The Photosynthetic Bacteria Rhodobacter Capsulatus and Synechocystis Sp. PCC 6803 as New Hosts for Cyclic Plant Triterpene Biosynthesis

PLoS One. 2017 Dec 27;12(12):e0189816. doi: 10.1371/journal.pone.0189816. eCollection 2017.

Abstract

Cyclic triterpenes constitute one of the most diverse groups of plant natural products. Besides the intriguing biochemistry of their biosynthetic pathways, plant triterpenes exhibit versatile bioactivities, including antimicrobial effects against plant and human pathogens. While prokaryotes have been extensively used for the heterologous production of other classes of terpenes, the synthesis of cyclic triterpenes, which inherently includes the two-step catalytic formation of the universal linear precursor 2,3-oxidosqualene, is still a major challenge. We thus explored the suitability of the metabolically versatile photosynthetic α-proteobacterium Rhodobacter capsulatus SB1003 and cyanobacterium Synechocystis sp. PCC 6803 as alternative hosts for biosynthesis of cyclic plant triterpenes. Therefore, 2,3-oxidosqualene production was implemented and subsequently combined with different cyclization reactions catalyzed by the representative oxidosqualene cyclases CAS1 (cycloartenol synthase), LUP1 (lupeol synthase), THAS1 (thalianol synthase) and MRN1 (marneral synthase) derived from model plant Arabidopsis thaliana. While successful accumulation of 2,3-oxidosqualene could be detected by LC-MS analysis in both hosts, cyclase expression resulted in differential production profiles. CAS1 catalyzed conversion to only cycloartenol, but expression of LUP1 yielded lupeol and a triterpenoid matching an oxidation product of lupeol, in both hosts. In contrast, THAS1 expression did not lead to cyclic product formation in either host, whereas MRN1-dependent production of marnerol and hydroxymarnerol was observed in Synechocystis but not in R. capsulatus. Our findings thus indicate that 2,3-oxidosqualene cyclization in heterologous phototrophic bacteria is basically feasible but efficient conversion depends on both the respective cyclase enzyme and individual host properties. Therefore, photosynthetic α-proteo- and cyanobacteria are promising alternative candidates for providing new bacterial access to the broad class of triterpenes for biotechnological applications.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cyclization
  • Gene Expression Regulation, Bacterial
  • Rhodobacter capsulatus / metabolism*
  • Synechocystis / metabolism*
  • Triterpenes / metabolism*

Substances

  • Triterpenes

Grant support

This work was supported by the Deutsche Forschungsgemeinschaft via CEPLAS - Cluster of Excellence on Plant Science (EXC1028) and by the Ministry of Innovation, Science and Research of the German State of North Rhine-Westphalia within the framework of the NRW Strategieprojekt BioSC (No. 313/323‐400‐00213). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.