl in a 4 mL scintillation vial and heated in an oil bath at 115° C. for 1.5 h. The acid was removed in vacuo and the material re-dissolved ˜1 mL in water. This was repeated 2× to completely remove the acid. Each sample was then loaded onto a PrepSep C8 cartridge (Fisher Scientific) and Ala was eluted with 10% MeCN in H2O. The eluant was dried in vacuo and Marfey's analysis was carried out as previously described. Bactobolins B-D only contained L-Ala, bactobolin A contained ˜18% D-Ala which presumably arose from racemization during extended storage of bactobolin A under acidic conditions (0.2%) TFA).
B) Export of Bactobolin.
The btaK mutant and btaI1, I2, I3 mutants have been described (see Table 3-B). The btaT deletion mutant was generated by a method previously described. Overlap extension PCR was used to generate PCR products with ˜1 kb of DNA homologous to the DNA flanking the btaT gene. The products were annealed and amplified by PCR. This step introduced primer-encoded XbaI and HindIII sites during amplification. The digested PCR products were ligated to XbaI-HindIII-digested pJRC115 and this suicide vector was introduced into B. thailandensis E264 to deliver the btaT deletion allele. Correct clones were identified by PCR amplification of the ΔbtaT allele. These were further confirmed by sequencing PCR products generated with primers targeting chromosomal DNA flanking the recombination sites.
Burkholderia thailandensis E264 wt
Burkholderia thailandensis ΔbtaK (BD20)
Burkholderia thailandensis ΔbtaT (BD44)
Burkholderia thailandensis ΔbtaI1, I2, I3 (JBT112)
Staphylococcus aureus COL
Enterococcus faecalis V583
Vibrio parahaemolyticus LM5674
Bacillus subtilis 3610
Escherichia coli ATCC25922
C) Antibiotic Assays of Bactobolin Analogs.
To test the potency of bactobolins, antibiotic assays were carried out against the human pathogens MRSA, VRE and V. parahaemo-lyticus, and against B. subtilis and E. coli (Table 3-C).
The minimum inhibitory concentrations (MIC) of each bactobolin was determined according to the 2003 guidelines of the Clinical and Laboratory Standards Institute (CLSI, formerly NCCLS) using the microtiter MIC method. Inocula for each test organism were prepared by suspending a colony from an LB agar plate into Tryptic Soy Broth and growing for 3-5 hours at 35° C. with shaking, then adjusting the culture turbidity in PBS to the equivalent of a 1.0 McFarland Standard (3×108 CFU/ml). These cell suspensions were used as inocula for microtiter MIC assays. 2.5 μL inoculum, which corresponded to 1×106 cells, was added to a 200 μL well containing each bactobolin diluted in cation-adjusted Mueller-Hinton II broth (actual CFU/ml ranged from 5×104 to 1×106), and these were incubated without shaking for 24 hours at 35° C. The MIC was defined as the lowest concentration of bactobolin (μg/ml) in which bacterial growth in the well was not measurable by determining the turbidity at 595 nm (A595) on a 96-well plate reader. See Table S8 for strains used in these assays.
S. aureus COL (MRSA)
E. faecalis V583 (VRE)
V. parahaemolyticus LM5674
B. subtilis 3610
E. coli ATCC25922
[a]The MIC was determined according to the CLSI assay (see SI). The values are the average from two independent experiments.
[b]nd, not determined.
The results show that bactobolins A and B, which contain the hydroxyl group at C5 are more potent than bactobolins C and D. Bactobolin D is the weakest analog consistent with its role as a biosynthetic precursor and its low isolation yield. Bactobolins A and B are potent antibiotics against MRSA and V. parahaemolyticus with MICs <1 μg/mL. Bactobolin C is also is also effective against these strains, though to a lesser extent. All three analogs displayed low activity against VRE.
Bactobolins have several interesting structural features. As suggested by the bta cluster, they are NRPS-PKS hybrids containing a di or tripeptide fused to a C6-polyketide. The fusion occurs at the carboxyl group of 2-hydroxy-3,3-dichlorovaline (OH—Cl2-Val), an unnatural amino acid with unknown biosynthesis that is thus far limited to the bactobolins. The enol lactone likely mimics a carboxyl group allowing bactobolins to enter the cell via di- and tripeptide transporters.35 Bactobolins differ from actinobolin, an antibiotic identified from Streptomyces griseoviridis, in that the OH—Cl2-Val residue is replaced by Thr.26a This switch renders actinobolin 21-fold, 1000-fold and 900-fold less active than bactobolin in E. coli kill-assays, in vitro inhibition of translation, and murine antitumor activity, respectively.36 Bta-based bioinformatic searches for its relatives show that bta is confined to the Burkholderia genus (FIG. S7). This suggests that the biosynthesis of OH—Cl2-Val and its incorporation into the actinobolin framework evolved only in the Burkholderia lineage.
The identification of bactobolin and its link to the bta cluster allows us to propose a scheme for its biosynthesis. The modular 44 kb PKS/NRPS cluster is highly fragmented with product(s) inconsistent with the co-linearity rule (Scheme 1A). Based on short intergenic sequences and a single Shine-Dalgarno sequence, the translation of btaK-btaS appears to occur from one polycistronic mRNA. The cluster contains six distinct elements: genes involved in regulation (I2 and R2), metabolite/product transporters (F, G, T, and V), synthesis of OH—Cl2-Val (B, C, D, E, and H), Ala-Ala (K and N) and the C6 polyketide (L, M, O, P, and S), and tailoring reactions (A, Q and U) (Scheme 1A). Results of homology searches and other bioinformatic analyses (e.g. FASTA search results for proteins in the bta cluster summarized in Table 4-1) lead us to propose the model in Scheme 1B.
Burkholderia multivorans CGD1,
Burkholderia pseudomallei MSHR346,
Burkholderia pseudomallei,
Burkholderia pseudomallei 1106b,
Burkholderia pseudomallei,
Burkholderia pseudomallei S13,
Burkholderia pseudomallei 406e,
Burtholderia pseudomallei,
Burkholderia pseudomallei,
Burkholderia pseudomallei,
Burkholeria pseudomallei,
Burkholderia pseudomallei Pakistan 9,
Burkholderia pseudomallei 668
Burkholderia pseudomallei Pakistan 9,
Burkholderia pseudomallei,
Burkholderia pseudomallei 1106a,
Burkholderia pseudomallei 1106 a,
Burkholderia pseudomallei MSHR346
aResults are from FASTA searches conducted between Jul. 5, 2009 and Sep. 1, 2009.
bInterProScan searches indicate similarities to an ACP-like domain.
cThe NRPS Predictor software indicates valine specificity for this domain.
dInterProScan searches indicate similarities to members of the α/β hydrolase superfamily.
eInterProScan searches indicate a domain architecture of C-A-TC, corroborated by analysis of the individual domains, where the initial C domain appears to be inactive. NRPS prediction software indicates Gly specificity for the A domain.
fInterProScan searches suggest a domain architecture of KS-AT-SDR-ACP. No homology was found for a ~300 amino acid stretch preceding the SDR domain.
gInterProScan searches suggest a domain architecture of KS-KR-ACP, the presence of the AT domain is inferred from results of secondary structure prediction searches using Phyre software.
hInterProScan and NRPS prediction software searches indicate an A-T-C-T domain architecture and Ala specificity for the A domain.
iInterProScan searches indicate a KS-AT-ACP architecture.
Biosynthesis of OH—Cl2-Val entails both dichlorination and hydroxylation reactions. Dichlorination likely involves BtaC, a Fe chlorinase (57% identical to CytC3). BtaC contains an Ala residue, typical for Fe chlorinases, in place of the carboxylate group that usually occupies the facial triad and coordinates Fe in α-KG-dependent oxygenases.37 Hydroxylation of Val (or Cl2-Val) could be catalyzed by either BtaA or BtaU, both non-heme Fe-dependent dioxygenases, or even by BtaC as recent studies have delineated the requirements for Cl vs OH insertion by these enzymes.38 BtaC is flanked by two free-standing thiolation (T) domains, where BtaB lacks the typical GXDS(L/I) sequence motif and may be inactive.39 Thus, BtaD is likely the active T domain and may be primed with Val by BtaE, a Val-specific, stand-alone adenylation (A) domain. BtaH contains a GXCX(G/S) motif and likely acts as an CmaE-like transacylase,40 by shuttling the assembled OH—Cl2-Val from one T domain to another within the NRPS assembly line (Scheme 1, inset).
BtaK and BtaN are NRPSs with the domain architecture shown in Scheme 1B. The first condensation (C) domain in BtaK does not contain the conserved active site motif (HHX3DG) and is likely inactive.39 Its A domain bears higher homology to Gly-specific domains, but no Gly-bearing bactobolins have been detected experimentally. In contrast the A domain of BtaN has an Ala-specific sequence. After formation of Ala-Ala (1), the C-terminal T domain of BtaN likely accepts the activated OH—Cl2-Val, which the adjacent C domain may assemble into an Ala-Ala-(OH—Cl2-Val) tripeptide (2). This is subsequently condensed to an acetyl group originating from malonyl CoA catalyzed by the ketosynthase (KS) domain of BtaO (3), followed by another acetyl group addition and ketoreduction (KR) by BtaM to give 4. The final C2 unit may be added by BtaL yielding 5. BtaL contains a non-canonical KR domain that is similar to short chain dehydrogenases (SDR). There is a 300 amino acid region between the AT and SDR domains with no recognizable sequence homology. The ensuing plausible reactions that may yield bactobolin, aldol condensation between OH—Cl2-Val and the final malonyl-CoA unit and elimination of water to give 6, followed by reduction of the resulting α,β-unsaturated ketone generating the enol moiety (Scheme 1A) lack clear precedents at this time. These reactions may be performed by the SDR domain in BtaL and the adjoining 300 amino acid region (Secondary structure predictions show that this 300 amino acid region is SDR-like. Recent studies have suggested that a KR domain (SDR-like) involved in erythromycin biosynthesis stabilizes an enolate intermediate, which is required for enzymatic aldol reactions proposed herein.);41 in this case, BtaS would act as a thioesterase to generate the lactone. Alternatively, they could be carried out by BtaS, BtaP (a “-lactamase), and the SDR domain in BtaL in accordance with studies by Fujii et al. who have shown a Claisen-like condensation catalyzed by a thioesterase,42 and recent reports implicating a “-lactamase in a thioesterase reaction (Table 4-1).43 Mutational analyses and biochemical studies are in progress to distinguish between these options.
Finally, the cluster appears to provide an indication of how four analogs are biosynthesized by one cluster.44 The hydroxyl group at C5 may be inserted by BtaA or BtaU. BtaQ and BtaJ bear high homology to an acetyltransferase and an oligopeptidase, respectively. Indeed acetylated bactobolins have previously been reported1 N-acetyl peptides are good substrates for oligo-peptidases,45 which could cleave Nacetyl-bactobolin B or D, generated by BtaQ, to yield bactobolin A or C, respectively. Thus, a promiscuous hydroxylase and an oligopeptidase may generate three additional congeners from bactobolin D.
To begin to test the predictions above, mutational and biochemical analyses have been carried out to examine the export of bactobolin as a means of host resistance. We investigated a markerless quorum sensing defective mutant, which does not express btaK-btaS and thus does not produce bactobolin.30, 29 Addition of exogenous bactobolin showed that the mutant was resistant indicating that the resistance genes lie outside of this region and that their expression is not quorum-controlled (Tables 4-2 & 4-3) (Exogenous bactobolin was added in the form of filtered culture supernatants of wt B. thailandensis E264, which contained 40±18 μg/mL bactobolin as quantitated by LC-MS using purified standards.).
B. thailandensis?
B. thailandensis E264 (wt)
B. thailandensis ΔbtaI1, I2, I3
B. thailandensis ΔbtaT (BD44)
aSusceptibility of B. thailandensis strains was determined using the CLSI microtiter MIC assay (described above) using purified bactobolin B or filter sterilized wild type culture fluid prepared from wt B. thailandensis E264 cultures grown shaking in LB + Mops, pH 7 at 30° C. for 24 hours.
BtaT is predicted to be a drug resistance transporter and may be involved in the export of bactobolin. To examine its function, a markerless btaT mutation was generated. Cultivation of this mutant under the same conditions as above for 12 or 24 h showed that export of bactobolin was reduced 18-fold or 10-fold, respectively, relative to wt supporting the role proposed for BtaT. The btaT mutant was also resistant to bactobolin indicating multiple pathways for bactobolin excretion. Consistent with previous analysis, the btaK mutation abrogated bactobolin production (
The following references are incorporated herein by reference in their entirety:
The present application claims priority to U.S. Provisional Application 61/166,729, filed Apr. 4, 2009, U.S. Provisional Application 61/168,589, filed Apr. 11, 2009, and U.S. Provisional Application 61/260,806, filed Nov. 12, 2009 which are incorporated herein by reference in their entirety.
The invention was made with Government support under Grant number U54A1057141, Grant number T32 GM07270 and Grant number GM086258 awarded by the National Institutes of Health. The U.S. Government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 61166729 | Apr 2009 | US | |
| 61168589 | Apr 2009 | US | |
| 61260806 | Nov 2009 | US |
