Nonproducer mutants support the assumption that epothilones A and B are syn
thesized by the same polyketide synthase (PKS). The endproducts of the PKS,
epothilones C and D, compete for the active site of a constitutively synth
esized monooxygenase which is regulated by product inhibition. The postulat
ed C-13 hydroxy-epothilones as direct precursors of epothilones C and D wer
e not detected.