Pradimicins A-C (1–3) are a group of antifungal and antiviral polyketides from Actinomadura hibisca. The sugar moieties in pradimicins are required for their biological activities. Consequently, the 5-OH that is used for glycosylation plays a critical role in pradimicin biosynthesis. A cytochrome P450 monooxygenase gene, pdmJ, was amplified from the genomic DNA of A. hibisca and expressed in Escherichia coli BL21(DE3). PdmJ introduced a hydroxyl group to G-2A (4), a key pradimicin biosynthetic intermediate, at C-5 to form JX134 (5). A D-Ala-containing pradimicin analog, JX137a (6) was tested as an alternative substrate, but no product was detected by LC-MS, indicating that PdmJ has strict substrate specificity. Kinetic studies revealed a typical substrate inhibition of PdmJ activity. The optimal substrate concentration for the highest velocity is 115 μM under the test conditions. Moreover, the conversion rate of 4 to 5 was reduced by the presence of 6, likely due to competitive inhibition. Coexpression of PdmJ and a glucose 1-dehydrogenase in E. coli BL21(DE3) provides an efficient method to produce the important intermediate 5 from 4.

Three key tailoring enzymes PdmJ, PdmW and PdmN in pradimicin biosynthesis were investigated. PdmW was characterized as the C-6 hydroxylase by structural characterization of the corresponding product 6-hydroxy-G-2A. The efficiencies of the C-5 and C-6 hydroxylations catalyzed respectively by PdmJ and PdmW were low when they were expressed individually with the early biosynthetic enzymes that form G-2A. When these two cytochrome P450 enzymes were co-expressed, a dihydroxylated product 5,6-dihydroxy-G-2A was efficiently produced, indicating that these two enzymes work synergistically in pradimicin biosynthesis. Heterologously expressed PdmN in Streptomyces coelicolor CH999 converted G-2A to JX137a by ligating a unit of D-alanine to the carboxyl group. PdmN has relaxed substrate specificity toward both amino acid donors and acceptors. Through combinatorial biosynthesis, a series of new pradimicin analogues were produced.

Pradimicins are antifungal and antiviral natural products from Actinomadura hibisca P157-2. The sugar moieties play a critical role in the biological activities of these compounds. There are two glycosyltransferase genes in the pradimicin biosynthetic gene cluster, pdmS and pdmQ, which are putatively responsible for the introduction of the sugar moieties during pradimicin biosynthesis. In this study, we disrupted these two genes using a double crossover approach. Disruption of pdmS led to the production of pradimicinone I, the aglycon of pradimicin A, which confirmed that PdmS is the O-glycosyltransferase responsible for the first glycosylation step and attaching the 4',6'-dideoxy-4'-amino-D-galactose or 4',6'-dideoxy-4'-methylamino-D-galactose moiety to the 5-OH. Disruption of pdmQ resulted in the production of pradimicin B, indicating that this enzyme is the second glycosyltransferase that introduces the D-xylose moiety to the 3'- OH of the first sugar moiety. Insertion of an integrative plasmid before pdmO might have interfered with the dedicated promoter, yielding a mutant that produces pradimicin C as the major metabolite, which suggested that PdmO is the enzyme that specifically methylates the 4'- NH2 of the 4',6'-dideoxy-4'-amino-D-galactose moiety. Functional characterization of these sugar-decorating and –incorporating enzymes thus facilitates the understanding of the pradimicin biosynthetic pathway.

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