**4. Conclusions**

Carotenoid pathways in plants have been described in great detail using genetic, biochemical, and molecular data, mainly from *Arabidopsis* and other higher plants; however, this is the first study in the unicellular microalga *C. protothecoides* CS-41. 

We successfully isolated and analyzed the *Cppsy* gene, which encodes the functional phytoene synthase—a vital enzyme for carotenoid biosynthesis in *C. protothecoides* CS-41—as well as its promoter. Computational analysis suggested that this protein belongs to the Isoprenoid\_Biosyn\_C1 superfamily. It contains one more substrate-Mg2+ binding site than other algae and higher plants. Analysis also demonstrated several candidate motifs for the promoter, which exhibited light- and MeJA-responsive characteristics. Light- and MeJA treatment showed that the *Cppsy* expression level was significantly enhanced by light and MeJA. 

These achievements will be helpful to understand more about the regulatory mechanism of the carotenoid biosynthesis pathway in algae and the mechanisms for accumulation of lutein and other important carotenoids. 

## **Acknowledgments**

This work was supported by the National 863 Program of China (2012AA101601), and the Agri-X Program of Shanghai Jiao Tong University (Agri-X2015005). 
