Polyketide synthase chimeras reveal key role of ketosynthase domain in chain branching.

Biosynthesis of rhizoxin in Burkholderia rhizoxinica affords an unusual polyketide synthase module with ketosynthase and branching domains that install the δ-lactone, conferring antimitotic activity. To investigate their functions in chain branching, we designed chimeric modules with structurally similar domains from a glutarimide-forming module and a dehydratase. Biochemical, kinetic and mutational analyses reveal a structural role of the accessory domains and multifarious catalytic actions of the ketosynthase.