Figure 2.

General model for PKS evolution. The multidomain architecture of type I PKS modules evidently arose through the fusion of genes encoding single-domain proteins of type II systems. The processes that led to the emergence of the multimodular architecture are less well understood. For instance, it is unclear whether assembly-line systems evolved from iterative PKSs that lost their ability to perform several consecutive condensation reactions on the same polyketide chain or from a separate subset of type II proteins. Once a set of assembly-line PKSs emerged, other processes allowed further diversification of these modular enzymes and their products.