Schematic diagram depicting the activity of type II PKS domains with actinorhodin biosynthesis as an example. Heterodimeric KS and CLF domains catalyze chain initiation and elongation through decarboxylative condensation of malonyl building blocks, an ACP domain delivers malonyl building blocks to the KS-CLF, and a MCAT domain supplies malonyl groups to the ACP domain. The collective action of these type II PKS domains lead to the formation of highly reactive poly-β-keto intermediates. This nascent polyketide chain is modified into a specific folding pattern by tailoring enzyme domains such as those of KR, ARO, and CYC. The KR domain reduces carbonyl group at a specific position of the polyketide chain, and the ARO and CYC domains control chain folding by catalyzing one or more regiospecific cyclization in the polyketide chain. Whereafter polyketide chain is modified by various tailoring enzymes into actinorhodin.