The unique structural features of marine-derived natural products, such as the abundant use of halogen atoms to create bromine, chlorine, and iodine containing natural products are but a reflection of the unique biochemical capabilities of these organisms. Through studies of the metabolic pathways used by marine cyanobacteria to create their diverse lipopeptide structures, we are gaining insights into other features of their metabolism that are novel. Using laboratory cultures of marine cyanobacteria from around the world and stable isotope-labeled precursors, such as sugars, amino acids and acetate, we have mapped out the fundamental pathways to several our most interesting natural products, such as curacin A, barbamide, hectochlorin, and the jamaicamides. In addition to revealing a number of surprising steps in the production of the aforementioned molecules, these efforts have shown that most are constructed from integrated non-ribosomal peptide synthatases and polyketide synthases.
However, to gain even deeper insight into these unique biosynthetic processes, and to capture and harness these pathways more effectively, we have added molecular genetic approaches to these studies. Knowledge of the pathways from precursor feeding experiments, especially the more unique features, has enhanced our rapid identification of the correct pathways from cosmid libraries, and shotgun cloning has rapidly allowed their efficient sequencing. Three pathways have been completely sequenced and three others are at various stages of completion. Indeed, at the genetic level, the secondary metabolite biosynthetic pathways of these ancient marine creatures are very different than any previously studied terrestrial organism. Currently, we are working on heterologous expression in E. coli andStreptomyces sp. as well as detailed mechanistic studies of the more intriguing steps. For example, as illustrated in the molluscicidal compound barbamide, how is a methyl group with three attached hydrogen atoms converted to a trichloromethyl group? What kind of enzyme catalyses this process, and mechanistically, how does it accomplish this amazing feat?