Ansatrienins are a group of ansamycins with an N-cyclohexanoyl d-alanyl side chain. Though ansatrienins have been identified for decades, the mechanism for the addition of this unique side chain was not established. Here, we report the biochemical characterization of a tridomain nonribosomal peptide synthetase (NRPS), AstC, and an N-acyltransferase, AstF1, encoded in the biosynthetic pathway of ansatrienins. We demonstrate that AstC can efficiently catalyze the transfer of d-alanine to the C-11 hydroxyl group of ansatrienins, and AstF1 is able to attach the cyclohexanoyl group to the amino group of d-alanine. Remarkably, AstC presents the first example that a modular NRPS can catalyze intermolecular d-alanylation of the hydroxyl group to form an ester bond, though alanyl natural products have been known for decades. In addition, both AstC and AstF1 have broad substrate specificity toward acyl donors, which can be utilized to create novel ansatrienins.

Genome mining is a rational approach to discovering new natural products. The genome sequence analysis of Streptomyces sp. LZ35 revealed the presence of a putative ansamycin gene cluster (nam). Constitutive overexpression of the pathway-specific transcriptional regulatory gene nam1 successfully activated the nam gene cluster, and three novel naphthalenic octaketide ansamycins were discovered with unprecedented n-pentylmalonyl-CoA or n-butylmalonyl-CoA extender units. This study represents the first example of discovering novel ansamycin scaffolds via activation of a cryptic gene cluster.

Ansamycins is a group of type I polyketides characterized by the unique starter unit 3-amino-5-hydroxybenzoic acid. This family of secondary metabolites shows diverse biological activities, well-known members of which include rifamycin, geldanamycin, and maytansine. Previously, we isolated an AHBA synthase gene-positive strain Streptomyces sp. XZQH13 containing a “silent” ansamycin biosynthetic gene cluster ast. The constitutive expression of the Large-ATP-binding regulators of the LuxR family regulator gene astG1 located within the cluster triggered the expression of the biosynthetic genes. Reverse transcription-PCR experiments showed that the expression of the key biosynthetic genes, astB4, astD1, and astF1, was induced in the astG1 overexpression mutant compared to the wild type. This led to the isolation of two known ansatrienins, hydroxymycotrienin A (1) and thiazinotrienomycin G (2), which were identified by analysis of the mass spectral and NMR spectral data, from the mutant. These observations suggest that astG1 is probably a pathway-specific positive regulator for the biosynthesis of ansatrienin.

Six hygrocins, polyketides of ansamycin class, were isolated from the gdmAI-disrupted Streptomyces sp. LZ35. The planar structure of hygrocins C–E (1–3) was determined by one-dimensional and two-dimensional NMR spectroscopy and high-resolution mass spectrometry. They are derivatives of hygrocin A but differ in the configuration at C-2 and the orientation of the C-3,4 double bond. Hygrocin F(4) and G(5) were shown to be isomers of hygrocin C (1) and B (6), respectively, due to the different alkyl oxygen participating in the macrolide ester linkage. Hygrocins C, D, and F were found to be toxic to human breast cancer MDA-MB-431 cells (IC50 = 0.5, 3.0, and 3.3 μM, respectively) and prostate cancer PC3 cells (IC50 = 1.9, 5.0, and 4.5 μM, respectively), while hygrocins B, E, and G were inactive.