Azicemicin B

A new structural class of the antibiotic, azicemicins A (1) and B (2) were isolated from the culture broth of the strain MJ126-NF4, which was closely related to Amycolatopsis sulphurea. They were purified by adsorption on Diaion HP-20, silica gel column chromatography and preparative TLC. The molecular formulas of 1 and 2 were determined to be C23H25O9N and C22H23O9N by HRFAB-MS, respectively. Azicemicins A and B have moderate growth inhibiting activity against Gram-positive bacteria and mycobacteria.

A new structural class of antibiotics, azicemicins A (1) and B (2) were isolated from the culture broth of Amycolatopsis sp. MJ126-NF4. Their structures were elucidated from their physico-chemical properties, various NMR experiments and chemical transformations and were shown to be 3-(1-methyl-2-aziridinyl)- and 3-(2-aziridinyl)-3,4-dihydro-3,7,8,10,12b-pentahydroxy-9,12-dimeth oxy-benz [a]anthracene-1,6(2H,5H)-dione, respectively.

The azicemicins, which are angucycline-type antibiotics produced by the actinomycete, Kibdelosporangium sp. MJ126-NF4, contain an aziridine ring attached to the polyketide core. Feeding experiments using [1-(13)C]acetate or [1,2-(13)C(2)]acetate indicated that the angucycline skeleton is biosynthesized by a type II polyketide synthase. Isotope-tracer experiments using deuterium-labeled amino acids revealed that aspartic acid is the precursor of the aziridine moiety. Subsequent cloning and sequencing efforts led to the identification of the azicemicin (azic) gene cluster spanning approximately 50 kbp. The cluster harbors genes typical for type II polyketide synthesis. Also contained in the cluster are genes for two adenylyl transferases, a decarboxylase, an additional acyl carrier protein (ACP), and several oxygenases. On the basis of the assigned functions of these genes, a possible pathway for aziridine ring formation in the azecimicins can now be proposed. To obtain support for the proposed biosynthetic pathway, two genes encoding adenylyltransferases were overexpressed and the resulting proteins were purified. Enzyme assays showed that one of the adenylyltransferases specifically recognizes aspartic acid, providing strong evidence, in addition to the feeding experiments, that aspartate is the precursor of the aziridine moiety. The results reported herein set the stage for future biochemical studies of aziridine biosynthesis and assembly.