Summary: Biosynthesis of novel carotenoid families based on unnatural carbon
backbones: A model for diversification of natural product pathways
Alexander V. Tobias 1
, Frances H. Arnold
Division of Chemistry and Chemical Engineering, California Institute of Technology 210-41, 1200 E. California Blvd., Pasadena, CA 91125, USA
Received 4 November 2005; received in revised form 3 January 2006; accepted 4 January 2006
Available online 30 January 2006
We show that the C40 carotenoid desaturase CrtI from Pantoea ananatis (Erwinia uredovora) is capable of desaturating unnaturally long C45
and C50 carotenoid backbones in recombinant E. coli. Desaturation step number in these pathways is not very specific, and at least ten new C45
and C50 carotenoids were synthesized. We also present evidence for a novel asymmetric C40 backbone formed by the condensation of farnesyl
diphosphate (C15PP) with farnesylgeranyl diphosphate (C25PP), and the subsequent desaturation of this backbone by CrtI in an atypical manner.
Under some conditions, the C40, C45, and C50 carotenoid backbones synthesized in E. coli were monohydroxylated; their desaturation by CrtI in
vitro led to yet more novel carotenoids. Challenging CrtI 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.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Molecular evolution; Carotenoid biosynthetic pathway
Carotenoids are lipid pigments that play vital roles in key