β-chain extension: a new mode of carbon-carbon bond formation and chain extension by polyketide synthases.

The PKS tree gets a new branch: Hertweck and co-workers have uncovered the enzymology behind the novel β-branching strategy used during Rhizoxin biosynthesis in Burholderia rhizoxinica. Both the KS and B domains are required for a δ-lactone to form by attack on the pre-existing polyketide chain; further chain extension must occur on the newly added β-branching carbonyl.