The myxobacterial strain Nannocystis pusilla B150 synthesizes the structurally new
polyketides phenylnannolone A–C. Apart from some common volatiles and
siderophores, these are the first natural products from the genus Nannocystis.
Phenylnannolone A shows inhibitory activity towards the ABCB1 gene product
P-glycoprotein and reverses
daunorubicin resistance in
cancer cells. To decipher the biochemical reactions leading to the formation of
phenylnannolone A, the putative biosynthetic genes were identified (phn1, phn2). Phn2 is a
polyketide synthase (PKS) with an NRPS-like loading module, and its domain order is consistent with the
phenylnannolone A structure. The functionality and substrate selectivity of the loading module were determined by means of a γ-18O4-ATP
pyrophosphate exchange and a
phosphopantetheine ejection assay. A specific activation of
cinnamic acid by the
AMP-
ligase was detected. Phn1 is a putative
butyryl-CoA carboxylase (BCC), providing
ethylmalonyl-CoA for the formation of the ethyl-substituted part of
phenylnannolone A. Phn1 is the first BCC found in biosynthetic genes for an ethyl-substituted natural compound. Biosynthesis of
phenylnannolone A, putatively encoded by phn1 and phn2, thus utilizes the first biosynthetic machinery in which both a BCC and a PKS are involved.