Genetic approaches for controlling ratios of related polyketide products in fermentation processes

Simple acyl thioesters are used as precursors for both the initiation and elongation steps in polyketide biosynthetic processes. Several structurally related polyketide products are sometimes made in these processes. These analogs are typically generated by a combination of two factors: availability of structurally similar biosynthetic precursors, and biosynthetic enzymes unable to effectively discriminate between them. Often, only one polyketide product Is desired from a fermentation process, requiring a method to control the ratio of these different analogs. Preferential production of one desired analog is accomplished using random mutagenesis and manipulation of fermentation conditions. A genetic enzymatic understanding of polyketide biosynthesis, as well as the pathways that provide the relevant precursors, allows for a rational and more contemporary approach for control of analogs produced in fermentation processes. This approach involves genetic manipulation of either the pathways that provide pools of the acyl CoA thioester precursors, or the function/specificity of the appropriate biosynthetic enzymes. Reviewed herein are three such examples where these approaches have been carried out successfully with polyketide biosynthetic processes.