Post-PKS Tailoring Steps of the Spiramycin Macrolactone Ring in Streptomyces ambofaciens

Abstract

dSpiramycins are clinically important 16-member macrolide antibiotics produced by Streptomyces ambofaciens. Biosynthetic studies have established that the earliest lactonic intermediate in spiramycin biosynthesis, the macrolactone platenolide I, is synthesized by a type I modular polyketide synthase (PKS). Platenolide I then undergoes a series of post-PKS tailoring reactions yielding the final products, spiramycins I, II, and III. We recently characterized the post-PKS glycosylation steps of spiramycin biosynthesis in S. ambofaciens. We showed that three glycosyltransferases, Srm5, Srm29, and Srm38, catalyze the successive attachment of the three carbohydrates mycaminose, forosamine, and mycarose, respectively, with the help of two auxiliary proteins, Srm6 and Srm28. However, the enzymes responsible for the other tailoring steps, namely, the C-19 methyl group oxidation, the C-9 keto group reduction, and the C-3 hydroxyl group acylation, as well as the timing of the post-PKS tailoring reactions, remained to be established. In this study, we show that Srm13, a cytochrome P450, catalyzes the oxidation of the C-19 methyl group into a formyl group and that Srm26 catalyzes the reduction of the C-9 keto group, and we propose a timeline for spiramycin-biosynthetic post-PKS tailoring reactions.

Evolutionary stability of antibiotic protection in a defensive symbiosis

“…After the release from the PKS protein, the polyketide is usually subjected to several post-PKS tailoring steps, some of which are encoded within the PKS gene cluster, like two methyltransferases, one aromatase, a monooxygenase, and an asparagin-synthase-like enzyme in the case of the piericidin cluster (40,41). Other enzymes involved in post-PKS modifications are often encoded by separate genes, yielding additional diversity in final compounds (65)(66)(67).…”

Genome mining of Streptomyces ambofaciens

“…All genes required for the synthesis of the three sugars mycaminose, forosamine and mycarose from glucose-1-phosphate are present in the spiramycin gene cluster. We recently identified the genes involved in most post-PKS tailoring steps [49,50]. On the basis of gene deletions and LC-MS-MS analyses, we proposed the following timeline for these various steps: platenolide I is first reduced at the C 9 position by Srm26.…”

Direct Synthesis of Nitriles from Aldehydes Using an O-Benzoyl Hydroxylamine (BHA) as the Nitrogen Source

“…A functional-group-enriched macrolide antibiotic, named spiramycin, which is widely used in clinics to treat toxoplasmosis and other soft tissue infections was employed to demonstrate the practicability of this new method (Scheme ). Commercially available spiramycin ( 10 , 1.69 g) was treated with 1b (1.5 equiv) and TFA (30 mol %) in DME at 80 °C to furnish the corresponding nitrile 11 (1.11 g) in a 66% yield without flash column chromatography.…”