Abstract
Biochemical studies of purified and dissected fungal polyketide synthase and nonribosomal peptide synthetase (PKS-NRPS) hybrid enzymes involved in biosynthesis of pseurotin and aspyridone indicate that one α-methylation step during polyketide synthesis is a prerequisite and a key checkpoint for chain transfer between PKS and NRPS modules. In the absence of the resulting γ-methyl feature, the completed polyketide intermediate is offloaded as an α-pyrone instead of being aminoacylated by the NRPS domain. These examples illustrate that precisely timed tailoring domain activities play critical roles in the overall programming of the iterative PKS (and NRPS) functions.