The emergence of resistant strains of human pathogens to current antibiotics, along with the demonstrated ability of polyketides as antimicrobial agents, provides strong motivation for understanding how polyketide antibiotics have evolved and diversified in nature. Insights into how bacterial polyketide synthases (PKSs) acquire new metabolic capabilities can guide future laboratory efforts in generating the next generation of polyketide antibiotics. Here, we examine phylogenetic and structural evidence to glean answers to two general questions regarding PKS evolution. How did the exceptionally diverse chemistry of present-day PKSs evolve? And what are the take-home messages for the biosynthetic engineer?biosynthesis ͉ metabolism ͉ engineering P olyketides are a large family of medicinally important natural products, which are formed through the condensation of acylthioester units such as malonyl-CoA and methylmalonyl-CoA to yield metabolites with diverse structures and biological activities. Broadly speaking, there are three separate types of polyketide synthases (PKSs) recognized in bacteria. Multimodular PKSs consist of one or more large multidomain polypeptides where the growing polyketide chain is sequentially passed from one active site to the next. Depending on the nature of their constituent catalytic domains, these megasynthases generate chemical variety and complexity in a stepwise fashion (reviewed in ref. 1). In contrast, iterative PKSs are comprised of a single set of catalysts that assemble a polyketide of controlled chain length through repetitive use of active sites (reviewed in ref.2). In both cases, the nascent polyketide product is frequently acted on by further tailoring enzymes to generate the antibiotic. A third type of PKS (called type III PKSs) is fundamentally different in that the growing polyketide chain is never directly attached to a protein (reviewed in ref.3). This article focuses on the evolution of only the first two PKS classes.Bacteria, in particular Actinomycetes and Cyanobacteria, are prolific sources of polyketides, many of which possess antibiotic activity. Erythromycin, tetracycline, and amphotericin B are three well known examples of antimicrobial warfare agents from this group of bacteria that have been found useful for treating human diseases. Polyketides have also been discovered that play other roles in the environment other than to defeat microbial competitors. One such polyketide, mycolactone, is a pathogenesis-enabling immunosuppressant produced by the bacterium Mycobacterium ulcerans. This human pathogen is the causative agent of Buruli ulcer, but M. ulcerans mycolactone-negative mutants are avirulent. Addition of mycolactone to the mycolactone-negative mutants (chemical complementation) restores virulence (4). Other polyketides have been discovered that support a symbiotic relationship. One such compound, rhizoxin, is produced by a Burkholderia sp. symbiont of the fungus Rhizopus sp. Without the symbiont, the fungus cannot make the polyketide that functions to inh...