Stambomycin A

Many polyketide antibiotics contain macrolactones that arise from polyketide synthase chain release via thioesterase (TE) domain-catalyzed macrolactonization. The hydroxyl groups utilized in such macrolactonization reactions typically derive from reduction of β-ketothioester intermediates in polyketide chain assembly. The stambomycins are a group of novel macrolide antibiotics with promising anticancer activity that we recently discovered via rational activation of a silent polyketide biosynthetic gene cluster in Streptomyces ambofaciens. Here we report that the hydroxyl group utilized for formation of the macrolactone in the stambomycins is derived from cytochrome P450-catalyzed hydroxylation of the polyketide chain rather than keto reduction during chain assembly. This is a novel mechanism for macrolactone formation in polyketide antibiotic biosynthesis.

There is a constant need for new and improved drugs to combat infectious diseases, cancer, and other major life-threatening conditions. The recent development of genomics-guided approaches for novel natural product discovery has stimulated renewed interest in the search for natural product-based drugs. Genome sequence analysis of Streptomyces ambofaciens ATCC23877 has revealed numerous secondary metabolite biosynthetic gene clusters, including a giant type I modular polyketide synthase (PKS) gene cluster, which is composed of 25 genes (nine of which encode PKSs) and spans almost 150 kb, making it one of the largest polyketide biosynthetic gene clusters described to date. The metabolic product(s) of this gene cluster are unknown, and transcriptional analyses showed that it is not expressed under laboratory growth conditions. The constitutive expression of a regulatory gene within the cluster, encoding a protein that is similar to Large ATP binding of the LuxR (LAL) family proteins, triggered the expression of the biosynthetic genes. This led to the identification of four 51-membered glycosylated macrolides, named stambomycins A-D as metabolic products of the gene cluster. The structures of these compounds imply several interesting biosynthetic features, including incorporation of unusual extender units into the polyketide chain and in trans hydroxylation of the growing polyketide chain to provide the hydroxyl group for macrolide formation. Interestingly, the stambomycins possess promising antiproliferative activity against human cancer cell lines. Database searches identify genes encoding LAL regulators within numerous cryptic biosynthetic gene clusters in actinomycete genomes, suggesting that constitutive expression of such pathway-specific activators represents a powerful approach for novel bioactive natural product discovery.

This review highlights cave habitats, cave microbiomes and their potential for drug discovery. Such studies face many challenges, including access to remote and pristine caves, and sample collection and transport. Inappropriate physical and chemical growth conditions in the laboratory for the isolation and cultivation of cave microorganisms pose many complications including length of cultivation; some cave microorganisms can take weeks and even months to grow. Additionally, DNA extraction from cave environmental samples may be difficult due to the high concentration of various minerals that are natural DNA blocking agents. Once cave microorganisms are grown in the lab, other problems often arise, such as maintenance of pure culture, consistency of antimicrobial activity and fermentation conditions for antimicrobial production. In this review, we suggest that, although based on what has been done in the field, there is potential in using cave microorganisms to produce antimicrobial agents, one needs to be highly committed and prepared.