Biosynthesis Of Astaxanthin In Synechocystis Sp. PCC 6803

Astaxanthin is a carotenoid widely used in food,cosmetics and aquaculture industry due to its superior antioxidant properties.The major sources of astaxanthin are chemical synthesis and extract from Haematococcus.Compared to synthetic astaxanthin,naturalastaxanthin has advantages of higher purity,less isomer and better bioavailability,and is more demanded by market.However,the production of naturalastaxanthin is still relative low and cannot meet the market demand.Fortunately,biosynthesis of astaxanthin through microbial cell factory provides an alternative choice.Nowadays,astaxanthin has been successfully biosynthesized in microorganisms,such as Escherichia coli and yeast,while there are few reports of biosynthesis of astaxanthin in the photosynthetic cyanobacteria.Astaxanthin isone of thederivatives of carotenoids,and its biosynthesis needs two key enzymes,?-caroteneketolase and ?-carotene hydroxylase,encoding bycrtW and crtZ,respectively.Many bacterial sources ?-caroteneketolase and ?-carotene hydroxylase are bifunctional enzymes,and can accept ?-carotene and its derivatives as substrates despite whether they are ketonisation or hydroxylation or not.As many as eight intermediate products can be generated in this process.So it is very important to choose appropriate ?-caroteneketolase and ?-carotene hydroxylase.Synechocystis sp.PCC 6803 has isoprenoid pathway,and can accumulate astaxanthin precursors such as ?-carotene and zeaxanthin naturally.In this study,crt W from Anabaena sp.PCC 7120 and crtZ from Pantoea agglomerans were selected.Firstly,we constructed the biosynthetic pathway of astaxanthin in E.coli to characterize the catalytic properties of ?-carotene ketolase and ?-carotene hydroxylase clonedfrom Anabaena sp.PCC 7120 and Pantoea agglomerans,respectively.The engineered stain E.coli BW-BWZ can accumulate astaxanthin and zeaxanthin,among them,the titer of astaxanthin is 971.21 ?g/g dcw after 2 days cultivation.Secondly,we constructed the biosynthetic pathway of astaxanthin in Synechocystis sp.PCC 6803 by introducing gene crtW into plasmid pJA2,which has a PsbA2 promoter.The engineered stain Synechocystis sp.PCC 6803-AX can accumulate astaxanthin and the titer is 112.16 ?g/g dcw after 4 days cultivation.In this study,we successfully accomplish the biosynthesis of astaxanthin from CO2 and light directly for the first time.In addition to astaxanthin,?-carotene,zeaxanthin and other five astaxanthin precursors can be detected in Synechocystis sp.PCC 6803-AX stain,indicating the expression strength of crtW was not enough.A stronger promoter should be adopted to improve its transformation rate.In addition,strategies such as overexpression of crtZ and crtB(encoding phytoene synthase)and inactivating the competing pathways can be adopted to improve the production of astaxanthin.