| dc.description | The most widely studied of all secondary metabolites, the polyketides (PKs), are low molecular weight compounds assembled via sequential condensations of small carboxylic acids. Biosynthetic and molecular genetic knowledge pertaining to PK production, mostly from studies of terrestrial microbes, have revealed the primary role of polyketide synthase (PKS) enzymes, which condense carboxylic acids to form highly functionalized metabolites. Studies on the biosynthesis of PKs produced by marine species revealed more complex pathways than those encountered in terrestrial species. In this study, we isolated PKS gene fragments from marine isolates of filamentous fungi via degenerate PCR. Phylogenetic comparison with corresponding sequences from terrestrial-derived fungi revealed a dichotomy, (consistent with a previous study involving terrestrial fungi only), between PKSs producing non-reduced (NR) PKs and those producing partially reduced (PR) PKs, but no clear division between sequences from marine and terrestrial fungi. PCR amplification of a PR-type gene fragment from a marine isolate of Apiospora montagnei is consistent with the production of an aromatic compound, papulacandin B (PapB) by this species. We have established that PapB is assembled via a PK pathway by way of experiments involving stable isotope precursor incorporation in conjunction with 13C NMR spectroscopy. We extended the investigation to the biosynthesis of PKs of marine dinoflagellates. Although attempts to isolate PKS genes from dinoflagellate DNA or a constructed cDNA library were unsuccessful, stable isotope incorporation studies of DTX-5, a putative PK produced by the dinoflagellate Prorocentrum maculosum, (1) established the PK mechanism of its biosynthesis, (2) revealed a rare carbon deletion step and (3) demonstrated the incorporation of an intact amino acid into the structure. Mechanisms are proposed for the biosyntheses of PapB and DTX-5. | en_US |
