Hygromycin A, an antitreponemal substance. II. Therapeutic effect for swine dysentery

Structural modifications of the neomycin class of aminoglycosides

This review encompasses comprehensive literature on synthetic modification and biological activities of clinically used neomycin-class aminoglycoside antibiotics to alleviate dose-related toxicity and pathogenic resistance.

Distinct tRNA Accommodation Intermediates Observed on the Ribosome with the Antibiotics Hygromycin A and A201A

The increase in multi-drug-resistant bacteria is limiting the effectiveness of currently approved antibiotics, leading to a renewed interest in antibiotics with distinct chemical scaffolds. We have solved the structures of the Thermus thermophilus 70S ribosome with A-, P-, and E-site tRNAs bound and in complex with either the aminocyclitol-containing antibiotic hygromycin A (HygA) or the nucleoside antibiotic A201A. Both antibiotics bind at the peptidyl transferase center and sterically occlude the CCA-end of the A-tRNA from entering the A site of the peptidyl transferase center. Single-molecule Förster resonance energy transfer (smFRET) experiments reveal that HygA and A201A specifically interfere with full accommodation of the A-tRNA, leading to the presence of tRNA accommodation intermediates and thereby inhibiting peptide bond formation. Thus, our results provide not only insight into the mechanism of action of HygA and A201A, but also into the fundamental process of tRNA accommodation during protein synthesis.

Biosynthesis of the aminocyclitol subunit of hygromycin A in Streptomyces hygroscopicus NRRL 2388

The antibacterial activity of hygromycin A (HA) arises from protein synthesis inhibition and is dependent upon a methylenedioxy bridged-aminocyclitol moiety. Selective gene deletions and chemical complementation in Streptomyces hygroscopicus NRRL 2388 showed that the hyg18 and hyg25 gene products, proposed to generate a myo-inositol intermediate, are dispensable for HA biosynthesis but contribute to antibiotic yields. Hyg8 and Hyg17, proposed to introduce the amine functionality, are essential for HA biosynthesis. Hyg6 is a methyltransferase acting on the aminocyclitol, and a Deltahyg6 mutant produces desmethylenehygromycin A. Deletion of hyg7, a metallo-dependant hydrolase homolog gene, resulted in methoxyhygromycin A production, demonstrating that the corresponding gene product is responsible for the proposed oxidative cyclization step of methylenedioxy bridge formation. The methyl/methylene group is not required for in vitro protein synthesis inhibition but is essential for activity against Escherichia coli.

Biosynthesis of aminocyclitol-aminoglycoside antibiotics and related compounds

This review covers the biosynthesis of aminocyclitol-aminoglycoside antibiotics and related compounds, particularly from the molecular genetic perspectives. 195 references are cited.

Production of hygromycin A analogs in Streptomyces hygroscopicus NRRL 2388 through identification and manipulation of the biosynthetic gene cluster

Hygromycin A, an antibiotic produced by Streptomyces hygroscopicus NRRL 2388, offers a distinct carbon skeleton structure for development of antibacterial agents targeting the bacterial ribosomal peptidyl transferase. A 31.5 kb genomic DNA region covering the hygromycin A biosynthetic gene cluster has been identified, cloned, and sequenced. The hygromycin gene cluster has 29 ORFs which can be assigned to hygromycin A resistance as well as regulation and biosynthesis of the three key moieties of hygromycin A (5-dehydro-alpha-L-fucofuranose, (E)-3-(3,4-dihydroxyphenyl)-2-methylacrylic acid, and 2L-2-amino-2-deoxy-4,5-O-methylene-neo-inositol. The predicted Hyg26 protein has sequence homology to short-chain alcohol dehydrogenases and is assigned to the final step in production of the 5-dehydro-alpha-L-fucofuranose, catalyzing the reduction of alpha-L-fucofuranose. A hyg26 mutant strain was generated and shown to produce no hygromycin A but 5''-dihydrohygromycin A, 5''-dihydromethoxyhygromycin A, and a 5''-dihydrohygromycin A product lacking the aminocyclitol moiety. To the best of our knowledge, these shunt metabolites of biosynthetic pathway intermediates have not previously been identified. They provide insight into the ordering of the multiple unusual steps which compromise the convergent hygromycin A biosynthetic pathway.

A bleaching herbicidal activity of methoxyhygromycin (MHM) produced by an actinomycete strain Streptomyces sp. 8E-12

AIMS

To screen bioherbicidal isolates and evaluate herbicidal activity of methoxyhygromycin (MHM) produced by Streptomyces sp. 8E-12.

METHODS AND RESULTS

Streptomyces sp. 8E-12 with herbicidal activity was selected through seed germination bioassay. An active metabolite, MHM was isolated from culture broth by carbon absorption, butanol extraction, silica gel, Sephadex LH-20 and G-10 chromatography, and preparative HPLC. The metabolite was identified by electrospray ionization mass spectra (ESI-MS) and 1H- and 13C-NMR spectral data analyses. In vivo herbicidal activity was examined against weeds and crops grown on pots.

CONCLUSIONS

Streptomyces sp. 8E-12 produced a selective herbicidal metabolite which was identified as MHM. The metabolite showed stronger in vivo activity against monocotyledonous plants than dicotyledonous plants, and caused a bleaching (albino symptom) on some weeds including Digitaria sanguinalis and Echinochloa crus-galli.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results showed that Streptomyces sp. 8E-12 produced a bioherbicidal metabolite, MHM and can be developed as a biocontrol agent (BCA) for weed control.

Biosynthetic origin of hygromycin A

Hygromycin A, an antibiotic produced by Streptomyces hygroscopicus, is an inhibitor of bacterial ribosomal peptidyl transferase. The antibiotic binds to the ribosome in a distinct but overlapping manner with other antibiotics and offers a different template for generation of new agents effective against multidrug-resistant pathogens. Reported herein are the results from a series of stable-isotope-incorporation studies demonstrating the biosynthetic origins of the three distinct structural moieties which comprise hygromycin A. Incorporation of [1-(13)C]mannose and intact incorporation of D-[1,2-(13)C(2)]glucose into the 6-deoxy-5-keto-D-arabino-hexofuranose moiety are consistent with a pathway in which mannose is converted to an activated L-fucose, via a 4-keto-6-deoxy-D-mannose intermediate, with a subsequent unusual mutation of the pyranose to the corresponding furanose. The aminocyclitol moiety was labeled by D-[1,2-(13)C(2)]glucose in a manner consistent with formation of myo-inositol and a subsequent unprecedented oxidation and transamination of the C-2 hydroxyl group to generate neo-inosamine-2. Incorporation of [carboxy-(13)C]-4-hydroxybenzoic acid and intact incorporation of [2,3-(13)C(2)]propionate are consistent with a polyketide synthase-type decarboxylation condensation to generate the 3,4-dihydroxy-alpha-methylcinnamic acid moiety of hygromycin A. No labeling of hygromycin A was observed when [3-(13)C]tyrosine, [3-(13)C]phenylalanine, or [carboxy-(13)C]benzoic acid was used, suggesting that the 4-hydroxybenzoic acid is derived directly from chorismic acid. Consistent with this hypothesis was the observation that hygromycin A titers could be reduced by addition of N-(phosphonomethyl)-glycine (an inhibitor of chorismic acid biosynthesis) and restored by coaddition of 4-hydroxybenzoic acid. The convergent biosynthetic pathway established for hygromycin A offers significant versatility for applying the techniques of combinatorial and directed biosynthesis to production of new antibiotics which target the ribosomal peptidyl transferase activity.

Application of the AAA reaction to the synthesis of the furanoside of C-2-epi-hygromycin A: a total synthesis of C-2-epi-hygromycin A

A strategy for stereocontrolled syntheses of furanoside type of natural products is developed for a glycosyl aryl ether. This strategy resolves the issue of low diastereoselectivity typical of normal glycosidation methods for furanosides. All the stereochemistry ultimately derives from a desymmetrization of a 2,5-diacyloxy-2,5-dihydrofuran using Pd catalyzed asymmetric allylic alkylation which sets both the absolute stereochemistry and 1,4-relative stereochemistry. Diastereo-controlled elaboration of the 3,4-double bond then completes the synthesis. A new conjunctive reagent, 1-nitro-1-phenylsulfonyl-ethane, is developed to serve as an acyl anion equivalent. The utility of a phenol as a nucleophile in the Pd catalyzed glycosylation is demonstrated. From this strategy emerged a short, practical synthesis of C-2-epi-hygromycin A.

Asymmetric Induction of Conduritols via AAA Reactions: Synthesis of the Aminocyclohexitol of Hygromycin A

Two synthetic routes towards the construction of the aminocyclohexitol moiety of hygromycin A have been developed based on palladium-catalyzed asymmetric alkylation of conduritol derivatives. A protocol has been established whereby this biologically relevant molecule is formed from benzoquinone. A conduritol A derivative is synthesized in eight steps from benzoquinone and is then subjected to the palladium reaction. From this flexible intermediate, four epimers of the aminocyclitol, including the natural one, can be obtained with complete stereoselectivity. Racemic conduritol B derivatives are available in four steps from benzoquinone, and these are then made enantiomerically pure by a palladium-catalyzed dynamic kinetic resolution. From the chiral conduritol B, the aminocyclitol is available in six steps. Excellent levels of enantio- and diastereoselectivity highlight these strategies.