Biosynthesis of novel carotenoid families based on unnatural carbon backbones: A model for diversification of natural product pathways

We show that the C-40 carotenoid desaturase Crtl from Pantoea ananatis (Erwinia uredovora) is capable of desaturating unnaturally long C-45 and C-50 carotenoid backbones in recombinant E. coli. Desaturation step number in these pathways is not very specific, and at least ten new C-45 and C-50 carotenoids were synthesized. We also present evidence for a novel asymmetric C-40 backbone formed by the condensation of farnesyl diphosphate (C15PP) with farnesylgeranyl diphosphate (C25PP), and the subsequent desaturation of this backbone by Crtl in an atypical manner. Under some conditions, the C-40, C-45, and C-50 carotenoid backbones synthesized in E. coli were monohydroxylated; their desaturation by Crtl in vitro led to yet more novel carotenoids. Challenging Crtl with larger-than-natural substrates in vivo has allowed us to show that this enzyme regulates desaturation step number by sensing the end groups of its substrate. Analysis of the mechanisms by which chemical diversity is generated and propagated through the nascent pathways provides insight into how natural product diversification occurs in nature. (c) 2006 Elsevier B.V. All rights reserved.