Two new tailoring enzymes, a glycosyltransferase and an oxygenase, involved in biosynthesis of the angucycline antibiotic urdamycin A in Streptomyces fradiae Tü2717

Urdamycin A, the principal product of Streptomyces fradiae Tu2717, is an angucycline-type antibiotic and anticancer agent containing C-glycosidically linked D-olivose. To extend knowledge of the biosynthesis of urdamycin A the authors have cloned further parts of the urdamycin biosynthetic gene cluster. Three new ORFs (urdK, urdJ and urdO) were identified on a 3.35 kb fragment, and seven new ORFs (urdL, urdM, urdJ2, urdZl, urdGT2, urdG and urdH) on an 8.05 kb fragment. The deduced products of these genes show similarities to transporters (urdJ and urdJ2), regulatory genes (urdK), reductases (urdO), cyclases (urdL) and deoxysugar biosynthetic genes (urdG, urdH and urdZ1). The product of urdM shows striking sequence similarity to oxygenases (N-terminal sequence) as well as reductases (C-terminal sequence), and the deduced amino acid sequence of urdGT2 resembles those of glycosyltransferases. To determine the function of urdM and urdGT2, targeted gene inactivation experiments were performed. The resulting urdM deletion mutant strains accumulated predominantly rabelomycin, indicating that UrdM is involved in oxygenation at position 12b of urdamycin A. A mutant in which urdGT2 had been deleted produced urdamycin I, urdamycin J and urdamycin K instead of urdamycin A. Urdamycins I, J and K are tetracyclic angucyclinones lacking a C-C connected deoxysugar moiety. Therefore UrdGT2 must catalyse the earliest glycosyltransfer step in the urdamycin biosynthetic pathway, the C-glycosyltransfer of one NDP-D-olivose.

Characterization of two glycosyltransferases involved in early glycosylation steps during biosynthesis of the antitumor polyketide mithramycin by Streptomyces argillaceus

A 2,580-bp region of the chromosome of Streptomyces argillaceus, the producer of the antitumor polyketide mithramycin, was sequenced. Analysis of the nucleotide sequence revealed the presence of two genes (mtmGIII and mtmGIV) encoding proteins that showed a high degree of similarity to glycosyltransferases involved in the biosynthesis of various antibiotics and antitumor drugs. Independent insertional inactivation of both genes produced mutants that did not synthesize mithramycin but accumulated several mithramycin intermediates. Both mutants accumulated premithramycinone, a non-glycosylated intermediate in mithramycin biosynthesis. The mutant affected in the mtmGIII gene also accumulated premithramycin A1, which contains premithramycinone as the aglycon unit and a D-olivose attached at C-12a-O. These experiments demonstrate that the glycosyltransferases MtmGIV and MtmGIII catalyze the first two glycosylation steps in mithramycin biosynthesis. A model is proposed for the glycosylation steps in mithramycin biosynthesis.

Analysis of two chromosomal regions adjacent to genes for a type II polyketide synthase involved in the biosynthesis of the antitumor polyketide mithramycin in Streptomyces argillaceus

Mithramycin is an aromatic antitumour polyketide synthesized by Streptomyces argillaceus. Two chromosomal regions located upstream and downstream of the locus for the mithramycin type II polyketide synthase were cloned and sequenced. Analysis of the sequence revealed the presence of eight genes encoding three oxygenases (mtmOI, mtmOII and mtmOIII), three reductases (mtmTI, mtmTII and mtmTIII), a cyclase (mtm Y) and an acyl CoA ligase (mtmL). The three oxygenase genes were each inactivated by gene replacement. Inactivation of one of them (mtmOII) generated a non-producing mutant, while inactivation of the other two (mtmOl and mtmOIII) did not affect the biosynthesis of mithramycin. The mtmOII gene may code for an oxygenase responsible for the introduction of oxygen atoms at early steps in the biosynthesis of mithramycin leading to 4-demethylpremithramycinone. One of the reductases may be responsible for reductive cleavage of an intermediate from an enzyme and another for the reduction of a keto group in the side-chain of the mithramycin aglycon moiety. A hypothetical biosynthetic pathway showing in particular the involvement of oxygenase MtmOII and of various other gene products in mithramycin biosynthesis is proposed.

Oxidative cleavage of premithramycin B is one of the last steps in the biosynthesis of the antitumor drug mithramycin

BACKGROUND

Mithramycin is a member of the clinically important aureolic acid group of antitumor drugs that interact with GC-rich regions of DNA nonintercalatively. These drugs contain a chromophore aglycon that is derived from condensation of ten acetate units (catalyzed by a type II polyketide synthase). The aglycones are glycosylated at two positions with different chain length deoxyoligosaccharides, which are essential for the antitumor activity. During the early stages of mithramycin biosynthesis, tetracyclic intermediates of the tetracycline-type occur, which must be converted at later stages into the tricyclic glycosylated molecule, presumably through oxidative breakage of the fourth ring.

RESULTS

Two intermediates in the mithramycin biosynthetic pathway, 4-demethyl-premithramycinone and premithramycin B, were identified in a mutant lacking the mithramycin glycosyltransferase and methyltransferase genes and in the same mutant complemented with the deleted genes, respectively. Premithramycin B contains five deoxysugars moieties (like mithramycin), but contains a tetracyclic aglycon moiety instead of a tricyclic aglycon. We hypothesized that transcription of mtmOIV (encoding an oxygenase) was impaired in this strain, preventing oxidative breakage of the fourth ring of premithramycin B. Inactivating mtmOIV generated a mithramycin nonproducing mutant that accumulated premithramycin B instead of mithramycin. In vitro assays demonstrated that MtmOIV converted premithramycin B into a tricyclic compound.

CONCLUSIONS

In the late stages of mithramycin biosynthesis by Strepyomyces argillaceus, a fully glycosylated tetracyclic tetracycline-like intermediate (premithramycin B) is converted into a tricyclic compound by the oxygenase MtmOIV. This oxygenase inserts an oxygen (Baeyer-Villiger oxidation) and opens the resulting lactone. The following decarboxylation and ketoreduction steps lead to mithramycin. Opening of the fourth ring represents one of the last steps in mithramycin biosynthesis.

The structure of mithramycin reinvestigated

A reinvestigation of the structure of mithramycin, the principal product of Streptomyces argillaceus ATCC 12956, is reported. The structure elucidation was carried out with mithramycin decaacetate (4) using 2D NMR methods, including TOCSY, HMBC, and HSQC experiments. The work resulted in structure 3being confirmed for mithramycin.

Identification of two genes from Streptomyces argillaceus encoding glycosyltransferases involved in transfer of a disaccharide during biosynthesis of the antitumor drug mithramycin

Mithramycin is an antitumor polyketide drug produced by Streptomyces argillaceus that contains two deoxysugar chains, a disaccharide consisting of two D-olivoses and a trisaccharide consisting of a D-olivose, a D-oliose, and a D-mycarose. From a cosmid clone (cosAR3) which confers resistance to mithramycin in streptomycetes, a 3-kb PstI-XhoI fragment was sequenced, and two divergent genes (mtmGI and mtmGII) were identified. Comparison of the deduced products of both genes with proteins in databases showed similarities with glycosyltransferases and glucuronosyltransferases from different sources, including several glycosyltransferases involved in sugar transfer during antibiotic biosynthesis. Both genes were independently inactivated by gene replacement, and the mutants generated (M3G1 and M3G2) did not produce mithramycin. High-performance liquid chromatography analysis of ethyl acetate extracts of culture supernatants of both mutants showed the presence of several peaks with the characteristic spectra of mithramycin biosynthetic intermediates. Four compounds were isolated from both mutants by preparative high-performance liquid chromatography, and their structures were elucidated by physicochemical methods. The structures of these compounds were identical in both mutants, and the compounds are suggested to be glycosylated intermediates of mithramycin biosynthesis with different numbers of sugar moieties attached to C-12a-O of a tetracyclic mithramycin precursor and to C-2-O of mithramycinone: three tetracyclic intermediates containing one sugar (premithramycin A1), two sugars (premithramycin A2), or three sugars (premithramycin A3) and one tricyclic intermediate containing a trisaccharide chain (premithramycin A4). It is proposed that the glycosyltransferases encoded by mtmGI and mtmGII are responsible for forming and transferring the disaccharide during mithramycin biosynthesis. From the structures of the new metabolites, a new biosynthetic sequence regarding late steps of mithramycin biosynthesis can be suggested, a sequence which includes glycosyl transfer steps prior to the final shaping of the aglycone moiety of mithramycin.

Experimental approaches towards interpreting dolphin-stimulated bioluminescence

Flow-induced bioluminescence provides a unique opportunity for visualizing the flow field around a swimming dolphin. Unfortunately, previous descriptions of dolphin-stimulated bioluminescence have been largely anecdotal and often conflicting. Most references in the scientific literature report an absence of bioluminescence on the dolphin body, which has been invariably assumed to be indicative of laminar flow. However, hydrodynamicists have yet to find compelling evidence that the flow remains laminar over most of the body. The present study integrates laboratory, computational and field approaches to begin to assess the utility of using bioluminescence as a method for flow visualization by relating fundamental characteristics of the flow to the stimulation of naturally occurring luminescent plankton. Laboratory experiments using fully developed pipe flow revealed that the bioluminescent organisms identified in the field studies can be stimulated in both laminar and turbulent flow when shear stress values exceed approximately 0.1 N m-2. Computational studies of an idealized hydrodynamic representation of a dolphin (modeled as a 6:1 ellipsoid), gliding at a speed of 2 m s-1, predicted suprathreshold surface shear stress values everywhere on the model, regardless of whether the boundary layer flow was laminar or turbulent. Laboratory flow visualization of a sphere demonstrated that the intensity of bioluminescence decreased with increasing flow speed due to the thinning of the boundary layer, while flow separation caused a dramatic increase in intensity due to the significantly greater volume of stimulating flow in the wake. Intensified video recordings of dolphins gliding at speeds of approximately 2 m s-1 confirmed that brilliant displays of bioluminescence occurred on the body of the dolphin. The distribution and intensity of bioluminescence suggest that the flow remained attached over most of the body. A conspicuous lack of bioluminescence was often observed on the dolphin rostrum and melon and on the leading edge of the dorsal and pectoral fins, where the boundary layer is thought to be thinnest. To differentiate between effects related to the thickness of the stimulatory boundary layer and those due to the latency of the bioluminescence response and the upstream depletion of bioluminescence, laboratory and dolphin studies of forced separation and laminar-to-turbulent transition were conducted. The observed pattern of stimulated bioluminescence is consistent with the hypothesis that bioluminescent intensity is directly related to the thickness of the boundary layer.

Iterative type II polyketide synthases, cyclases and ketoreductases exhibit context-dependent behavior in the biosynthesis of linear and angular decapolyketides

BACKGROUND

Iterative type II polyketide synthases (PKSs) produce polyketide chains of variable but defined length from a specific starter unit and a number of extender units. They also specify the initial regiospecific folding and cyclization pattern of nascent polyketides either through the action of a cyclase (CYC) subunit or through the combined action of site-specific ketoreductase (KR) and CYC subunits. Additional CYCs and other modifications may be necessary to produce linear aromatic polyketides. The principles of the assembly of the linear aromatic polyketides, several of which are medically important, are well understood, but it is not clear whether the assembly of the angular aromatic (angucyclic) polyketides follows the same rules.

RESULTS

We performed an in vivo evaluation of the subunits of the PKS responsible for the production of the angucyclic polyketide jadomycin (jad), in comparison with their counterparts from the daunorubicin (dps) and tetracenomycin (tcm) PKSs which produce linear aromatic polyketides. No matter which minimal PKS was used to produce the initial polyketide chain, the JadD and DpsF CYCs produced the same two polyketides, in the same ratio; neither product was angularly fused. The set of jadABCED PKS plus putative jadl CYC genes behaved similarly. Furthermore, no angular polyketides were isolated when the entire set of jad PKS enzymes and Jadl or the jad minimal PKS, Jadl and the TcmN CYC were present. The DpsE KR was able to reduce decaketides but not octaketides; in contrast, the KRs from the jad PKS (JadE) or the actinorhodin PKS (ActIII) could reduce octaketide chains, giving three distinct products.

CONCLUSIONS

It appears that the biosynthesis of angucyclic polyketides cannot be simply accomplished by expressing the known PKS subunits from artificial gene cassettes under the control of a non-native promoter. The characteristic structure of the angucycline ring system may arise from a kinked precursor during later cyclization reactions involving additional, but so far unknown, components of the extended decaketide PKS. Our results also suggest that some KRs have a minimal chain length requirement and that CYC enzymes may act aberrantly as first-ring aromatases that are unable to perform all of the sequential cyclization steps. Both of these characteristics may limit the widespread application of CYC or KR enzymes in the synthesis of novel polyketides.

Traditional risk factors and ischemic stroke in young adults: the Baltimore-Washington Cooperative Young Stroke Study

OBJECTIVE

To determine the association of hypertension, diabetes, and cigarette smoking with incidence of ischemic stroke in young adults.

DESIGN

Case-control study.

SETTING

Population-based sample of cases and controls.

SUBJECTS

The study included 296 cases of incident ischemic stroke among black and white adults aged 18 to 44 years in central Maryland counties from the Baltimore-Washington Cooperative Young Stroke Study and 1220 black and white adults aged 18 to 44 years from the Maryland Behavioral Risk Factor Survey, a telephone survey of a random sample of the same region, to serve as controls.

MAIN OUTCOME MEASURES

Logistic regression models were developed to determine the age-adjusted odds ratios for each risk factor. Population-attributable risk percent were computed based on the odds ratios and prevalence of each risk factor.

RESULTS

The age-adjusted odds ratios (95% confidence intervals) for white men (WM), white women (WW), black men (BM), and black women (BW) were as follows: current cigarette smoking: WM, 2.0 (1.1-3.8), WW, 2.1 (1.1-4.3), BM, 3.3 (1.6-6.6), and BW, 2.2 (1.3-3.9); history of diabetes mellitus: WM, 22.9 (5.8-89.6), WW, 6.2 (1.9-20.2), BM, 4.2 (0.8-21.9), and BW, 3.3 (1.4-7.7); and history of hypertension: WM, 1.6 (0.7-3.2), WW, 2.5 (1.1-5.9), BM, 3.8 (1.8-7.9), and BW, 4.2 (2.4-7.5). The population-attributable risk percent (95% confidence intervals) were as follows: current cigarette smoking: WM, 22.6 (3.1-38.2), WW, 17.2 (4.0-34.0), BM, 40.5 (23.1-54.0), and BW, 29.1 (13.5-41.9); history of diabetes mellitus: WM, 19.0 (8.2-28.5), WW, 15.8 (3.8-26.3), BM, 13.2 (5.3-20.4), and BW, 22.1 (12.5-30.7); and history of hypertension: WM, 21.7 (6.2-34.6), WW, 21.3 (5.4-34.5), BM, 53.5 (39.0-64.4), and BW, 50.5 (37.1-61.1).

CONCLUSIONS

Hypertension, diabetes mellitus, and current cigarette smoking are important risk factors in a biracial young adult population. Cigarette smoking and hypertension, the 2 most modifiable risk factors, were particularly important risk factors in young blacks.

Biosynthetic capacities of actinomycetes. 5. Dioxolides, novel secondary metabolites from Streptomyces tendae

Dioxolides, a novel class of secondary metabolites were detected by a HPLC-diode array screening technique in the culture filtrate of Streptomyces tendae Tu 4042. The compounds show no biological activity against Gram-positive and Gram-negative bacteria, yeasts and fungi. Besides dioxolides, which consist of an unusual substituted dioxolane ring, anhydroshikimate and para-hydrobenzamide were detected by this technique. Both compounds were not yet described as natural products.

Identification of Streptomyces olivaceus Tü 2353 genes involved in the production of the polyketide elloramycin

The genes for the production of elloramycin (ELM) from Streptomyces olivaceus (So) Tü2353 were cloned using a polyketide synthase gene probe from the tetracenomycin pathway. A cosmid clone (16F4) isolated from a gene library of So Tü2353 conferred tetracenomycin C and ELM resistance to S. lividans TK64 and complemented a mutation in So Tü2353R. Introduction of cosmid 16F4 into S. lividans TK64 resulted in the production of 8-demethyl-tetracenomycin C, an intermediate of ELM biosynthesis.

Structure-activity relationships of elloramycin and tetracenomycin C

The structure-activity relationships of the anthracycline-related antibiotics of the tetracenomycin C/elloramycin-type were investigated by derivatization of elloramycin (1) and elloramycinone (2). During hydrolysis experiments a unique transglycosylation reaction was discovered, converting elloramycin (1) into isoelloramycin (10) by treatment with anhydrous trifluoroacetic acid. Following the proposed structure-activity relationship concept, 8-O-methylelloramycinone (14) was synthesized from elloramycinone (2), and was shown to be the most active derivative according to the proliferation inhibition assay against murine L1210 leukemia cells.

Landomycins, new angucycline antibiotics from Streptomyces sp. I. Structural studies on landomycins A-D

The chemical structure of the new angucycline antibiotic landomycin A has been elucidated via chemical and spectroscopic methods, in particular by 2D NMR correlation spectroscopy, e.g., 1H, 1H-COSY, 13C, 1H-COSY, correlation spectroscopy via long-range-couplings and heteronuclear multiple bond connectivity spectroscopy sequences. The spectroscopic investigations were carried out principally with the octaacetyl derivative of landomycin A, which is more soluble in organic solvents than landomycin A itself. The structure consists of a new, unusual angucyclinone, landomycinone A, and of six deoxy sugars, four D-olivoses and two L-rhodinoses, which are all assembled in one chain thus forming the sequence (olivose-4----1-olivose-3----1-rhodinose)2. This long sugar chain is bonded as a phenolic glycoside to the aglycone moiety, a unique structural feature among quinone glycoside antibiotics. By comparison with the main component landomycin A, the structures of three minor congeners, namely landomycins B, C and D, could be proposed.

Urdamycins, new angucycline antibiotics from Streptomyces fradiae. VI. Structure elucidation and biosynthetic investigations on urdamycin H

A new angucycline antibiotic has been discovered as a small side product of Streptomyces fradiae (strain Tü 2717), the producer of the urdamycin complex, during screening for biosynthetic relatives of urdamycins C and D. The structure was elucidated after isolation, via strain selection, of a mutant of S. fradiae that produces this new congener in larger amounts. The structure includes a new chromophore containing aglycone that has not been found before among the angucyclines nor as a natural product generally. In urdamycin H (1) the angucycline four-ring system is enlarged by a (p-OH-phenyl)furan moiety and is closely related to urdamycin C (2). The structure was elucidated by comparison of the physico-chemical data with those of known urdamycins, especially with those of urdamycin C (2), and was confirmed by intensive 2D NMR analysis. Biosynthetic studies showed that tyrosine and not the smaller p-OH-phenylglycine is the precursor of the (p-OH-phenyl)furan moiety.

Urdamycins, new angucycline antibiotics from Streptomyces fradiae. IV. Biosynthetic studies of urdamycins A-D

The biogenetic origin of the angucycline antibiotics urdamycins A-D was studied by feeding experiments with isotope labeled precursors and by NMR analysis. Feeding experiments with [1-13C]acetate and [1,2-13C2]acetate show that the chromophores of urdamycins A and B and the angucycline 4-ring skeleton of the urdamycins C and D chromophores are formed from a single decapolyketide chain. The chromophores of the urdamycins C and D contain additional structural elements which derived from the amino acids tyrosine and tryptophan, respectively. The latter was shown by feeding deuterium-labeled tyrosine and 13C-labeled tryptophan derivatives. Feeding of [1-13C]glucose and of [U-13C3]glycerol proved that the C-glycosidic moiety and the three sugars (2 x L-rhodinose, 1 x D-olivose each) of the urdamycins arise from glucose. Experiments with 14C-labeled urdamycin A, obtained by biosynthesis from [14C]acetate, showed this compound to be a late precursor of the urdamycins C and D.

Urdamycins, new angucycline antibiotics from Streptomyces fradiae. V. Derivatives of urdamycin A

Derivatives of the angucycline urdamycin A (1) were prepared in order to study structure-activity relationships in this group of antitumor antibiotics. Derivatives of 1 formed by methanolysis, O-acylation, hydrogenation and treatment with diazomethane were isolated and characterized by their spectroscopic data. Urdamycin G (20) was isolated from Streptomyces fradiae by shortening the fermentation time. The different glycosidation pattern of the aglycone 14 did not lead to significant differences in the biological activity. O-Acylation was shown to enhance the in vitro activity of 1 against stem cells of murine L1210 leukemia depending on the lipophilicity of the molecules. The importance of the 5,6-double bond of 1 with regard to the antitumor activities is discussed.

Metabolic products of microorganisms. 249. Tetracenomycins B3 and D3, key intermediates of the elloramycin and tetracenomycin C biosynthesis

Tetracenomycins B3 and D3, besides tetracenomycin D (D1), were produced by a blocked mutant of the elloramycin producer Streptomyces olivaceus TU 2353. The compounds were isolated as red powders, and their structures were elucidated by comparing their physicochemical data with those of the known tetracenomycins A2, B1, B2, D and E. Tetracenomycin B3 (2), the main compound, and tetracenomycin D (3) were antibiotically inactive against Gram-positive and Gram-negative bacteria, whereas tetracenomycin D3 (1) showed a moderate activity against Bacillus subtilis and Arthrobacter aurescens. Tetracenomycin B3 (2) is the key intermediate where the biosynthesis of the elloramycins branches off from the line leading to tetracenomycin C (5) as the final product of the tetracenomycin biosynthesis branch.

[Echocardiographic discoveries in 102 patients with vascular cerebral accidents]

The causes of vascular ischaemic accidents are numerous, and when the brain is involved management is limited to the prevention of similar events. Since cardiac sources of embolism potentially curable, we have prospectively analyzed the results of cardiovascular examinations (including ECG and radiography of the chest) and of echocardiography in 102 patients with cerebral or peripheral vascular ischaemic event in order to determine the impact of echocardiography and the influence of different diagnoses on the need for anticoagulant therapy. Intracardiac thrombi, mitral stenosis, dilated cardiomyopathy, severe left ventricular dysfunction with or without aneurysm and cardiac valve vegetations were regarded as diseases carrying a high risk of embolism, the low risk diseases being mitral valve prolapse, mitral annulus calcification and isolated left atrial dilatation. Atrial fibrillation was treated separately, as it may be associated with several of the diseases listed above. We found 14 diseases with a high risk of embolism (14 p. 100) and 35 diseases with a low risk of embolism (34 p. 100). 10/91 patients with cerebral vascular accident (11 p. 100) and 4/11 patients with peripheral vascular accident presented with a heart disease carrying a high risk of embolism. The most common heart disease with a high risk of embolism (10/14, 71 p. 100) was severe left ventricular dysfunction secondary to a coronary disease or a dilated cardiomyopathy. We did not find more cases of mitral valve prolapse or mitral annulus calcification than in the normal population. 20/29 patients with normal cardiac examination had a normal echocardiogram. The anticoagulant treatment was modified after echocardiography in only one case.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic products of microorganisms. 240. Urdamycins, new angucycline antibiotics from Streptomyces fradiae. II. Structural studies of urdamycins B to F

The structures of the angucycline antibiotics urdamycin B (5), E (2) and F (9) were established by comparing of their spectra with those of urdamycin A (1). The structures of urdamycins C and D, the largest compounds of this series, are still incomplete (10 and 11). The aglycones urdamycinone C, D and E can be liberated by methanolysis of the corresponding urdamycins. The liberation of urdamycinone B (6) requires an alcohol-free medium, to prevent its rearrangement to the naphthacenequinone 7 or 8. The urdamycins differ from other O-glycoside series in their variety of aglycones.