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Background   Euglena gracilis , a unicellular phytoflagellate within Euglenida, has attracted much attention as a potential feedstock for renewable energy production. In outdoor open-pond cultivation for biofuel production, excess direct sunlight can inhibit photosynthesis in this alga and decrease its productivity. Carotenoids play important roles in light harvesting during photosynthesis and offer photoprotection for certain non-photosynthetic and photosynthetic organisms including cyanobacteria, algae, and higher plants. Although, Euglenida contains β-carotene and xanthophylls (such as zeaxanthin, diatoxanthin, diadinoxanthin and 9′- cis  neoxanthin), the pathway of carotenoid biosynthesis has not been elucidated.    Results  To clarify the carotenoid biosynthetic pathway in  E. gracilis , we searched for the putative  E. gracilis  geranylgeranyl pyrophosphate (GGPP) synthase gene ( crtE ) and phytoene synthase gene ( crtB ) by tblastn searches from RNA-seq data and obtained their cDNAs. Complementation experiments in  Escherichia coli  with carotenoid biosynthetic genes of  Pantoea ananatis  showed that  E. gracilis crtE  ( EgcrtE ) and  EgcrtB  cDNAs encode GGPP synthase and phytoene synthase, respectively. Phylogenetic analyses indicated that the predicted proteins of  EgcrtE  and  EgcrtB  belong to a clade distinct from a group of GGPP synthase and phytoene synthase proteins, respectively, of algae and higher plants.  In addition, we investigated the effects of light stress on the expression of  crtE  and  crtB  in  E. gracilis.  Continuous illumination at 460 or 920 μmol m −2  s −1  at 25 °C decreased the  E. gracilis  cell concentration by 28–40 % and 13–91 %, respectively, relative to the control light intensity (55 μmol m −2  s −1 ). When grown under continuous light at 920 μmol m −2  s −1 , the algal cells turned reddish-orange and showed a 1.3-fold increase in the  crtB  expression. In contrast,  EgcrtE  expression was not significantly affected by the light-stress treatments examined.    Conclusions  We identified genes encoding CrtE and CrtB in  E. gracilis  and found that their protein products catalyze the early steps of carotenoid biosynthesis. Further, we found that the response of the carotenoid biosynthetic pathway to light stress in  E. gracilis  is controlled, at least in part, by the level of  crtB  transcription. This is the first functional analysis of  crtE  and  crtB  in  Euglena .    Electronic supplementary material  The online version of this article (doi:10.1186/s12870-015-0698-8) contains supplementary material, which is available to authorized users.

bmc plant biology

Identification and functional analysis of the geranylgeranyl pyrophosphate synthase gene (crtE) and phytoene synthase gene (crtB) for carotenoid biosynthesis in Euglena gracilis