Paulomycin A

Paulomycins (PAUs) refer to a group of glycosylated antibiotics with attractive antibacterial activities against Gram-positive bacteria. They contain a special ring A moiety that is prone to dehydrate between C-4 and C-5 to a quinone-type form at acidic condition, which will reduce the antibacterial activities of PAUs significantly. Elucidation of the biosynthetic mechanism of the ring A moiety may facilitate its structure modifications by combinatorial biosynthesis to generate PAU analogues with enhanced bioactivity or stability. Previous studies showed that the ring A moiety is derived from chorismate, which is converted to 3-hydroxyanthranilic acid (3-HAA) by a 2-amino-2-deoxyisochorismate (ADIC) synthase, a 2,3-dihydro-3-hydroxyanthranilic acid (DHHA) synthase, and a DHHA dehydrogenase. Unfortunately, little is known about the conversion process from 3-HAA to the highly decorated ring A moiety of PAUs. In this work, we characterized Pau17 as an unprecedented 3-HAA 6-hydroxylase responsible for the conversion of 3-HAA to 3,6-DHAA by in vivo and in vitro studies, pushing one step forward toward elucidating the biosynthetic mechanism of the ring A moiety of PAUs.

Four novel paulomycin derivatives have been isolated from S. albus J1074 grown in MFE culture medium. These compounds are structural analogs of antibiotics 273a2α and 273a2β containing a thiazole moiety, probably originated through an intramolecular Michael addition. The novel, thiazole, moiety-containing paulomycins show a lower antibiotic activity than paulomycins A and B against Gram-positive bacteria. However, two of them show an improved activity against Gram-negative bacteria. In addition, the four novel compounds are more stable in culture than paulomycins A and B. Thus, the presence of an N-acetyl-l-cysteine moiety linked to the carbon atom of the paulic acid isothiocyanate moiety, via a thioester bond, and the subsequent intramolecular cyclization of the paulic acid to generate a thiazole heterocycle confer to paulomycins a higher structural stability that otherwise will conduce to paulomycin degradation and into inactive paulomenols.

The interspecies communication roles of γ-butyrolactones (GBLs) have been described for a long time but are still poorly understood. Herein, we analyzed more than 1000 Streptomyces strains and noticed a big quantitative gap between the strains with GBL biosynthetic genes and the strains with GBL receptor genes, which implies the wide-spread of GBLs as interspecies signals in Streptomyces and their great potential in the activation of silent natural product gene clusters. Streptomyces albidoflavus J1074, which has one GBL receptor gene but no GBL biosynthetic gene, was chosen as a target to study the possible interspecies communication roles of GBLs. At first, the GBL biosynthetic genes from Streptomyces coelicolor M145 were expressed in S. albidoflavus J1074, which enabled the S. albidoflavus strains to synthesize Streptomyces coelicolor butanolides (SCBs) and activated the production of paulomycins. Further studies showed that this activation process requires the participation of the GBL receptor gene XNR_4681. The results suggest that the expression of exogenous GBL biosynthetic genes can modulate the metabolisms of GBL non-producing strains, and this regulation role might be meaningful for silent gene cluster activation in Streptomyces. At final, we synthesized racemic-SCB2 and tried to simplify the activation process by adding SCB2 directly to S. albidoflavus J1074, which unfortunately failed to induce paulomycin production.