Natural Product Discovery through Improved Functional Metagenomics in <i>Streptomyces</i>

Because the majority of environmental bacteria are not easily culturable, access to many bacterially encoded secondary metabolites will be dependent on the development of improved functional metagenomic screening methods. In this study, we examined a collection of diverse <i>Streptomyces</i> species for the best innate ability to heterologously express biosynthetic gene clusters. We then optimized methods for constructing high quality metagenomic cosmid libraries in the best <i>Streptomyces</i> host. An initial screen of a 1.5 million-membered metagenomic library constructed in <i>Streptomyces albus</i>, the species that exhibited the highest propensity for heterologous expression of gene clusters, led to the identification of the novel natural product metatricycloene (<b>1</b>). Metatricycloene is a tricyclic polyene encoded by a reductive, iterative polyketide-like gene cluster. Related gene clusters found in sequenced genomes appear to encode a largely unexplored collection of structurally diverse, polyene-based metabolites