P139: Discovery of structurally diverse metabolites from the functional analysis of soil derived type II polyketide gene clusters

A single gram of soil is predicted to contain thousands of unique bacterial species.  The majority of these species remain recalcitrant to standard culture methods, prohibiting their use as sources of novel bioactive small molecules. The cloning and analysis of DNA extracted directly from environmental samples provides a means of exploring the biosynthetic capacity of natural bacterial populations. Environmental DNA libraries should contain large reservoirs of bacterial genetic diversity from which new secondary metabolite biosynthetic gene clusters can be systematically recovered and studied. Here, we report the screening of metagenomic libraries using degenerate primers targeting genes found in the minimal type II polyketide synthases.  Recovered gene clusters were introduced into multiple Streptomyces hosts.  In these experiments S. albus J1074 was found to be the best background for heterologous expression of environmental pathways. This functional analysis revealed metabolites with novel and rare carbon skeletons. Two new metabolites show activity against methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecalis.   These results suggest that culture-independent approaches are capable of accessing biosynthetic diversity that has not yet been extensively explored using culture based methods.  Studies are underway to scale-up the recovery of functional Type II polyketide synthase containing clones from large metagenomic libraries.