Chemistry, Biochemistry and Genetics of Antibiotics Biosynthesis in Actinomycetes
Microorganisms produce a large variety of biologically active substances representing a vast diversity of fascinating molecular architecture not available in any other systems. Our research centers on the chemistry, biochemistry and genetics of the biosynthesis of these secondary metabolites. Our method blends organic chemistry, biochemistry, and molecular biology. Our objectives are to understand the secondary metabolism by asking the following questions: which reactions are available in nature, what are the enzymatic mechanisms of these reactions, how they are linked to produce complex structures, and what are the regulatory mechanisms of these pathways. Our ultimate goal is to manipulate nature's biosynthetic machinery for the discovery of new drugs and the improvement of the existing ones. Currently we are studying two types of molecules: polyketides such as thermorubin produced by Thermoactinomyces vulgaris and peptides such as the bleomycins produced by Streptomyces vercitillus.
Engineered biosynthesis of peptide/polyketide-derived antibiotics:
Anticancer drug bleomycin, produced by Streptomyces verticillus, is a natural hybrid metabolite of peptide and polyketide biosynthesis and the bleomycin peptide/polyketide backbone is assembled by the bleomycin synthetase that should bear the characteristics of both peptide synthetase and polyketide synthase (PKS). Therefore, understanding the biosynthesis of bleomycins should shed light on the mechanism of how peptide synthetase and PKS could be integrated into a biosynthetic machinery to make complex natural products. We have recently cloned several S. verticillus DNA fragments encoding the putative bleomycin synthetase genes and further characterizations of the entire bleomycin biosynthesis gene cluster by gene walking, gene expression, gene inactivation, and biosynthetic pathway engineering are under vigorous investigation. The outcome of our studies could potentially lead to the discovery of new anticancer drugs or impact directly on the production of bleomycin at lower cost through modifications of the bleomycin biosynthetic genes, and will lay the foundation for rational designing of hybrid polyketide synthase/peptide synthetase enzymes from other peptide and polyketide pathways for the biosynthesis of novel classes of natural products from amino acids and short fatty acids.
Polyketide biosynthesis in thermophilic microorganisms:
Thermorubin, a polyketide antibiotic produced by a thermphilic actinomycete Thermoactinomyces vulgaris, is assembled from one molecule of salicylic acid and eleven molecules of malonyl CoA, presumably catalyzed by the thermorubin PKS. We are currently cloning the thermorubin PKS genes by DNA hybridization with homologous PKS genes and by polymerase chain reactions with primers designed according to the conserved PKS sequences. Our long-term goal is to study the biosynthesis of thermorubin in T. vulgaris as a model for polyketide biosynthesis in thermophilic microorganisms and to explore the potential to use thermostable enzymes in engineered biosynthesis. Since engineered PKS has already demonstrated a great potential in making novel compounds and the thermorubin PKS utilizes a salicylic acid as the starter unit and is capable of catalyzing polyketide biosynthesis at an elevated temperature, the outcome of this research promises to reveal new insight about polyketide biosynthesis, to provide thermostable proteins to facilitate the biochemical and enzymological characterization of PKS complex, and to open access to novel polyketides via engineered PKS complexes.