| Structurally diverse polyketides are a class of small molecular weight natural products produced by bacteria, fungi and plants, which are widely used in human and veterinary medicine, agriculture and animal nutrition. In this research, type I polyketide antibiotic biosynthesis was studied at the levels of initiation, extension/termination and post-polyketide modification. Streptomyces collinus produces two type I polyketides, ansatrienin and naphthomycin, using a common starter unit 3-amino-5-hydroxybenzoic acid (AHBA). The gene AHBA synthase was used to identify two separate AHBA biosynthetic gene clusters in S. collinus. Each cluster is adjacent to a type I polyketide synthase (PKS) gene cluster. These gene clusters encode proteins responsible for loading the AHBA onto the polyketide synthase and for catalyzing unusual macrolactam formation at the end of polyketide chain extension. The putative ansatrienin biosynthetic gene cluster also contains genes involved in the cyclohexanecarboxylic acid biosynthetic pathway used in the formation of the ansatrienin side chain.;Pikromycin/methymycin production in Streptomyces venezuelae was used to study unusual extension and termination mechanisms. The terminal module PikAIV plays a pivotal role in producing two different ring size polyketide products. A series of expression plasmid and chromosomal mutants carrying genetically altered pikAIV were constructed. The presence of mutated PikAIV and heterodimeric PikAIV significantly varied the relative efficiency of polyketide chain extension/termination processes, providing a new insight into functional molecular interaction and interdomain recognition in modular PKSs. The pikAV gene encoding a thioesterase II (TEII) enzyme has an unknown function in polyketide metabolism. A pikAV mutant was constructed and found to produce normal amounts of polyketides, but only in aglycone forms. Subsequent sequence alignment of TEIIs from different sources suggested a promotor region inside the pikAV. This region deleted in the mutant presumably is responsible for initiating transcription of critical sugar biosynthesis genes in the pik cluster. Three different genetic approaches were used to demonstrate the proposed promotor activity of pikAV. The functional role of TEII enzyme itself, however, remains to be determined. |
