Demonstration of the catalytic roles and evidence for the physical association of type I fatty acid synthases and a polyketide synthase in the biosynthesis of aflatoxin B1

BACKGROUND

Aflatoxin B1 (compound 5. ) is a potent environmental carcinogen produced by certain Aspergillus species. Its first stable biosynthetic precursor is the anthraquinone norsolorinic acid (compound 3. ), which accumulates in the Aspergillus mutant strain NOR-1. Biochemical and genetic evidence suggest that this metabolite is synthesized in vivo by a specialized pair of fatty acid synthases (FAS-1 and FAS-2) and a separately transcribed polyketide synthase (PKS-A).

RESULTS

The N-acetylcysteamine (NAC) thioester of hexanoic acid was shown to efficiently support the biosynthesis of norsolorinic acid (compound 3. ) in the NOR-1 strain. In contrast, the mutants Dis-1 and Dis-2, which are derived from NOR-1 by insertional inactivation of fas-1, produced unexpectedly low amounts of norsolorinic acid in the presence of hexanoylNAC. Controls eliminated defects in the parent strain or enhancement of degradative beta-oxidation activity as an explanation for the low level of production. Southern blots and restriction mapping of Dis-1 and Dis-2 suggested normal levels of expression of the PKS-A and FAS-2 proteins should be observed because the genes encoding these proteins are not physically altered by disruption of fas-1.

CONCLUSIONS

The impaired ability of Dis-1 and Dis-2, harboring modified FAS-1 enzymes, to carry out norsolorinic acid synthesis implies the need for FAS-1 (and possibly also FAS-2) to physically associate with the PKS before biosynthesis can begin. The failure of the unaffected PKS alone to be efficiently primed by hexanoylNAC, and the presumed requirement for at least one of the FAS proteins to bind and transfer the C6 unit to the PKS, is in contrast to behavior widely believed to occur for type I PKSs.

Identification, cloning, sequencing, and overexpression of the gene encoding proclavaminate amidino hydrolase and characterization of protein function in clavulanic acid biosynthesis

Proclavaminate amidino hydrolase (PAH) catalyzes the reaction of guanidinoproclavaminic acid to proclavaminic acid and urea, a central step in the biosynthesis of the beta-lactamase inhibitor clavulanic acid. The gene encoding this enzyme (pah) was tentatively identified within the clavulanic acid biosynthetic cluster in Streptomyces clavuligerus by translation to a protein of the correct molecular mass (33 kDa) and appreciable sequence homology to agmatine ureohydrolase (M.B.W. Szumanski and S.M. Boyle, J. Bacteriol. 172:538-547, 1990) and several arginases, a correlation similarly recognized by Aidoo et al. (K. A. Aidoo, A. Wong, D. C. Alexander, R. A. R. Rittammer, and S. E. Jensen, Gene 147:41-46, 1994). Overexpression of the putative open reading frame as a 76-kDa fusion to the maltose-binding protein gave a protein having the catalytic activity sought. Cleavage of this protein with factor Xa gave PAH whose N terminus was slightly modified by the addition of four amino acids but exhibited unchanged substrate specificity and kinetic properties. Directly downstream of pah lies the gene encoding clavaminate synthase 2, an enzyme that carries out three distinct oxidative transformations in the in vivo formation of clavulanic acid. After the first of these oxidations, however, no further reaction was found to occur in vitro without the intervention of PAH. We have demonstrated that concurrent use of recombinant clavaminate synthase 2 and PAH results in the successful conversion of deoxyguanidinoproclavaminic acid to clavaminic acid, a four-step transformation. PAH has a divalent metal requirement, pH activity profile, and kinetic properties similar to those of other proteins of the broader arginase class.

Cleavage by calicheamicin gamma 1I of DNA in a nucleosome formed on the 5S RNA gene of Xenopus borealis

The cleavage by calicheamicin gamma 1I (CLM gamma 1I) of a nucleosome formed on the 5S RNA gene of Xenopus borealis was studied in vitro as a first step toward the understanding of CLM gamma 1I-chromatin interactions within the cell. The drug does not cleave in the region of the dyad axis of the nucleosome. Outside of this region, double-stranded cleavage occurs with a periodicity of 10-11 bp. The sites of cleavage correspond to DNA sequences facing outward in the nucleosome. The drug shows some sequence preference of cleavage within these accessible sites. The predominant cleavage event within this nucleosome occurs at -1 helical turn from the dyad axis. This site constitutes a "hot spot" for CLM gamma 1I cleavage within the 5S nucleosome. We observe an overall footprinting effect whereby the drug preferentially cleaves DNA located outside the nucleosome core (analogous to the linker DNA of chromatin) as compared to cleavage within the nucleosome core. We discuss the importance of accessibility, structural deformations of DNA within the nucleosome, and steric constraints posed by sequence, in the recognition and cleavage of nucleosomal DNA by calicheamicin.

Purification and characterization of clavaminate synthase from Streptomyces antibioticus. A multifunctional enzyme of clavam biosynthesis

Clavaminate synthase (CS), a key enzyme in the clavulanic acid biosynthetic pathway, has been purified to electrophoretic homogeneity from Streptomyces antibioticus (Tü 1718), a species that does not produce clavulanic acid. A comparison of the physical and kinetic properties of clavaminate synthase from S. antibioticus (CS3) and the two isozymes from Streptomyces clavuligerus (CS1 and CS2) has been conducted. In oxidative reactions requiring the co-substrates O2, alpha-ketoglutaric acid, and catalytic Fe2+, both CS1 and CS2 catalyze three distinct transformations, the hydroxylation of deoxyguanidinoproclavaminic acid to guanidinoproclavaminic acid, and the cyclization and desaturation of proclavaminic acid to clavaminic acid. We have demonstrated that CS3 from S. antibioticus also catalyzes these three oxidations. The apparent molecular mass of CS3 from matrix-assisted laser desorption mass spectrometry is 35,839 +/- 36 Da. The enzyme is a monomer in solution as determined by gel filtration chromatography. Analysis of the four possible proclavaminic acid diastereomers confirmed the absolute configuration of the substrate to be 2S,3R. Based upon N-terminal sequence comparisons among the three proteins, CS3 possesses the higher degree of homology with the CS1 isozyme from S. clavuligerus. Although previously associated solely with clavulanic acid biosynthesis, we propose these findings and recent precursor incorporation data support the view that clavaminate synthase plays a critical role in the biosynthesis of the clavam metabolites.

Expression and purification of two isozymes of clavaminate synthase and initial characterization of the iron binding site. General error analysis in polymerase chain reaction amplification

Clavaminate synthase is an Fe(2+)-, O2-, and alpha-ketoglutarate-dependent oxygenase that catalyzes three transformations in the biosynthesis of the important beta-lactamase inhibitor clavulanic acid. The genes from Streptomyces clavuligerus encoding two isoenzymes of clavaminate synthase have been over-expressed in Escherichia coli to give soluble proteins whose reactions, kinetic properties, and molecular masses are in excellent agreement with the wild-type isozymes. Preliminary investigation of the active site of clavaminate synthase was undertaken using diethyl pyrocarbonate and N-ethylmaleimide. Each was inhibitory to catalytic activity. Protection from inactivation in the presence of these reagents by Fe2+, O2, and alpha-ketoglutaric acid was thwarted by the rapid self-inactivation of the enzyme in the absence of substrate. However, protection was achieved when Co2+, a potent competitive inhibitor of clavaminate synthase 2 with respect to Fe2+, was substituted. This is consistent with the presence of histidine and cysteine, respectively, at or near the active site and possibly involved in iron binding. In the course of constructing the expression vector, a simply applied general error analysis of the polymerase chain reaction was formulated to calculate the proportion of correctly replicated DNA and guide the design of experiments using this method.

Hydroxyl radical footprinting of calicheamicin. Relationship of DNA binding to cleavage

The binding to DNA by calicheamicin epsilon (CLM epsilon), the rearranged and reduced product of the diynene antitumor antibiotic calicheamicin gamma 1I (CLM gamma 1I), was studied using the method of hydroxyl radical footprinting. The drug binding sites determined in this way were compared to locations of double-stranded DNA cleavage by thiol-activated CLM gamma 1I. The results of these experiments show that CLM epsilon lies in the minor groove in an extended conformation protecting approximately four nucleotides on each strand of DNA. Sites of CLM epsilon binding correlate to sites of CLM gamma 1I cleavage with protection by CLM epsilon occurring mainly to the 3' side of the site of C5' hydrogen abstraction. From these results, it is possible to propose global structures of the drug/DNA complexes such that the oligosaccharide side chain is arrayed to the 3' side of the site of C5' hydrogen abstraction. This conclusion is entirely consistent with the results of recent atom-transfer experiments [Hangeland, J.J., De Voss, J.J., Heath, J.A., Townsend, C.A., Ding, W.-D., Ashcroft, J., & Ellestad, G.A. (1992) J. Am. Chem. Soc. 114, 9200-9202]. Somewhat greater protection on the strand undergoing C5' hydrogen abstraction was observed to the 5' side of the site of attack owing presumably to proximity of the methyl carbamate portion of the drug with DNA. Overall, binding is seen where cleavage is seen in accord with thermodynamics of drug association to DNA being important in determining the sites of cleavage.

The potential role of fatty acid initiation in the biosynthesis of the fungal aromatic polyketide aflatoxin B1

Earlier work in this laboratory has shown the intact incorporation of [1-13C]hexanoate into averufin (1), a key intermediate in aflatoxin B1 biosynthesis. Parallel experiments with equimolar amounts of [1-13C]butyrate, [1-13C]-3-oxo-octanoate, and [1-13C]-5-oxo-hexanoate gave no detectable specific incorporation of heavy isotope but low and equivalent background incorporation comparable to [1-13C]acetate. Three of these potential intermediates in polyketide formation were reexamined as their corresponding N-acetylcysteamine (NAC) thioesters. The NAC thioester of [1-13C]hexanoic acid gave a remarkably high 22% intact incorporation while the NAC thioester of [1-13C]-3-oxo-octanoic acid afforded nearly 5% when an equimolar amount was administered to the producing organism Aspergillus parasiticus (ATCC 24551). In contrast, the NAC thioester of [1-13C]butyric acid showed no selective enrichment of averufin. This negative result was tested further in a more sensitive experiment with the NAC thioester of [2,3-13C2]butyric acid. No 1JCC coupling was detectable, indicating an incorporation efficiency of <0.1%. [1-13C,18O2]Hexanoate was prepared and gave a 53% retention of 18O relative to the I3C internal standard in keeping with previous experiments with [1-13C,18O2]acetate. It is concluded from these data that the initial C6 segment of polyketide biosynthesis is unlikely to arise by β-oxidation of a higher fatty acid but more probably is generated by a specialized fatty acid synthase (FAS) that provides this unit either separately to the polyketide synthase (PKS) or as part of a larger FAS/PKS fusion. While these two physical arrangements cannot be distinguished by these experiments, both must accommodate comparatively efficient exchange of the NAC thioesters of both hexanoic and 3-oxo-octanoic acid, but not the NAC thioester of butyric acid.

Synthesis and reaction of potential alternate substrates and mechanism-based inhibitors of clavaminate synthase

Clavaminate synthase is an FeII/alpha-ketoglutarate-dependent enzyme central to the biosynthesis of the beta-lactamase inhibitor clavulanic acid. In the presence of dioxygen it catalyzes the oxidative cyclization/desaturation of proclavaminic acid to clavaminic acid in a two-step process. Samples of (4'R)- and (4'S)-D,L-[4'-2H]proclavaminic acid have been prepared and used to demonstrate that oxazolidine ring formation occurs with retention of configuration. The stereochemical course of oxygen insertion from substrate that takes place in this oxidative cyclization is the same as that observed from molecular oxygen in several hydroxylation reactions catalyzed by other FeII/alpha-ketoglutarate-dependent enzymes. The ferryl (FeIV = O) species thought to be transiently involved in each of these processes was investigated in the present work with clavaminate synthase and three structural analogues of proclavaminic acid bearing vinyl or ethynyl groups at C-4' or a cyclopropyl at C-4. In the synthesis of the former two derivatives and proclavaminic acid stereoselectively labeled with deuterium at C-4', introduction of the unsaturated substituents in a stereochemically defined manner at C-4' relied upon ready access to (4R)-4-thiophenyl-2-azetidinone. Trimethylsilyl substitution could be easily achieved at C-3 of the optically pure starting material to give the readily separable cis and trans diastereomers. In radical chain reactions in which the thiophenyl was replaced by deuterium or in anionic reactions in which the thiophenyl was eliminated as its sulfone and replaced by addition of carbanions, the steric bulk of the trimethylsilyl group at C-3 governed the approach of incoming reagents to give the trans product. The enzymatic fate, however, of these derivatives was disappointing, yielding neither detectable reaction nor hoped-for inactivation of clavaminate synthase. Finally, as mixed competitive/noncompetitive inhibitors of catalysis, they gave unexceptional inhibition constants in the range 2-10 mM.

Two isozymes of clavaminate synthase central to clavulanic acid formation: cloning and sequencing of both genes from Streptomyces clavuligerus

Clavaminate synthase (CS) is an alpha-ketoglutarate-dependent oxygenase central to the biosynthesis of clavulanic acid, a potent inhibitor of beta-lactamases. CS catalyzes the oxidative cyclization/desaturation of proclavaminic acid to clavaminic acid in a two-step process involving the intermediacy of dihydroclavaminic acid [Salowe, S. P., Krol, W. J., Iwata-Reuyl, D., & Townsend, C. A. (1991) Biochemistry 30, 2281-2292]. During the purification of CS to homogeneity from Streptomyces clavuligerus [Salowe, S. P., Marsh, E. N., & Townsend, C. A. (1990) Biochemistry 29, 6499-6508], two forms of the enzyme capable of carrying out the complete reaction having very similar molecular weights and kinetic properties were isolated by Mono-Q chromatography. The gene for each has been cloned, sequenced, and found to be significantly homologous (87% identity). The two genes so isolated, cs1 and cs2, have open reading frames of 975 and 978 nucleotides, respectively, encoding proteins of M(r) 35,347 and 35,774. These genes are located in different loci of the genome separated by > 20 kbp. This separation is large for a natural product biosynthetic pathway in bacteria where gene duplication and limited divergence are typically observed to occur within narrower confines of a gene cluster. Sequence comparisons made between cs1/cs2 and other genes encoding iron-dependent proteins involved in penicillin and cephalosporin biosynthesis in the same organism show minimal homology. Further sequence alignments made to other non-heme iron oxygenases reveal unexpected dissimilarity within the alpha-ketoglutarate-dependent class itself. The limited data available suggests evolutionary convergence among these proteins.

Elucidation of the order of oxidations and identification of an intermediate in the multistep clavaminate synthase reaction

The enzyme clavaminate synthase (CS) catalyzes the formation of the first bicyclic intermediate in the biosynthetic pathway to the potent beta-lactamase inhibitor clavulanic acid. Our previous work has led to the proposal that the cyclization/desaturation of the substrate proclavaminate proceeds in two oxidative steps, each coupled to a decarboxylation of alpha-ketoglutarate and a reduction of dioxygen to water [Salowe, S. P., Marsh, E. N., & Townsend, C. A. (1990) Biochemistry 29, 6499-6508]. We have now employed kinetic isotope effect studies to determine the order of oxidations for CS purified from Streptomyces clavuligerus. By using (4'RS)-[4'-3H,1-14C]-rac-proclavaminate, a primary T(V/K) = 8.3 +/- 0.2 was measured from [3H]water release data, while an alpha-secondary T(V/K) = 1.06 +/- 0.01 was determined from the changing 3H/14C ratio of the product clavaminate. Values for the primary and alpha-secondary effects of 11.9 +/- 1.7 and 1.12 +/- 0.07, respectively, were obtained from the changing 3H/14C ratio of the residual proclavaminate by using new equations derived for a racemic substrate bearing isotopic label at both primary and alpha-secondary positions. Since only the first step of consecutive irreversible reactions will exhibit a V/K isotope effect, we conclude that C-4' is the initial site of oxidation in proclavaminate. As expected, no significant changes in the 3H/14C ratio of residual substrate were observed with [3-3H,1-14C]-rac-proclavaminate. However, two new tritiated compounds were produced in this incubation, apparently the result of isotope-induced branching brought about by the presence of tritium at the site of the second oxidation. One of these compounds was identified by comparison to authentic material as dihydroclavaminate, a stable intermediate that normally remains enzyme-bound. On the basis of the body of information available and the similarities to alpha-ketoglutarate-dependent dioxygenases, a comprehensive mechanistic scheme for CS is proposed to account for this unusual enzymatic transformation.

Purification and characterization of clavaminate synthase from Streptomyces clavuligerus: an unusual oxidative enzyme in natural product biosynthesis

A pivotal step in the biosynthetic pathway to the beta-lactamase inhibitor clavulanic acid is the conversion of proclavaminic acid to clavaminic acid in a reaction requiring Fe2+, alpha-ketoglutarate, and oxygen [Elson, S. W., Baggaley, K. H., Gillett, J., Holland, S., Nicholson, N. H., Sime, J. T., & Woroniecki, S. R. (1987) J. Chem. Soc., Chem. Commun., 1736-1738]. Clavaminate synthase, the enzyme that catalyzes this oxidative cyclization/desaturation, has been purified to homogeneity from clavulanic acid producing cells of Streptomyces clavuligerus (ATCC 27064). The enzyme behaved as a monomer during gel filtration and migrated with Mr 47,000 during denaturing gel electrophoresis. After ion-exchange FPLC two active forms of the protein were resolved that differed slightly in kinetic constants and apparent molecular weight. Kinetic comparisons with the four possible diastereomers of proclavaminate confirmed the absolute configuration of the substrate to be 2S,3R. The stoichiometry of the overall transformation was determined to be proclavaminate + 2(alpha-ketoglutarate) + 2O2----clavaminate + 2(succinate) + 2CO2 + 2H2O. In the absence of proclavaminate a slow decarboxylation of alpha-ketoglutarate to succinate and CO2 was observed in an uncoupled reaction which resulted in enzyme inactivation. Steady-state kinetic studies were undertaken for an initial description of the enzyme's catalytic cycle. The double-reciprocal plot with alpha-ketoglutarate as the variable substrate was linear; this supports the proposal that two stepwise oxidations of proclavaminate occur, each with the consumption of alpha-ketoglutarate and oxygen and the release of succinate, CO2, and H2O. The intersecting initial velocity plots obtained from pairwise variation of substrate concentrations were consistent with a sequential kinetic mechanism for the first oxidation. Similarities observed between clavaminate synthase and alpha-ketoglutarate-dependent dioxygenases argue for a common mechanism of oxygen activation. However, the nature of the interactions of the substrates in the active site of clavaminate synthase apparently redirects the conventional hydroxylase activity of dioxygenases to the construction of a strained bicyclic skeleton driven by the overall reduction of dioxygen.

Hemoptysis in sarcoidosis

The medical records of 433 patients with sarcoidosis were reviewed to determine the prevalence and significance of hemoptysis in this disease. Twenty-five patients (6%) were found to have hemoptysis. Nineteen of the 25 patients had mild hemoptysis, four had moderate, and two had massive hemoptysis. The clinical, roentgenographic, and laboratory features in patients with hemoptysis were compared with those from an age, race, and sex matched control group of sarcoidosis patients without hemoptysis. With the exception of eye involvement which occurred with greater frequency in control patients, no significant differences were found between the two groups. While bronchoscopy may be useful in establishing a diagnosis of endobronchial sarcoidosis, identification of a specific bleeding site is not likely in the absence of massive hemoptysis or localized radiographic abnormalities. Corticosteroid therapy may be useful to control hemoptysis in some patients.