Functional characterizations of novWUS involved in novobiocin biosynthesis from Streptomyces spheroides

Broadened glycosylation patterning of heterologously produced erythromycin

The biosynthetic flexibility associated with the antibiotic natural product erythromycin is both remarkable and utilitarian. Product formation is marked by a modular nature where directing compound variation increasingly spans both the secondary metabolite core scaffold and adorning functionalization patterns. The resulting molecular diversity allows for chemical expansion of the native compound structural space. Accordingly, associated antibiotic bioactivity is expected to expand infectious disease treatment applications. In the enclosed work, new glycosylation patterns spanning the deoxysugars d-forosamine, d-allose, l-noviose, and d-vicenisamine were engineered within the erythromycin biosynthetic system established through an Escherichia coli heterologous production platform. The resulting analogs highlight the expanded flexibility of the erythromycin biosynthetic process. In addition, the new compounds demonstrated bioactivity against multiple Gram-positive tester strains, including erythromycin-resistant Bacillus subtilis, and limited activity against a Gram-negative bacterial target.

Non-enzymatic pyridine ring formation in the biosynthesis of the rubrolone tropolone alkaloids

The pyridine ring is a potent pharmacophore in alkaloid natural products. Nonetheless, its biosynthetic pathways are poorly understood. Rubrolones A and B are tropolone alkaloid natural products possessing a unique tetra-substituted pyridine moiety. Here, we report the gene cluster and propose a biosynthetic pathway for rubrolones, identifying a key intermediate that accumulates upon inactivation of sugar biosynthetic genes. Critically, this intermediate was converted to the aglycones of rubrolones by non-enzymatic condensation and cyclization with either ammonia or anthranilic acid to generate the respective pyridine rings. We propose that this non-enzymatic reaction occurs via hydrolysis of the key intermediate, which possesses a 1,5-dione moiety as an amine acceptor capable of cyclization. This study suggests that 1,5-dione moieties may represent a general strategy for pyridine ring biosynthesis, and more broadly highlights the utility of non-enzymatic diversification for exploring and expanding natural product chemical space.

The biosynthesis of pyridine rings is still poorly understood. Here the authors propose a biosynthetic pathway for pyridine-containing rubrolones, which is characterized by a non-enzymatic condensation and cyclization of the pyridine moiety.

Structural insight into MtmC, a bifunctional ketoreductase-methyltransferase involved in the assembly of the mithramycin trisaccharide chain

More and more post-PKS tailoring enzymes are recognized as being multifunctional and codependent on other tailoring enzymes. One of the recently discovered intriguing examples is MtmC, a bifunctional TDP-4-keto-d-olivose ketoreductase-methyltransferase, which-in codependence with glycosyltransferase MtmGIV-is a key contributor to the biosynthesis of the critical trisaccharide chain of the antitumor antibiotic mithramycin (MTM), produced by Streptomyces argillaceus. We report crystal structures of three binary complexes of MtmC with its methylation cosubstrate SAM, its coproduct SAH, and a nucleotide TDP as well as crystal structures of two ternary complexes, MtmC-SAH-TDP-4-keto-d-olivose and MtmC-SAM-TDP, in the range of 2.2-2.7 Å resolution. The structures reveal general and sugar-specific recognition and catalytic structural features of MtmC. Depending on the catalytic function that is conducted by MtmC, it must bind either NADPH or SAM in the same cofactor binding pocket. A tyrosine residue (Tyr79) appears as a lid covering the sugar moiety of the substrate during the methyl transfer reaction. This residue swings out of the active site by ~180° in the absence of the substrate. This unique conformational change likely serves to release the methylated product and, possibly, to open the active site for binding the bulkier cosubstrate NADPH prior to the reduction reaction.

Molecular architecture of a C-3'-methyltransferase involved in the biosynthesis of D-tetronitrose

S-Adenosylmethionine (SAM)-dependent methyltransferases are involved in a myriad of biological processes, including signal transduction, chromatin repair, metabolism, and biosyntheses, among others. Here we report the high-resolution structure of a novel C-3'-methyltransferase involved in the production of D-tetronitrose, an unusual sugar found attached to the antitumor agent tetrocarcin A or the antibiotic kijanimicin. Specifically, this enzyme, referred to as TcaB9 and cloned from Micromonospora chalcea, catalyzes the conversion of dTDP-3-amino-2,3,6-trideoxy-4-keto-D-glucose to dTDP-3-amino-2,3,6-trideoxy-4-keto-3-methyl-D-glucose. For this analysis, two structures were determined to 1.5 A resolution: one in which the enzyme was crystallized in the presence of SAM and dTMP and the other with the protein complexed to S-adenosylhomocysteine and its dTDP-linked sugar product. The overall fold of the monomeric enzyme can be described in terms of three domains. The N-terminal domain harbors the binding site for a zinc ion that is ligated by four cysteines. The middle domain adopts the canonical "SAM-binding" fold with a seven-stranded mixed beta-sheet flanked on either side by three alpha-helices. This domain is responsible for anchoring the SAM cofactor to the protein. Strikingly, the C-terminal domain also contains a seven-stranded beta-sheet, and it appears to be related to the middle domain by an approximate 2-fold rotational axis, thus suggesting TcaB9 arose via gene duplication. Key residues involved in sugar binding include His 181, Glu 224, His 225, and Tyr 222. Their possible roles in catalysis are discussed.

Genetic engineering of antibiotic biosynthesis for the generation of new aminocoumarins

The aminocoumarin antibiotics novobiocin, clorobiocin and coumermycin A(1) are inhibitors of gyrase and highly effective antibacterial agents. Their biosynthetic gene clusters have been cloned from the respective Streptomyces producer strains, and the function of nearly all genes contained therein has been elucidated by genetic and biochemical methods. Efficient methods have been developed for the genetic manipulation and the heterologous expression of the clusters, and more than 100 new derivatives of these antibiotics have been generated by metabolic engineering, mutasynthesis and chemoenzymatic synthesis, providing a model for the power of genetic and genomic methods for the generation of new bioactive compounds.

Biosynthesis of sibiromycin, a potent antitumor antibiotic

Pyrrolobenzodiazepines, a class of natural products produced by actinomycetes, are sequence selective DNA alkylating compounds with significant antitumor properties. Among the pyrrolo[1,4]benzodiazepines (PBDs) sibiromycin, one of two identified glycosylated PBDs, displays the highest affinity for DNA and the most potent antitumor properties. Despite the promising antitumor properties clinical trials of sibiromycin were precluded by the cardiotoxicity effect in animals attributed to the presence of the C-9 hydroxyl group. As a first step toward the development of sibiromycin analogs, we have cloned and localized the sibiromycin gene cluster to a 32.7-kb contiguous DNA region. Cluster boundaries tentatively assigned by comparative genomics were verified by gene replacement experiments. The sibiromycin gene cluster consisting of 26 open reading frames reveals a "modular" strategy in which the synthesis of the anthranilic and dihydropyrrole moieties is completed before assembly by the nonribosomal peptide synthetase enzymes. In addition, the gene cluster identified includes open reading frames encoding enzymes involved in sibirosamine biosynthesis, as well as regulatory and resistance proteins. Gene replacement and chemical complementation studies are reported to support the proposed biosynthetic pathway.

Natural-product sugar biosynthesis and enzymatic glycodiversification

Many biologically active small-molecule natural products produced by microorganisms derive their activities from sugar substituents. Changing the structures of these sugars can have a profound impact on the biological properties of the parent compounds. This realization has inspired attempts to derivatize the sugar moieties of these natural products through exploitation of the sugar biosynthetic machinery. This approach requires an understanding of the biosynthetic pathway of each target sugar and detailed mechanistic knowledge of the key enzymes. Scientists have begun to unravel the biosynthetic logic behind the assembly of many glycosylated natural products and have found that a core set of enzyme activities is mixed and matched to synthesize the diverse sugar structures observed in nature. Remarkably, many of these sugar biosynthetic enzymes and glycosyltransferases also exhibit relaxed substrate specificity. The promiscuity of these enzymes has prompted efforts to modify the sugar structures and alter the glycosylation patterns of natural products through metabolic pathway engineering and enzymatic glycodiversification. In applied biomedical research, these studies will enable the development of new glycosylation tools and generate novel glycoforms of secondary metabolites with useful biological activity.

Biosynthesis of the unusual 5,5-gem-dimethyl-deoxysugar noviose: investigation of the C-methyltransferase gene cloU

The aminocoumarin antibiotic clorobiocin contains an unusual branched deoxysugar with a 5,5-gem-dimethyl structure. Inactivation of the putative C-methyltransferase gene cloU was carried out, which led to the loss of the axial methyl group at C-5 of this deoxysugar moiety. This result establishes the function of cloU, and at the same time it proves that the biosynthesis of the deoxysugar moiety of clorobiocin proceeds via a 3,5-epimerization of the dTDP-4-keto-6-deoxyglucose intermediate. The inactivation was carried out on a cosmid which contained the entire clorobiocin biosynthetic gene cluster. Expression of the modified cluster in a heterologous host led to the formation of desmethyl-clorobiocin and a structural isomer thereof. Both compounds were isolated on a preparative scale, their structures were elucidated by 1H-NMR and mass spectroscopy and their antibacterial activity was assayed.

Characterisation of Streptomyces spheroides NovW and revision of its functional assignment to a dTDP-6-deoxy-d-xylo-4-hexulose 3-epimerase

Characterisation of recombinant Streptomyces spheroides NovW in vitro suggests that it is not a kinetically competent dual action dTDP-6-deoxy-d-xylo-4-hexulose 3,5-epimerase, but possesses only significant 3-epimerase activity.

Acyl Transfer in Clorobiocin Biosynthesis: Involvement of Several Proteins in the Transfer of the Pyrrole-2-carboxyl Moiety to the Deoxysugar

Clorobiocin is an aminocoumarin antibiotic containing a pyrrole-2-carboxyl moiety, attached through an ester bond to a deoxysugar. The pyrrole moiety is important for the binding of the antibiotic to its biological target, gyrase. The complete biosynthetic gene cluster for clorobiocin has been cloned and sequenced from the natural producer, Streptomyces roseochromogenes DS 12.976. In this study, the genes cloN1 and cloN7 were deleted separately from a cosmid containing the complete clorobiocin cluster. The modified cosmids were introduced into the genome of the heterologous host Streptomyces coelicolor M512 by using the integration functions of the PhiC31 phage. While a heterologous producer strain harbouring the intact clorobiocin biosynthetic gene cluster accumulated clorobiocin, the cloN1- and cloN7-defective integration mutants accumulated a clorobiocin derivative that lacked the pyrrole-2-carboxyl moiety, while also producing free pyrrole-2-carboxylic acid. The structures of these metabolites were confirmed by NMR and MS analysis. These results showed that CloN1 and CloN7, together with the previously investigated CloN2, are involved in the transfer of the pyrrole-2-carboxyl moiety to the deoxysugar of clorobiocin. A possible mechanism for the role of these three proteins in the acyl-transfer process is suggested.