Isolation and Biosynthesis of Phenazine-Polyketide Hybrids from Streptomyces sp. KIB-H483.

A phenazine-polyketide hybrid compound, nexphenazine A (1), was isolated from Streptomyces sp. KIB-H483. The bioinformatic analysis of the draft genome of the producing strain and gene inactivation experiments revealed that the biosynthesis of 1 involves a phenazine-polyketide hybrid gene cluster. The abolished production of 1 as well as the accumulation of shunt metabolites 4-7 in mutant strain ΔnpzI revealed the key role of the npzI gene, which encodes an NAD(P)H-dependent ketoreductase, in nexphenazine biosynthesis. The structures and absolute configurations of the isolated intermediates were established on the basis of spectroscopic data analysis, single-crystal X-ray diffraction, chiral chromatography, and chemical conversion experiments. NpzI exhibited stereochemical selectivity in reducing the carbonyl group of 4. Nexphenazine biosynthesis is proposed to involve a condensation of the carboxyl group of phenazine with one molecule of methylmalonyl-CoA by a type I PKS, followed by a ketone reduction by NpzI and an unknown methylation reaction.

Mono-/Bis-Alkenoic Acid Derivatives From an Endophytic Fungus Scopulariopsis candelabrum and Their Antifungal Activity.

During a screening for antifungal secondary metabolites, six new mono-/bis-alkenoic acid derivatives (2-7) and one known alkenoic acid derivative (1) were isolated from an endophytic fungi Scopulariopsis candelabrum. Their chemical structures were identified by 1H-NMR, 13C-NMR, 2D NMR, and high-resolution mass spectrometry, as well as comparisons with previously reported literatures. Among them, fusariumesters C‒F (2-5) are bis-alkenoic acid derivatives dimerized by an ester bond, while acetylfusaridioic acid A (6) and fusaridioic acid D (7) are alkenoic acid monomers. All the isolates were submitted to an antifungal assay against Candida albicans and the corn pathogen Exserohilum turcicum using the filter paper agar diffusion method. As a result, only compound 1 decorating with ß-lactone ring turned out to be active against these two tested fungi. The broth microdilution assay against Candida albicans showed the minimum inhibitory concentration (MIC) value of 1 to be 20 µg/ml, while the minimum inhibitory concentration value of the positive control (naystatin) was 10 µg/ml. And the half maximal inhibitory concentration (IC50) value (21.23 µg/ml) of 1 against Exserohilum turcicum was determined by analyzing its inhibition effect on the mycelial growth, using cycloheximide (IC50 = 46.70 µg/ml) as the positive control.