Strategies for combinatorial biosynthesis with modular polyketide synthases

Polyketides are assembled by the polyketide synthases (PKS) through a common mechanism, the condensation of small carboxylic acids. However, a large structural variety exists within these molecules, paralleled by their different bioactivities. Structural differences in polyketides mostly stem from variations in the number of elongation cycles, in the extender unit incorporated and the extent of processing occurring during each cycle. A significant fraction of polyketides is made in bacteria by modular PKSs, which direct polyketide synthesis on a protein template, where each module is responsible for selecting, incorporating and processing the appropriate carboxylate unit. Since their discovery in the early nineties, the architecture of modular PKSs and their modus operandi have attracted efforts by several laboratories to reprogram PKSs to produce tailor-made polyketides. The availability of a growing number of modular PKSs of defined sequence, and of well-developed model systems for the in vitro and in vivo analysis of these enzymes, has led to the successful production of many novel polyketides after genetic manipulation of the appropriate PKS. We discuss the different strategies that are followed for the construction of functional "hybrid" systems, with particular emphasis on what can be done in terms of generating chemical diversity, highlighting also the limitations of our current understanding. The prospects of generating novel useful polyketides by genetic engineering are also discussed.