SEARCH · PubMed Health

Results for “Polyketides”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 163 records · Page 9Linked to original sources

PubMed2004

Polyketide biosynthesis: understanding and exploiting modularity.

Polyketide-based pharmaceuticals are some of our most important medicines. They are constructed in micro-organisms (typically bacteria and fungi) by gigantic enzyme catalysts called polyketide synthases (PKSs). The organization of PKSs into molecular assembly lines makes them particularly appealing targets for genetic engineering because, in principle, an alteration in the enzyme organization might translate into a predictable change in polyketide structure. Excitingly, this has been shown repeatedly to work in practice, but the efficiency of the engineered PKSs is frequently too low to be useful for large-scale drug synthesis. To reach this goal, researchers need a deeper understanding of the structure and function of these proteins, which are among the most complex in nature. This review highlights some recent experiments which are providing key information about the molecular organization, mechanism and orchestration of these magnificent catalysts, and opening up fresh prospects of truly combinatorial biosynthesis of novel polyketides as leads in drug discovery.

Anti-Infective Agents↗

PubMed2001

Feedback control of polyketide metabolism during tylosin production.

Tylosin is produced by Streptomyces fradiae via a combination of polyketide metabolism and synthesis of three deoxyhexose sugars, of which mycaminose is the first to be added to the polyketide aglycone, tylactone (protylonolide). Previously, disruption of the gene (tylMII) encoding attachment of mycaminose to the aglycone unexpectedly abolished accumulation of the latter, raising the possibility of a link between polyketide metabolism and deoxyhexose biosynthesis in S. fradiae. However, at that time, it was not possible to eliminate an alternative explanation, namely, that downstream effects on the expression of other genes, not involved in mycaminose metabolism, might have contributed to this phenomenon. Here, it is shown that disruption of any of the four genes (tylMI--III and tylB) specifically involved in mycaminose biosynthesis elicits a similar response, confirming that production of mycaminosyl-tylactone directly influences polyketide metabolism in S. fradiae. Under similar conditions, when mycaminose biosynthesis was specifically blocked by gene disruption, accumulation of tylactone could be restored by exogenous addition of glycosylated tylosin precursors. Moreover, certain other macrolides, not of the tylosin pathway, were also found to elicit qualitatively similar effects. Comparison of the structures of stimulatory macrolides will facilitate studies of the stimulatory mechanism.

Bacterial Proteins↗

PubMed2004

Blockage of methylcitrate cycle inhibits polyketide production in Aspergillus nidulans.

Aspergillus nidulans produces the polyketide toxin sterigmatocystin (ST) of which the biosynthetic and pathway specific regulatory genes compose a stc gene cluster. A previous mutagenesis screen identified 23 mutants defective in production of ST. Five mutants constitute a single locus. Genetic complementation and sequencing analysis revealed the mutant locus to be mcsA encoding methylcitrate synthase that converts propionyl-CoA to methylcitrate. Feeding downstream products of methylcitrate synthase, methylcitrate and pyruvate, did not restore ST production in mcsA mutants, indicating that loss of methylcitrate cycle products is not the cause of the ST defect. However, propionate, a precursor for propionyl-CoA, inhibited ST production and induced transcription of mcsA in the wild type. Furthermore, propionate impaired formation of two polyketide spore pigments whereas overexpression of mcsA relieved inhibition of ST production by propionate. Transcription analyses revealed that disruption of mcsA did not affect expression of the specialized fatty acid synthase genes (stcJ and stcK) or polyketide synthase gene (stcA) required for formation of norsolorinic acid (NOR), the first stable intermediate in the ST biosynthetic pathway. Feeding studies showed that NOR but not hexanoic acid (the fatty acid produced by StcJ/StcK and primer unit of StcA) or malonate (source of the extender unit of StcA) restored ST production in the mcsA mutant. We hypothesize that excess buildup of propionyl-CoA in mcsA mutants interferes with polyketide synthase activity.

Aspergillus nidulans↗

PubMed1997

Structural organization of a multifunctional polyketide synthase involved in the biosynthesis of the macrolide immunosuppressant FK506.

The immunosuppressant FK506 is a 23-membered macrocyclic polyketide produced by several Streptomyces species. Sequencing of a 19.5-kb contiguous segment of DNA from the FK506 gene cluster of Streptomyces sp. MA6548 revealed the presence of a single 19.3-kb open reading frame designated fkbA. fkbA encodes a component of the FK506 polyketide synthase, a complex enzyme system which catalyzes synthesis of the polyketide portion of FK506. The predicted product of gene fkbA is a 630,660-Da protein (6420 amino acids) that contains 19 independent domains with a high degree of amino acid sequence similarity to the catalytic activities of known fatty acid synthases. The identified domains are arranged into four repeated modules with a linear organization precisely as that of animal fatty acid synthase and type I polyketide synthase. Each module participates in one round of chain extension and subsequent processing and thus FkbA polypeptide catalyzes four of the ten condensation steps required for synthesis of the FK506 macrolactone ring. Disruption of fkbA results in the generation of an FK506 non-producing mutant demonstrating direct involvement of fkbA in the biosynthesis of FK506.

Amino Acid Sequence↗

PubMed1992

Cloning, analysis, and heterologous expression of a polyketide synthase gene cluster of Streptomyces curacoi.

Streptomyces curacoi produces curamycin, an antibiotic based on a modified orsellinic acid skeleton that is synthesized by the polyketide pathway. We have cloned, characterized, and partly sequenced a polyketide synthase gene cluster of S. curacoi. The sequence data reveal an organization of open reading frames that is similar to those of other polyketide synthetic clusters, although the biosynthetic products differ considerably in size and structure. We propose that one of the predicted open reading frames (curA) encodes polykeptide synthase, on the basis of its homology with other enzymes with similar functions. Expression of the cloned chromosomal fragment in the heterologous host S. lividans leads to the production of a brown pigment in large quantities. The analysis and expression of the cur genes for detailed molecular studies of the mechanism of polyketide biosynthesis is discussed.

Amino Acid Sequence↗

PubMed2001

On the presence of peptide synthetase and polyketide synthase genes in the cyanobacterial genus Nodularia.

Nodularin is a hepatotoxin produced by the bloom-forming cyanobacterial species Nodularia spumigena. Putative peptide synthetase and polyketide synthase genes were detected in toxic strains of Nodularia by degenerate PCR. Using specific primer sets, peptide synthetase and polyketide synthase gene homologues were detected in nodularin-producing strains indicating a possible role of peptide synthetase and polyketide synthase enzyme complexes in the biosynthesis of nodularin. Strains of Nodularia isolated from around the world were also analyzed for the production of nodularin by the protein phosphatase 2A inhibition assay. The protein phosphatase inhibition assay and the molecular detection of peptide synthetase and polyketide synthase genes in Nodularia may be useful techniques for the assessment of nodularin-producing cyanobacteria in the environment.

Amino Acid Sequence↗

PubMed1991

Modular organization of genes required for complex polyketide biosynthesis.

In Saccharopolyspora erythraea, the genes that govern synthesis of the polyketide portion of the macrolide antibiotic erythromycin are organized in six repeated units that encode fatty acid synthase (FAS)-like activities. Each repeated unit is designated a module, and two modules are contained in a single open reading frame. A model for the synthesis of this complex polyketide is proposed, where each module encodes a functional synthase unit and each synthase unit participates specifically in one of the six FAS-like elongation steps required for formation of the polyketide. In addition, genetic organization and biochemical order of events appear to be colinear. Evidence for the model is provided by construction of a selected mutant and by isolation of a polyketide of predicted structure.

Amino Acid Sequence↗

PubMed1999

Modulation of polyketide synthase activity by accessory proteins during lovastatin biosynthesis.

Polyketides, the ubiquitous products of secondary metabolism in microorganisms, are made by a process resembling fatty acid biosynthesis that allows the suppression of reduction or dehydration reactions at specific biosynthetic steps, giving rise to a wide range of often medically useful products. The lovastatin biosynthesis cluster contains two type I polyketide synthase genes. Synthesis of the main nonaketide-derived skeleton was found to require the previously known iterative lovastatin nonaketide synthase (LNKS), plus at least one additional protein (LovC) that interacts with LNKS and is necessary for the correct processing of the growing polyketide chain and production of dihydromonacolin L. The noniterative lovastatin diketide synthase (LDKS) enzyme specifies formation of 2-methylbutyrate and interacts closely with an additional transesterase (LovD) responsible for assembling lovastatin from this polyketide and monacolin J.

Aspergillus↗

PubMed1995

Repositioning of a domain in a modular polyketide synthase to promote specific chain cleavage.

Macrocyclic polyketides exhibit an impressive range of medically useful activities, and there is great interest in manipulating the genes that govern their synthesis. The 6-deoxyerythronolide B synthase (DEBS) of Saccharopolyspora erythraea, which synthesizes the aglycone core of the antibiotic erythromycin A, has been modified by repositioning of a chain-terminating cyclase domain to the carboxyl-terminus of DEBS1, the multienzyme that catalyzes the first two rounds of polyketide chain extension. The resulting mutant markedly accelerates formation of the predicted triketide lactone, compared to a control in which the repositioned domain is inactive. Repositioning of the cyclase should be generally useful for redirecting polyketide synthesis to obtain polyketides of specified chain lengths.

Base Sequence↗

PubMed1993

Engineered biosynthesis of novel polyketides.

Polyketide synthases (PKSs) are multifunctional enzymes that catalyze the biosynthesis of a huge variety of carbon chains differing in their length and patterns of functionality and cyclization. Many polyketides are valuable therapeutic agents. A Streptomyces host-vector system has been developed for efficient construction and expression of recombinant PKSs. Using this expression system, several novel compounds have been synthesized in vivo in significant quantities. Characterization of these metabolites has provided new insights into key features of actinomycete aromatic PKS specificity. Thus, carbon chain length is dictated, at least in part, by a protein that appears to be distinctive to this family of PKSs, whereas the acyl carrier proteins of different PKSs can be interchanged without affecting product structure. A given ketoreductase can recognize and reduce polyketide chains of different length; this ketoreduction always occurs at the C-9 position. The regiospecificity of the first cyclization of the nascent polyketide chain is either determined by the ketoreductase, or the chain-extending enzymes themselves. However, the regiospecificity of the second cyclization is determined by a distinct cyclase, which can discriminate between substrates of different chain lengths.

Base Sequence↗

PubMed2003

Biosynthesis of Yersiniabactin, a complex polyketide-nonribosomal peptide, using Escherichia coli as a heterologous host.

The medicinal value associated with complex polyketide and nonribosomal peptide natural products has prompted biosynthetic schemes dependent upon heterologous microbial hosts. Here we report the successful biosynthesis of yersiniabactin (Ybt), a model polyketide-nonribosomal peptide hybrid natural product, using Escherichia coli as a heterologous host. After introducing the biochemical pathway for Ybt into E. coli, biosynthesis was initially monitored qualitatively by mass spectrometry. Next, production of Ybt was quantified in a high-cell-density fermentation environment with titers reaching 67 +/- 21 (mean +/- standard deviation) mg/liter and a volumetric productivity of 1.1 +/- 0.3 mg/liter-h. This success has implications for basic and applied studies on Ybt biosynthesis and also, more generally, for future production of polyketide, nonribosomal peptide, and mixed polyketide-nonribosomal peptide natural products using E. coli.

Bacterial Outer Membrane Proteins↗

PubMed1995

Sterigmatocystin biosynthesis in Aspergillus nidulans requires a novel type I polyketide synthase.

A filamentous fungus, Aspergillus nidulans, produces the carcinogenic mycotoxin sterigmatocystin (ST), which is a polyketide-derived secondary metabolite. A gene (pksST) encoding the ST polyketide synthase (PKSst) in A. nidulans was cloned, sequenced, and characterized. Large induced deletion mutants, which did not make ST or any ST intermediates, were used to identify genes associated with ST biosynthesis. Among the transcripts detected within the deletion region, which showed developmental expression with ST production, was a 7.2-kb transcript. Functional inactivation of the gene encoding the 7.2-kb transcript blocked production of ST and all ST intermediate substrates but did not affect transcription of the pathway genes, indicating that this gene was involved in a very early step of ST biosynthesis. These results also indicate that PKSst was not associated with activation of other ST genes. Sequencing of the region spanning this gene revealed that it encoded a polypeptide with a deduced length of 2,181 amino acids that had high levels of similarity to many of the known polyketide synthases and FASs. This gene, pksST, encodes a multifunctional novel type I polyketide synthase which has as active sites a beta-ketoacyl acyl carrier protein synthase, an acyltransferase, duplicated acyl carrier proteins, and a thioesterase, all of these catalytic sites may be multiply used. In addition, a 1.9-kb transcript, which also showed developmental expression, was mapped adjacent to pksST, and the sequence of this gene revealed that it encoded a cytochrome P-450 monooxygenase-like peptide.

Amino Acid Sequence↗

PubMed1995

Characterization of the polyketide synthase gene (pksL1) required for aflatoxin biosynthesis in Aspergillus parasiticus.

Aflatoxins are potent toxic and carcinogenic compounds, produced by Aspergillus parasiticus and A. flavus as secondary metabolites. In this research, a polyketide synthase gene (pksL1), the key gene for aflatoxin biosynthesis initiation in A. parasiticus, has been functionally identified and molecularly characterized. PCR-derived DNA probes were used to find the pksL1 gene from subtracted, aflatoxin-related clones. Gene knockout experiments generated four pksL1 disruptants which lost both the ability to produce aflatoxins B1, B2, and G1 and the ability to accumulate norsolorinic acid and all other intermediates of the aflatoxin biosynthetic pathway. A pksL1 DNA probe detected a 6.6-kb poly(A)+ RNA transcript in Northern (RNA) hybridizations. This transcript, associated with aflatoxin production, exhibited a regulated expression that was influenced by growth phase, medium composition, and culture temperature. DNA sequencing of pksL1 revealed an open reading frame for a polypeptide (PKSL1) of 2,109 amino acids. Sequence analysis further recognized four functional domains in PKSL1, acyl carrier protein, beta-ketoacyl-acyl carrier protein synthase, acyltransferase, and thioesterase, all of which are usually present in polyketide synthases and fatty acid synthases. On the basis of these results, we propose that pksL1 encodes the polyketide synthase which synthesizes the backbone polyketide and initiates aflatoxin biosynthesis. In addition, the transcript of pksL1 exhibited heterogeneity at the polyadenylation site similar to that of plant genes.

Aflatoxins↗

PubMed1999

The Streptomyces peucetius dpsC gene determines the choice of starter unit in biosynthesis of the daunorubicin polyketide.

The starter unit used in the biosynthesis of daunorubicin is propionyl coenzyme A (CoA) rather than acetyl-CoA, which is used in the production of most of the bacterial aromatic polyketides studied to date. In the daunorubicin biosynthesis gene cluster of Streptomyces peucetius, directly downstream of the genes encoding the beta-ketoacyl:acyl carrier protein synthase subunits, are two genes, dpsC and dpsD, encoding proteins that are believed to function as the starter unit-specifying enzymes. Recombinant strains containing plasmids carrying dpsC and dpsD, in addition to other daunorubicin polyketide synthase (PKS) genes, incorporate the correct starter unit into polyketides made by these genes, suggesting that, contrary to earlier reports, the enzymes encoded by dpsC and dpsD play a crucial role in starter unit specification. Additionally, the results of a cell-free synthesis of 21-carbon polyketides from propionyl-CoA and malonyl-CoA that used the protein extracts of recombinant strains carrying other daunorubicin PKS genes to which purified DpsC was added suggest that this enzyme has the primary role in starter unit discrimination for daunorubicin biosynthesis.

Acyl Coenzyme A↗

PubMed1993

Polyketide synthesis: prospects for hybrid antibiotics.

Polyketides fall into two structural classes: aromatic and complex. The former are built mainly from acetate units through a reiterative process wherein the beta-carbonyl groups formed after each condensation cycle are left largely unreduced. Complex polyketides are composed of acetates, propionates, or butyrates, and the extent of beta-carbonyl reduction varies from one cycle to the next. Two themes for polyketide synthases are emerging. Aromatic PKSs are determined by four to six genes encoding mono- or bifunctional enzymes; one PKS complex is used for all synthesis steps. Complex PKSs are composed of several multifunctional polypeptides that contain enzymatic domains for the condensation and reduction steps; each domain is used at a unique step in the pathways, and the extent of beta-carbonyl processing depends on the functional domains operating at that cycle. Mutations rendering certain domains nonfunctional have been introduced into genes for complex polyketides, resulting in the production of novel molecules.

Amino Acid Sequence↗

PubMed1999

POLYKETIDE PRODUCTION BY PLANT-ASSOCIATED PSEUDOMONADS.

Polyketides constitute a huge family of structurally diverse natural products including antibiotics, chemotherapeutic compounds, and antiparasitics. Most of the research on polyketide synthesis in bacteria has focused on compounds synthesized by Streptomyces or other actinomycetes; however, plant-associated pseudomonads also produce a variety of compounds via the polyketide pathway including the phytotoxin coronatine, the antibiotic mupirocin, and the antifungal compounds pyoluteorin and 2,4-diacetylphloroglucinol. This review focuses on the mode of action, regulation, biosynthesis, and genetics of these four compounds and the potential use of Pseudomonas-derived polyketide synthases in the generation of novel compounds with unique activities.

Journal Article↗

PubMed1997

Cloning and nucleotide sequence of the putative polyketide synthase genes for pradimicin biosynthesis from Actinomadura hibisca.

We cloned the putative polyketide synthase genes (pms genes) for pradimicin A biosynthesis from Actinomadura hibisca using an oligonucleotide probe designed on the basis of conserved amino acid sequences of other polyketide synthases (PKSs). By DNA sequencing of an 8.2-kb SacI fragment that hybridized with the oligonucleotide probe, 11 open reading frames (ORFs) were found. All of the ORFs except for ORF10 were predicted to be translated in the same direction. Each of the deduced ORFs has significant sequence similarity to the protein responsible for polyketide biosynthesis or spore pigmentation. In particular, ORF1, ORF2, and ORF3 were 50-70% identical with genes coding for PKSs for actinorhodin biosynthesis. Specific DNA regions similar in sequence to pms genes were found with genomic Southern hybridization in all of the pradimicin producers examined, but were not found in pradimicin nonproducers, suggesting that the genes cloned in this study encode polyketide synthase for pradimicin biosynthesis.

Actinomycetales↗

PubMed2006

[Polyketides obtained using metagenome library technique].

Polyketides are an important class of bioactive secondary metabolites. Since the recurrence of the known polyketides is very high during the discovery of novel natural polyketide compounds using traditional methods, new approaches are developed for the biosynthesis of these compounds. This review discusses discovery of new polyketides via metagenome library technique.

Biosynthetic Pathways↗