Genetic engineering of modular PKSs: from combinatorial biosynthesis to synthetic biology.

Multienzyme polyketide synthases (PKSs) are molecular-scale assembly lines which construct complex natural products in bacteria. The underlying modular architecture of these gigantic catalysts inspired, from the moment of their discovery, attempts to modify them by genetic engineering to produce analogues of predictable structure. These efforts have resulted in hundreds of metabolites new to nature, as detailed in this review. However, in the face of many failures, the heady days of imagining the possibilities for a truly 'combinatorial biosynthesis' of polyketides have faded. It is now more appropriate to talk about 'PKS synthetic biology' with its more modest goals of delivering specific derivatives of known structure in combination with and as a complement to synthetic chemistry approaches. The reasons for these failures will be discussed in terms of our growing understanding of the three-dimensional architectures and mechanisms of these systems. Finally, some thoughts on the future of the field will be presented.