The widely found fungal iterative PKS-NRPS hybrid megasynthetases are highly programmed biosynthetic machines involved in the synthesis of 3-acyltetramic acids and related natural products. In vitro analysis of iterative PKS-NRPS has been hampered by the difficulties associated with obtaining pure and functional forms of these large enzymes (>400 kDa). We successfully expressed Aspergillus nidulans aspyridone synthetase (ApdA) from an engineered Saccharomyces cerevisiae strain. The complete functions of ApdA and its enoylreductase partner ApdC are reconstituted in vitro and in S. cerevisiae with the production of preaspyridone 7. The programming rules of both the PKS and NAPS modules were then examined in vitro. The key interaction between the PKS and the NRPS was dissected and reconstituted in trans by using stand-alone modules. Analogs of 7 were synthesized through heterologous combinations of PKS and NRPS modules from different sources. Our results represent one of the largest, multidomain enzyme reconstituted to date and offer new opportunities for engineered biosynthesis of fungal natural products.