Partial In Vitro Reconstitution of an Orphan Polyketide Synthase Associated with Clinical Cases of Nocardiosis

Although a few well-characterized polyketide synthases (PKSs) have been functionally reconstituted in vitro from purified protein components, the use of this strategy to decode orphan assembly line PKSs has not been described. To begin investigating a PKS found only in Nocardia strains associated with clinical cases of nocardiosis, we reconstituted in vitro its five terminal catalytic modules. In the presence of octanoyl-CoA, malonyl-CoA, NADPH, and S-adenosyl methionine, this pentamodular PKS system yielded unprecedented octaketide and heptaketide products whose structures were partially elucidated using mass spectrometry and NMR spectroscopy. The PKS has several notable features, including a split, stuttering module and a terminal reductive release mechanism. Our findings pave the way for further analysis of this unusual biosynthetic gene cluster whose natural product may enhance the infectivity of its producer strains in human hosts.