Natural products containing the α-pyrone moiety
are produced
by polyketide synthases (PKSs) in bacteria, fungi, and plants. The
conserved biosynthetic logic for the production of the α-pyrone
moiety involves the cyclization of a triketide intermediate which
also off-loads the polyketide from the activating thioester. In this
study, we show that truncating a tetraketide natural product producing
PKS assembly line allows for a thioesterase-independent off-loading
of an α-pyrone polyketide natural product, one which we find
to be natively present in the extracts of the bacterium that otherwise
furnishes the tetraketide natural product. By engineering the truncated
PKSin vitro, we demonstrate that a ketosynthase
(KS) domain with relaxed substrate selectivity when coupled with in
trans acylation of polyketide extender units can expand the chemical
space of α-pyrone polyketide natural products. Findings from
this study point toward heterologous intermolecular protein–protein
interactions being detrimental to the efficiency of engineered PKS
assembly lines.