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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Am Chem Soc. Author manuscript; available in PMC May 19, 2011.
Published in final edited form as:
PMCID: PMC2876980
NIHMSID: NIHMS200703
Functional Characterization of TtnD and TtnF Unveiling New Insights into Tautomycetin Biosynthesis
Yinggang Luo,Ω Wenli Li, Jianhua Ju, Qiuping Yuan,[perpendicular] Noel R. Peters,[perpendicular] F. Michael Hoffmann,[perpendicular] Shengxiong Huang, Tim S. Bugni, Scott Rajski, Hiroyuki Osada,¥ and Ben Shen*§
Division of Pharmaceutical Sciences, University of Wisconsin, Madison, Wisconsin 53705-2222
[perpendicular]University of Wisconsin Paul P. Carbone Comprehensive Cancer Center Small Molecule Screening Facility, University of Wisconsin, Madison, Wisconsin 53705-2222
§University of Wisconsin National Cooperative Drug Discovery Group, University of Wisconsin, Madison, Wisconsin 53705-2222
Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53705-2222
ΩCenter for Natural Products Research, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China
¥Antibiotics Laboratory, Chemical Biology Department, Advanced Science Institute, RIKEN, Wako-shi, Saitama 351-0198, Japan
To whom correspondence should be addressed: Ben Shen, Division of Pharmaceutical Sciences, School of Pharmacy, University of Wisconsin-Madison, 777 Highland Ave., Madison, WI 53705-2222, USA. Tel: (608) 263-2673, Fax: (608) 262-5345, bshen/at/pharmacy.wisc.edu
The biosynthetic gene cluster for tautomycetin (TTN), a highly potent and selective protein phosphatase (PP) inhibitor isolated from Streptomyces griseochromogenes, has recently been cloned and sequenced. To better understand the transformations responsible for converting the post-polyketide synthase product into the exciting anticancer and immunosuppressive chemotherapeutic candidate TTN, we produced and characterized new analogues resulting from inactivation of two genes, ttnD and ttnF, in S. griseochromogenes. Inactivation of ttnD and ttnF, which encode for putative decarboxylase and dehydratase enzymes, respectively, afforded mutant strains SB13013 and SB13014. The ΔttnD mutant SB13013 accumulated four new TTN analogues, TTN D-1, TTN D-2, TTN D-3, and TTN D-4 whereas the ΔttnF mutant accumulated only one new TTN analogue, TTN F-1. The accumulation of these new TTN analogues defines the function of TtnD and TtnF and the timing of their chemistries in relation to installation of the C5 ketone moiety within TTN. Notably, all new analogues possess a structurally distinguishing carboxylic acid moiety, revealing that TtnD apparently cannot catalyze decarboxylation in the absence of TtnF. Additionally, cytotoxicity and PP inhibition assays reveal the importance of the functional groups installed by TtnDF and, consistent with earlier proposals, the C2"-C5 fragment of TTN to be a critical structural determinant behind the important and unique PP-1 selectivity displayed by TTN.
Tautomycetin (TTN, 1, Figure 1) and tautomycin (TTM, 2) are potent cell permeable inhibitors of protein phosphatases (PPs) PP-1 and PP-2A and are recognized as potent inducers of apoptosis. TTN, first isolated from Streptomyces griseochromogenes,1 displays nearly a 40-fold preference for PP-1 inhibition over PP2A and is the most selective PP1 inhibitor reported to date.2 The PP-1 selectivity of 1 likely plays a role in the agent's extraordinary immunosuppressive activity3 and sharply contrasts the PP-2A selective inhibition by the natural product okadaic acid, another potent phosphatase inhibitor, making it a particularly useful tool.4 Indeed, 1 has been instrumental in dissecting the role of PP-1 in the MEK-ERK pathway.5 TTM, isolated from Streptomyces spiroverticillatus shares significant structural features with TTN yet displays only a weak preference for PP1 inhibition relative to PP2A2.
Figure 1
Figure 1
Structures of tautomycetin (TTN, 1) and tautomycin (TTM, 2).
We recently cloned and sequenced the biosynthetic gene clusters for both 1 and 2.6,7 In the case of the highly selective PP-1 inhibitor 1, the ttn biosynthetic gene cluster from S. griseochromogenes was characterized, and its involvement in 1 biosynthesis confirmed by gene inactivation and complementation experiments.6 The ttn cluster was localized to a 79-kb DNA region, consisting of nineteen open reading frames that encode two modular type I polyketide synthases (TtnAB), one type II thioesterase (TtnH), eight proteins for dialkylmaleic anhydride biosynthesis (TtnKLMNOPRS), four tailoring enzymes (TtnCDFI), two regulatory proteins (TtnGQ), and one resistance protein (TtnJ). On the basis of functional assignments for each gene in the ttn cluster obtained from sequence analysis we have formulated a model for biosynthesis of 1 that agrees well with previous feeding experiments, has been supported by in vivo gene inactivation experiments, and is supported by analogy to the recently reported ttm cluster. These findings set the stage to fully interrogate biosynthesis of 1. Of particular interest is the means by which the C2"-C5 component (right hemisphere) is installed (Figure 1). This component of 1 differs significantly from the corresponding right hemisphere of 2 and has been proposed as a crucial determinant dictating the greater PP-1 selectivity of 1 relative to 2.811 This postulate has been substantiated by the recent crystal structure elucidation of PP-1 bound to 2 although high resolution structural information relating to PP-1 inhibition by 1 remains elusive.12 Both 1 and 2 exist as equilibrating mixtures of anhydride and ring opened diacids1b,13,14; the PP-1-to-2 crystal structure reveals that the diacid form of 2 is the active PP-1 inhibitor and implies, by analogy, that the diacid form of anhydride 1 is the species directly responsible for PP-1 inhibition.12
Here, we report that inactivation of two genes, ttnD and ttnF, in S. griseochromogenes abolishes production of 1, instead leading to five new analogues, TTN F-1 (3), TTN D-1 (4), TTN D-2 (5), TTN D-3 (6), and TTN D-4 (7), all of which lack the terminal C1"-C2" olefin, a critical feature of the right hemisphere of 1 (Figure 2). These findings support the proposed functions of TtnF and TtnD as a dehydratase and decarboxylase, respectively.6 Evaluation of the cytotoxicity and PP inhibitory activities of the new analogues highlight the importance of the C2"-C5 component in providing 1 with its ability to potently inhibit PP-1 in a highly selective fashion. These data significantly improve our understanding of TTN biosynthesis and PP inhibition by TTN.
Figure 2
Figure 2
TTN biosynthetic intermediates and shunt metabolites accumulated in the ΔttnF and ΔttnD mutant strains SB13013 and SB13014. (A) HPLC traces of metabolite profiles from S. griseochromogenes wild-type and mutant strains: (I) S. griseochromogenes (more ...)
Construction and Evaluation of the ΔttnD and ΔttnF Mutant Strains SB13013 and SB13014
To confirm the proposed function of TtnD and TtnF, in vivo gene inactivations were performed by using REDIRECT Technology (Table S1, Supporting Information) as described previously.6 The mutant cosmids were introduced into S. griseochromogenes by conjugation, and the resultant double crossover mutants were confirmed by PCR and Southern blot analysis (Table S2, Supporting Information). Genetic complementations to the mutant strains were subsequently carried out to eliminate the possibility of polar effects (Table S3, Supporting Information).
Isolation and Characterization of TTN Analogue 3 from SB13014 and TTN Analogues 4–7 from SB13013
Mutant strains SB13013 (ΔttnD) and SB13014 (ΔttnF) were fermented according to TTN production methods previously reported for the S. griseochromogenes wild-type strain, and corresponding metabolites were analyzed by HPLC with authentic TTN as a control.6 Only one TTN analogue 3 was accumulated and isolated from the ΔttnF mutant strain SB13014 (Figure 2). Its molecular formula C34H54O12 was established from the quasi-molecular ion at m/z 653.3532 ([M-H]), requiring an additional CH4O2 moiety relative to 1. Instead of the C2"-C5 moiety present in the right fragment of 1, only one trisubstituted olefin was deduced in 3 based on characteristic NMR signals at δH 5.48 (1H, t, J=7.0), δC 127.1 (d) and 141.4 (s) (Table 1). Two substituted groups, one β-hydroxypropanoic acid moiety (C1″-C3″) and one ethyl moiety (C1-C2), were observed in the 1-D and 2-D NMR data with connectivity to the carbon observed at 141.4 ppm (s, C-3) of the only double bond (Table 1). The third substituent of this double bond was assigned as a methylene group in view of the characteristic 1H NMR triplet signal observed at δH 5.48 (1H, t, J=7.0). This assignment was confirmed by HMQC and HMBC signals (Figure 3A). Thus, the C5 of 3 was assigned as a methylene group and not a ketone. The upfield NMR signals were consistent with those previously observed for 1 and were assigned on the basis of 2-D NMR cross signals (Table 1 and Figure 3A). The stereochemical configurations at C7, C9, C12, C13, C16, C17, C18, and C3′ are suggested to be identical to those of 1 on the basis of their shared biosynthetic origin and the very similar optical rotations observed for 1 and 3. Although we could predict a R-configuration for C1″ of 3 on the basis of bioinformatics comparisons of conserved amino acid residues of the TTN polyketide synthase KR domain and those of KR domains associated with stereochemically defined natural products,15 the absolute stereochemistry was not established experimentally. Attempts to make the Mosher ester of 3 were unsuccessful, with 3 undergoing rapid dehydration to 4 under all conditions examined.16
Table 1
Table 1
Summary of 1H and 13C NMR data for compounds 3, 4, and 7 in CDCl3ab
Figure 3
Figure 3
(A) Key 1H-1H COSY and HMBC correlations observed and applied to structure determination for TTN analogs 3–7. (B) Determination of the S-configuration at C5 of 5 on the basis of HETLOC, gHSQMBC, and gDQCOSY analyses.
Four TTN analogues 4, 5, 6, and 7 were accumulated and isolated from the ΔttnD mutant strain SB13013 (Figure 2). Their structures were elucidated on the basis of 1-D and 2-D NMR (including 1H-and 13C-NMR, 1H-1H COSY, HMQC, and HMBC data), UV, IR, and HRMS data. The molecular formula of 4, C34H52O11, was established from the quasi-molecular ion at m/z 659.3412 ([M+Na]+), requiring one H2O less than 3. The only difference between 4 and 3 was that 4 is not a β-hydroxypropanoic acid moiety, but rather, an acrylic acid moiety attached to C3 as deduced from its 1-D and 2-D NMR data (Table 1). The characteristic coupling constant 15.2 Hz between the two protons at δH 7.28 and 5.78 (each 1H, d, J=15.2) suggests a trans-double bond within the acrylic acid moiety (Table 1). The upfield NMR signals were consistent with those of 3 and were assigned by 2-D NMR cross signals (Table 1 and Figure 3A). Stereochemical configurations at C7, C9, C12, C13, C16, C17, C18, and C3' are predicted to be identical to those of 1 based on the shared biosynthetic origin of 1 and 4.
The molecular formula C34H52O12 of 5 was established from the quasi-molecular ion at m/z 651.3400 ([M-H]), requiring one more oxygen atom than 4. The only difference between 5 and 4 was that the characteristic 1H NMR signal of the proton of C4 is a doublet at δH 5.77 (1H, d, J=8.8), instead of the triplet signal observed in 3 and 4. Thus, C5 was assigned as a methine group instead of a methylene moiety, which is the case for 3 and 4. In view of the NMR signals at δH 4.58 (1H, q-like, J=7.6) and δC 66.6 (d), the oxygenation at C5 was deduced. C5 oxygenation was confirmed by 1H-1H COSY, HMQC, and HMBC cross signals (Table 2 and Figure 3A). The upfield NMR signals were consistent with those of 4 and were assigned by 2-D NMR cross signals (Table 2 and Figure 3A). Stereochemical configurations spanning C3' to C7 (with the exception of C5) were assigned in a fashion analogous to that used for compounds 3 and 4.
Table 2
Table 2
Summary of 1H and 13C NMR data for compounds 5 and 6 in CDCl3ab
The molecular formula of C34H52O12 of 6, deduced on the basis of the quasi-molecular ion at m/z 651.3399 ([M-H]), is identical to that of 5. The NMR spectra of 5 and 6 were almost superimposable (Table 2 and Figure 3A). The only discernible difference between the two compounds was their 1H and 13C NMR signals around C5, suggestive of a diastereomeric relationship between 5 and 6, the result of opposite configurations at C5. The exact configuration at C5 for 5 was subsequently assigned on the basis of extensive HETLOC, gHSQMBC, and gDQCOSY experiments.17,18 Thus, the anti-orientations of Hh-6/H-5 was suggested by the observed large 3J value (9.0 Hz), while the gauche orientations of Hl-6/H-5, Hl-6/H-7, and Hh-6/H-7 were supported by the small 3J values. The anti-orientations of Hl-6/OH and Hl-6/7-Me were shown by the small 2J value for Hl-6/C5 and the large 3J value (6.0 Hz) for Hl-6/7-Me, respectively. The gauche orientation of Hh-6/5-OH was suggested by the large 2J value (6.8 Hz) (Figure 3B). Taken together, an S-configuration was assigned to C5 of 5, thereby a R-configuration at C5 for 6 on the basis of their a diastereomeric relationship (Table 2 and Figure 2B).
For 7, a molecular formula of C34H50O12 was established from the quasi-molecular ion at m/z 649.3219 ([M-H]), requiring two protons less than 5. The only difference between 7 and 5 was that the characteristic 1H resonance for the C4 proton in 7 was a singlet at δH 6.34 (1H, s), instead of a triplet as observed for 3 and 4, or a doublet as observed for 5 and 6. The suggestion, on the basis of these data, that C5 of 7 was a carbonyl carbon was prompted by the observed 13C NMR signal at δC 201.5 (s) indicative of a quaternary carbon. The identity of C5 in 7 as a ketone carbon was confirmed by HMBC cross signals (Table 1 and Figure 3A) and is the only significant structural difference between 7 and its putative methylene precursor compound 4 (Figure 2B). The upfield NMR signals for 7 were consistent with those of 1 and its analogues and were assigned by 2-D NMR cross signals (Table 1 and Figure 3A). Stereochemical configurations at C7, C9, C12, C13, C16, C17, C18, and C3′ in 7 are likely identical to those observed in 1 on the basis of the compounds' shared biosynthetic origin.
Evaluation of PP Inhibitory Activity and Cytotoxicity of 3–7 in Comparison with 1
Compounds 3–7 were subjected to PP inhibition and cytotoxicity assays with 1 as a control.19 Assays focused specifically on the inhibition of PP-1 and PP-2A (Table 3), while cytotoxicity assays exploited the use of selected human cancer cell lines Du145, MCF7, and HCT-115 (Table 4).
Table 3
Table 3
Summary of in vitro inhibition data (IC50 in µM) for TTN and analogs against PP-1 and PP-2.
Table 4
Table 4
Summary of in vitro cytotoxicity data (IC50 in µM) for TTN and analogs against selected human cancer cell lines.
The C2"-C5 fragment of 1 is not consistent with structural expectations for the nascent polyketide resulting from the TtnAB polyketide synthase as predicted previously.6 The involvement of post-polyketide synthase steps en route to 1 was supported by the presence of four genes associated with putative tailoring enzymes: TtnC, a flavoprotein decarboxylase homolog; TtnD, a UbiD family decarboxylase homolog; TtnF, an L-carnitine dehydratase homolog; and TtnI, a putative cytochrome P450.6 On the basis of these functional assignments and the predicted synthetic capabilities of the TtnAB polyketide synthases, we have proposed previously 1 to arise through the intermediacy of a β-hydroxy acid intermediate (Figure 4).6 Concomitant decarboxylation and dehydration to form the terminal olefin has been previously suggested,17,18 and olefin installation in this manner was postulated to benefit substantially from the presence of the C5 ketone and its conjugation with the C3-C4 olefin.
Figure 4
Figure 4
Proposed biosynthesis of 1 predicated on a linear biosynthetic logic leading to intermediate 3 and subsequent processing by tailoring enzymes TtnD and TtnF. Tailoring steps involving TtnF, TtnI and TtnD and their ordering are assigned on the basis of (more ...)
The true functions of putative dehydratase TtnF and decarboxylase TtnD were evaluated using gene inactivation strategies followed by examining metabolite profiles of the resultant mutant strains. It is significant to note that under no circumstances was the previously postulated β-hydroxy acid intermediate observed, and this finding is consistent with the alternative idea that C5 oxidation proceeds after the chemistries of TtnD and TtnF (Figure 4). The ΔttnF mutant SB13014 accumulated 3 whereas the ΔttnD mutant SB13013 accumulated compounds 4–7. The accumulation of 3 in the ΔttnF mutant reveals three important details about the biosynthesis of 1 (Figure 2). First, it confirms the functional assignment of TtnF as a dehydratase. Second, and perhaps more surprising, is that retention of the terminal acid in 3 suggests that the decarboxylase activity of TtnD requires the presence of TtnF; TtnD alone is not sufficient to effect decarboxylation. Conventional reactivity considerations dictate that dehydration of the allylic C1" in 3 or related compounds might occur upon TtnD-catalyzed decarboxylation. However, the accumulation of 3 in the presence of TtnD but the absence of TtnF reveals the flaw in this thinking as does the fact that no trace of 1, 4, 5, 6, or 7 could be found in fermentations of the ΔttnF mutant SB13014. The biosynthetic transformations catalyzed by TtnD and TtnF appear to occur in concert. Conversely, TtnF-catalyzed dehydration of C1" is in no way dependent upon the decarboxylase activity of TtnD as reflected by the absence of the C1" OH moiety in all compounds accumulated by the ΔttnD mutant SB13013. Third, the accumulation of 3 by ΔttnF mutant SB13014 reveals that dehydration chemistry precedes polyketide C-5 oxidation needed for ketone installation. This is not the case for the ΔttnD mutant SB13013, which accumulated the C5 ketone 7 in addition to compounds 4–6. TtnI, a cytochrome P450 homolog, the only oxygenase within the ttn cluster and a putative C5 oxidase, is likely responsible for conversion of 4 into 7. That 4–6, and not just 7, accumulate in SB13013 suggests that the ΔttnD mutant accumulates impaired TtnI substrates. It is clear that the activities of TtnD, TtnF and TtnI are, to varying extents, impacted by the chemistries catalyzed by each other.
The accumulation of 3 by the ΔttnF mutant SB13014 and 4–7 by the ΔttnD mutant SB13013 allows us to more accurately predict the biosynthesis of 1 (Figure 4). Previous inactivation experiments indicate that TTN biosynthesis proceeds by a linear pathway.6 The dialkylmaleic anhydride unit is coupled to the growing TTN polyketide intermediate prior to its release from the polyketide synthase with the dialkylmaleic anhydride being constructed via an independent pathway that relies on TtnLMNOPRS.6 Hence, we envision a biosynthetic pathway in which acetyl CoA, malonyl CoA, methylmalonyl CoA and ethylmalonyl CoA are used by the two polyketide synthases TtnAB to produce, after TtnK-mediated dialkylmaleic anhydride coupling, 3 (Figure 4). The absence of any C5 oxygenated analogues of 3 accumulated by the ΔttnF mutant SB13014 suggests that TtnF-catalyzed chemistry precedes that of TtnI, an observation leading us now to postulate that, once formed, 3 is dehydrated by TtnF to provide diene 4.6 The findings would also be consistent with an alternative scenario wherein TtnF and TtnD act in concert to produce a diene intermediate, a substrate then for C5 oxidation by TtnI (Figure 4). Both biosynthetic hypotheses for ultimate conversion of 3 to 1 relegate C5 oxidation to a late stage transformation although further inactivation efforts are warranted to determine the precise timing and coordination of the steps catalyzed by TtnF, TtnI, and TtnD. Finally, the accumulation of C5 alcohols 5 and 6 in the ΔttnD mutant SB13013 could have resulted from C5 oxidation of compound 4 or C5 reduction of compound 7 by adventitious enzymes. However, regardless of the precise means by which 5 and 6 are produced, that both stereoisomers at C5 are observed correlates well with the production of 5 and 6 as shunt metabolites rather than intermediates formed by the stereospecific biosynthetic machinery driving production of 1.
A critical distinction between 1 and 2 is the significantly greater selectivity of 1 for inhibition of PP-1 over PP-2A relative to 2.811 Yet surprisingly, little attention has been directed to the generation of TTN analogues able to shed insight into the structural basis for this selectivity; 3–7 are among the first TTN analogues reported. In light of extensive efforts to produce analogues of 2 as possible drug candidates, the lack of interest in analogue generation with 1 is truly remarkable.811,2326 To investigate the impact of right hemisphere modification upon TTN bioactivity 1, 3–7 were subjected to PP inhibition and cytotoxicity assays as previously described.6 Assays focused specifically on the inhibition of PP-1 and PP-2A, while cytotoxicity assays exploited the use of selected human cancer cell lines Du145, MCF7, and HCT-115.19 Thus, as summarized in Table 4, the impact of right hemisphere modifications on cytotoxicity appeared mixed. While the new analogues 3, 4, and 5 were inactive, 6 and 7 retained significant, albeit reduced, cytoxicity (within 3-folds of reduction relative to 1). In contrast, modifications of the right hemisphere of 1 clearly had a profound, uniform impact on its PP-1 selectivity. As shown in Table 3, 1 potently inhibited both PP-1 and PP-2 and did so with a PP-1 selectivity of about 27-fold. Analogues 3, 4, 6, and 7 inhibited PP-1 less efficiently than 1 by approximately one order of magnitude yet inhibited PP-2A with about the same potency as 1. In effect, any change to the C2"-C5 portion of 1 led to a significant decrease in PP-1 selective inhibition, a key trademark of 1. Not only was this the case for 3, 4, 6, and 7 but this was observed also for 5, which was a significantly poorer PP inhibitor than any other right hemisphere congener tested. PP-1 inhibition by 5 was approximately three orders of magnitude worse than for 1, and PP-2A inhibition by 5 was about two orders of magnitude worse than for 1. Hence, although the PP-1 selectivity of 5 is on par with all other analogues tested, the absolute inhibitory activity of 5 was markedly less than for all other analogues, even its diastereomer 6. The precise molecular origins of the more dramatically altered activity of 5 relative to other TTN analogues are uncertain. However, the results of these studies support proposals implicating the right hemisphere of 1 as providing much of the compound's PP-1 selectivity relative to PP-2A.811
Taken together, our ability to correlate inactivation of the ttnD and ttnF genes with specific structural modifications to 1 supports the significance of the genetic system developed for the TTN producer S. griseochromogenes during sequencing of the ttn biosynthetic gene cluster and reinforces current functional assignments for all genes in the ttn cluster. The accumulation of compounds 3–7 in ΔttnD and ΔttnF mutant strains SB13013 and SB13014 sheds significant new insight into how the C2"-C5 fragment of 1 is produced and how these chemistries might be applied in a combinatorial biosynthetic fashion to produce new analogues of 1. Production of 3–7 has also allowed us to critically evaluate some of the structural determinants responsible for the PP-1 selectivity of 1 relative to other PP inhibitors and general cytotoxicites against selected human cancer cells. These data establish an excellent stage for future investigations of TTN biosynthesis and the future generation of TTN analogues by manipulating the 1 biosynthetic machinery.
General
IR spectra were measured on a Bruker EQUINOX 55/S FT-IR/NIR spectrophotometer (Ettlingen, DE). Optical rotations were determined on a Perkin-Elmer 241 instrument at the sodium D line (589 nm). High resolution mass spectrum (HRMS) analyses were acquired on an IonSpec HiResMALDI FT-Mass spectrometer (Lake Forest, CA) for HRMALDIMS or on an Agilent 1100 series LC/MSD Trap SL for HRESIMS (Santa Clara, CA). NMR data were recorded on a Varian Unity Inova 400 or 500 MHz NMR Spectrometer (Varian, Inc., Palo Alto, CA). 1H- and 13C-NMR chemical shifts were referenced to residual solvent signals: δH 7.26 ppm and δC 77.7 ppm for CDCl3. 1H-1H COSY, HMQC, HMBC, HETLOC, gHSQMBC, and gDQCOSY were performed using either standard VARIAN pulse sequences or literature pulse sequences.17,18 High performance liquid chromatography (HPLC) analysis was carried out a Varian HPLC system equipped with ProStar 210 pumps and a photodiode detector. Mobile phases used were buffer A (H2O) and buffer B (CH3CN). Analytical and semi-preparative HPLC columns used were Alltech Alltima C18 column 250 × 4.6 mm, 5µm and 250 × 10 mm, 5 µm, respectively. Cytotoxity assays and PP inhibition assays for TTN and related analogues were performed as previously described for compound 2 and related congeners.19 Medium components and all other chemical solvents and reagents were commercially from Fisher Scientific (Fairlawn, NJ). Silica gel 60 RP-18 (230–400 mesh, EMD Chemical Inc., Gibbstown, NJ) was used for standard bench-top column chromatography. Amberlite XAD-16 resin was purchased from Sigma.
Bacterial Strains and Plasmids
Escherichia coli DH5α was used as the host for general subcloning.27 E. coli ET12567/pUZ800228 was used as the cosmid donor host for E. coli-Streptomyces conjugation. E. coli BW25113/pIJ790 and E. coli DH5α/pIJ773 were provided by John Innes Center (Norwich, UK) as a part of the REDIRECT Technology kit.29 The S. griseochromogenes wild-type strain has been described previously.1b,14
Biochemicals, Chemicals, and Media
Common biochemicals and chemicals were from standard commercial sources. E. coli strains carrying plasmids were grown in Luria-Bertani (LB) medium with appropriate antibiotics selection.27 All media for Streptomyces growth were prepared according to standard protocols.30 ISP-4 and tryptic soy broth (TSB) were from Difco Laboratories (Detroit, MI), and modified ISP-4 is ISP-4 supplemented with 0.05% yeast extract and 0.1% tryptone.31 ISP-4 medium and MS medium were used for S. griseochromogenes sporulation at 30°C for 5–7 days.
S griseochromogenes Strain Sporulation and Growth Conditions
The S griseochromogenes wild-type and ΔttnD and ΔttnF mutant strains SB13013 and SB13014 were grown on MS medium (consisting of autoclaved 2% mannitol, 3% soybean flour, and 1.8% agar in tap water) at 30 °C until they were well sporulated (7 days). Spores were then harvested and stored in 20% glycerol at −80°C using previously reported standard procedures.
Plasmids and DNA Manipulation
Plasmid extraction and DNA purification were carried out using commercial kits (Qiagen, Santa Clarita, CA) and genomic DNAs isolated according to literature protocol.30 The digoxigenin-11-dUTP labeling and detection kit (Roche Diagnostics Corp, Indianapolis, IN.) was used for preparation of DNA probes, and Southern hybridization was carried out as per manufacturer instructions.
Construction of ΔttnD and ΔttnF Mutant Strains SB13013 and SB13014
The ttnD and ttnF genes were inactivated by application of REDIRECT Technology according to the literature protocols.6,29 An apramycin (Apr) resistance gene aac(3)IV/oriT cassette was used to replace an internal region of each target gene. Mutant cosmids pBS13025 (ΔttnD) and pBS13026 (ΔttnF) for gene inactivation were constructed (Table S1, Supporting Information) and then introduced into S. griseochromogenes by conjugation from E. coli ET12567/pUZ8002 according to the literature procedure with the following modifications.6,29,30 Thus, S. griseochromogenes spores were suspended in TSB medium and heat-shocked at 45°C for 15 min, followed by incubation at 30°C for 6 hr. Germinated spores (as conjugation recipients) were mixed with E. coli ET12567/pUZ8002 harboring mutant cosmid (as conjugation donor) and spread onto modified ISP-4 plates freshly supplemented with 20 mM MgCl2. After incubation at 28°C for 16 to 22 h, each plate was overlaid with 1 mL of sterilized water containing Apr at a final concentration of 10 µg/mL and nalidixic acid at a final concentration of 50 µg/mL. Incubation continued at 28°C until exconjugants appeared. The double crossover mutants found to be apramycin resistant and kanamycin sensitive were selected, named SB 13013 (ΔttnD) and SB13014 (ΔttnF), and verified by PCR and subsequently confirmed by Southern analysis (Figure S1 and S2, Supporting Information).
Complementation of the ΔttnD Mutation in SB13013 and the ΔttnF Mutation in SB13014
To construct expression plasmids for genetic complementation experiments, ttnD and ttnF were amplified and digested by NsiI and XbaI, and then cloned into the same sites of pBS60277 to give pBS13029 (for ttnD expression) and pBS13030 (for ttnF expression). They were introduced into the corresponding mutant strains by conjugation to yield complemented strains SB13015 (i.e., SB13103/pBS13029) and SB13016 (i.e., SB13104/pBS13030), respectively (Table S3).
Fermentation of S. griseochromogenes wild-type and Recombinant Strains and Production of TTN and Analogues
A two-stage fermentation procedure was utilized to grow the S. griseochromogenes wild-type and recombinant strains SB13013, SB13014, SB13015, and SB13016 for TTN and analogue production as previously described.6 Thus, seed medium (50 mL in 250-mL flask) was inoculated with spores, and the flasks were incubated on a rotary shaker at 250 rpm (Innova® 44 Incubator Shaker Series, New Brunswick Scientific Co., Inc., Edison, NJ) and 28°C for 2 days. This seed culture (50 mL) was then transferred into the fermentation medium (500 mL in 2-L flask), and the flasks were incubated on a rotary shaker at 250 rpm and 28°C for 5 days. Both seed and production media consist of glucose 2% (separately autoclaved), soluble starch 0.5%, beef extract 0.05%, yeast extract 0.3%, soybean flour 1%, NaCl 0.1%, K2HPO4 0.0025%, and distilled water and tap water (1:1), pH 7.0, and were sterilized by autoclaving at 121°C for 35 min.
Extraction and Isolation of TTN (1) and Analogues (3–7) from S. griseochromogenes Fermentation
The typical procedure for extraction and isolation of TTN and analogues from S. griseochromogenes wild-type and recombinant strain fermentation is as follows. The fermentation broth (10 L) was harvested by first bringing the broth pH to 4.0 via dropwise addition of 1 N HCl. Fermentation mixtures were then centrifuged at 3800 RPM (SLC-6000 rotor, Sorvall® Evolution RC, Thermo Scientific Inc., Waltham, MA) at 4°C for 20 min to pellet the mycelia. Broth supernatants were then collected and filtered to afford transparent amber colored supernatants. Supernatants were then adsorbed onto 1.8 L of XAD-16 resin twice. Resins (now bearing secondary metabolites) were then washed with 5.4 L distilled water to remove residual cells and broth components and then subjected to 3.6 L acetone to elute the absorbed compounds. Acetone was removed under vacuum to give the crude products, and these products were then dissolved into 600 mL acidic water (pH = 4.0). Acidic aqueous fractions were then extracted three times with 900 mL ethyl acetate (300 mL fresh solvent each time). The resulting organic layers were combined and dried over anhydrous sodium sulfate. Following removal of all solids, the ethyl acetate was removed under reduced pressure to afford the crude syrups containing TTN and analogues. The syrups were then subjected to column chromatography over silica gel 60 RP-18 eluted with acetonitrile and water (from 2:8 to 9:1; 300 mL each) gradient. Each 100 mL fraction was analyzed by analytical HPLC, employing a detection wavelength of 264 nm and a linear gradient running from a buffer A/buffer B composition of 70:30 to 100% buffer B over the course of 24 min and continued at 100 % buffer B for an additional 3 min, at a flow rate of 1 mL/min. Fractions containing TTN or analogues were combined and the solvents removed under reduced pressure for further purification by HPLC on analytic or semi-preparative C-18 column. Precise purification procedures for each compound are noted below. Following collection of relevant fractions from HPLC, samples were frozen in dry ice and then solvent lyophilized for 12h.
For purification of TTN (1) and TTN F-1 (3) semi-preparative HPLC was carried out on an Alltech Alltima C-18 column (250 × 10.0 mm, 5µm), employing a linear gradient from buffer A/buffer B (70:30) to 100% buffer B over 24 min and continued at 100% buffer B for an additional 3 min, at a flow rate of 3 mL/min and monitored by UV detection at 264 nm.
For purification of TTN D-1 (4) a linear gradient from buffer A/buffer B (90:10) to 100% buffer B over 20 min and continued at 100% buffer B for an additional 3 min, at a flow rate of 3 mL/min and UV detection at 264 nm.
For purification of diastereomers 5 and 6, an effective linear gradient involved ramping from buffer A/buffer B (60:40) to buffer A/buffer B (20:80) over 16 min with continued flow at 100% buffer B for an additional 2 min, at a flow rate of 3 mL/min and UV detection at 264 nm. The first peak corresponded to compound 5, and the slightly slower moving peak corresponded to compound 6.
For purification of TTN D-4 (7) a linear gradient from buffer A/buffer B (70:30) to 100% buffer B over 16 min and continued at 100% buffer B for an additional 2 min, at a flow rate of 3 mL/min and UV detection at 264 nm.
TTN F-1 (3)
Absolute yield: 16 mg from 16 L fermentation broth of SB13014. Yellowish gum; equation M1 (c 1.0, Acetone); APCI-MS (positive mode) m/z: 637 ([M-H2O+H]+, 15), 619 ([M-2H2O+H]+, 50), 601 ([M-3H2O+H]+, 15), 281 (80), 263 (100), and 139 (65); HR-ESI-MS (negative mode) m/z: 653.3532 [M-H] (calcd for C34H53O12, 653.3543, −1.6 ppm Error); IR: 3415, 2930, 1766, 1707, 1457, 1427, 1364, 1269, 1225, 1180, 1110, 1073, 1048, 910, 824, 794, and 706 cm−1; For 1H- and 13C-NMR data, see Table 1.
TTN D-1 (4)
Absolute yield: 17 mg from 40 L fermentation broth of SB 13013. Off-yellowish gum; equation M2 (c 1.0, acetone); APCI-MS (negative mode) m/z: 635 ([M-H], 100); HR-MALDI-MS (positive mode) m/z: 659.3412 [M+Na]+ (calcd for C34H52O11Na, 659.3402, 1.58 ppm Error); IR: 3422, 2930, 1766, 1706, 1621, 1515, 1456, 1364, 1259, 1222, 1177, 1089, 1062, 1029, 985, 907, 852, 764, and 731 cm−1; For 1H- and 13C-NMR data, see Table 1.
TTN D-2 (5)
Absolute yield: 30 mg from 40 L fermentation broth of SB13013. Off-yellowish gum; equation M3 (c 2.0, acetone); APCI-MS (negative mode) m/z: 651 ([M-H], 100); HR-ESI-MS (negative mode) m/z: 651.3400 [M-H] (calcd for C34H51O12, 651.3375, 3.83 ppm Error); IR: 3407, 2931, 1830, 1765, 1703, 1621, 1456, 1365, 1260, 1223, 1179, 1032, 986, 957, 907, 854, and 732 cm−1; For 1H-and 13C-NMR data, see Table 2.
TTN D-3 (6)
Absolute yield: 12 mg from 40 L fermentation broth of SB13013. Off-yellowish gum; equation M4 (c 1.0, acetone); APCI-MS (negative mode) m/z: 651 ([M-H], 100); HR-ESI-MS (negative mode) m/z: 651.3399 [M-H] (calcd for C34H51O12, 651.3375, 3.68 ppm Error); IR: 3406, 29671, 1830, 1765, 1703, 1621, 1456, 1365, 1260, 1223, 1179, 1040, 985, 956, 908, 855, and 732 cm−1; For 1H-and 13C-NMR data, see Table 2.
TTN D-4 (7)
Absolute yield: 4 mg from 40 L fermentation broth of SB13013. Off-yellowish gum; equation M5 (c 2.0, acetone); APCI-MS (negative mode) m/z: 649 ([M-H], 100); HR-ESI-MS (negative mode) m/z: 649.3239 [M-H] (calcd for C34H49O12, 649.3219, 3.15 ppm Error); IR: 3416, 2966, 1829, 1765, 1704, 1625, 1581, 1457, 1378, 1261, 1181, 1090, 1033, 986, 957, 908, and 732 cm−1; for 1H-and 13C-NMR data, see Table 1.
Supplementary Material
1_si_001
Acknowledgment
We thank the Analytical Instrumentation Center of the School of Pharmacy, UW-Madison for support in obtaining MS and NMR data. This work is supported in part by NIH grants CA106150 and CA113297. Y.L. is the recipient of the visiting scholar fellowship from Chinese Academy of Sciences.
Footnotes
Supporting Information Available. Full experimental details describing production and confirmation of ΔttnD and ΔttnF mutant strains SB13013 and SB13014 and 1H and 13C NMR spectra for new TTN analogues 3–7. This material is available free of charge via the Internet at http://pubs.acs.org.
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