Nucleoside antibiotics. Biosynthesis of the maleimide nucleoside antibiotic, showdomycin, by Streptomyces showdoensis

Characterization of the Oxazinomycin Biosynthetic Pathway Revealing the Key Role of a Nonheme Iron-Dependent Mono-oxygenase

Oxazinomycin is a C-nucleoside natural product with antibacterial and antitumor activities. In addition to the characteristic C-glycosidic linkage shared with other C-nucleosides, oxazinomycin also features a structurally unusual 1,3-oxazine moiety, the biosynthesis of which had previously been unknown. Herein, complete in vitro reconstitution of the oxazinomycin biosynthetic pathway is described. Construction of the C-glycosidic bond between ribose 5-phosphate and an oxygen-labile pyridine heterocycle is catalyzed by the C-glycosidase OzmB and involves formation of an enzyme-substrate Schiff base intermediate. The DUF4243 family protein OzmD is shown to catalyze oxygen insertion and rearrangement of the pyridine C-nucleoside intermediate to generate the 1,3-oxazine moiety along with the elimination of cyanide. Spectroscopic analysis and mutagenesis studies indicate that OzmD is a novel nonheme iron-dependent enzyme in which the catalytic iron center is likely coordinated by four histidine residues. These results provide the first example of 1,3-oxazine biosynthesis catalyzed by an unprecedented iron-dependent mono-oxygenase.

Biosynthesis of C-nucleoside antibiotics in actinobacteria: recent advances and future developments

Epidemic diseases and antibiotic resistance are urgent threats to global health, and human is confronted with an unprecedented dilemma to conquer them by expediting development of new natural product related drugs. C-nucleoside antibiotics, a remarkable group of microbial natural products with diverse biological activities, feature a heterocycle base linked with a ribosyl moiety via an unusual C-glycosidic bond, and have played significant roles in healthcare and for plant protection. Elucidating how nature biosynthesizes such a group of antibiotics has provided the basis for engineered biosynthesis as well as targeted genome mining of more C-nucleoside antibiotics towards improved properties. In this review, we mainly summarize the recent advances on the biosynthesis of C-nucleoside antibiotics, and we also tentatively discuss the future developments on rationally accessing C-nucleoside diversities in a more efficient and economical way via synthetic biology strategies.

Identification of the C-Glycoside Synthases during Biosynthesis of the Pyrazole-C-Nucleosides Formycin and Pyrazofurin

C-Nucleosides are characterized by a C-C rather than a C-N linkage between the heterocyclic base and the ribofuranose ring. While the biosynthesis of pseudouridine-C-nucleosides has been studied, less is known about the pyrazole-C-nucleosides such as the formycins and pyrazofurin. Herein, genome screening of Streptomyces candidus NRRL 3601 led to the discovery of the pyrazofurin biosynthetic gene cluster pyf. In vitro characterization of gene product PyfQ demonstrated that it is able to catalyze formation of the C-glycoside carboxyhydroxypyrazole ribonucleotide (CHPR) from 4-hydroxy-1H-pyrazole-3,5-dicarboxylic acid and phosphoribosyl pyrophosphate (PRPP). Similarly, ForT, the PyfQ homologue in the formycin pathway, can catalyze the coupling of 4-amino-1H-pyrazole-3,5-dicarboxylic acid and PRPP to form carboxyaminopyrazole ribonucleotide. Finally, PyfP and PyfT are shown to catalyze amidation of CHPR to pyrazofurin 5'-phosphate thereby establishing the latter stages of both pyrazofurin and formycin biosynthesis.

Biosynthesis of the polyoxins, nucleoside peptide antibiotics: glutamate as an origin of 2-amino-2-deoxy-L-xylonic acid (polyoxamic acid)

The biosynthetic origin of the carbon skeleton of 2-amino-2-deoxy-L-xylonic acid (polyoxamic acid) is described. This aminoaldonic acid is the N terminus of the nucleoside peptide antibiotics, the polyoxins, produced by Streptomyces cacaoi var. asoensis. In vivo experiments concerning incorporation and distribution of radioactivity from a number of 14C-labeled compounds have clearly shown that the carbon skeleton of glutamate is a precursor for this aminoaldonic acid and sugars are incorporated only after their conversion into gluamate through the glycolytic and the tricarboxylic acid cycle pathways. Experiments utilizing [14C]-acetate and succinate have also indicated multiple passages through the Krebs cycle are operating before their incorporation into polyoxamic acid via glutamate. The distribution of 14C between C-1 and C-5 of polyoxamic acid from [5-14C]glutamate experiment has indicated that 40% of glutamate incorporated only after the reversible conversion into alpha-ketoglutarate followed by the passage through the Krebs cycle. Lack of incorporation of 3H in the [1-14C;2-3H]- and [5-14C;2-3H]glutamate experiments is discussed in terms of a reaction(s) between glutamate and polyoxamic acid.

A9145, a new adenine-containing antifungal antibiotic: fermentation

A9145 is a basic, water-soluble, antifungal antibiotic which is produced in a complex organic medium by Streptomyces griseolus. The metabolite has a molecular weight of 510, and contains adenine as well as sugar hydroxyl and amino groups. Although glucose, fructose, glucose polymers, and some long-chain fatty acid methyl esters supported biosynthesis, oils were superior, with cottonseed oil being preferred. Several ions and salts, especially Co(2+), PO(4) (3-), and CaCO(3), were stimulatory. Adenine, nucleosides, and some amino acids increased the accumulation of A9145 in shaken-flask fermentors. Enrichment of the culture medium with tyrosine afforded maximal enhancement of antibiotic production in both flask and tank fermentors. Control of the dissolved O(2) level was also critical, the optimal concentration being 3 x 10(-2) to 4.5 x 10(-2) mumole of O(2)/ml. Optimization of various fermentation parameters increased antibiotic titers approximately 135-fold in shaken flask fermentors and 225-fold in stirred vessels.