Synthetic promoters capable of driving robust nuclear gene expression in the green alga Chlamydomonas reinhardtii

Algae have enormous potential as bio-factories for the efficient production of a wide array of high-value products, and eventually as a source of renewable biofuels. However, tools for engineering the nuclear genomes of algae remain scarce and limited in functionality. In this study, synthetic algal promoters (saps)were generated as a tool for increasing nuclear gene expression and as a model for understanding promoter elements and structure in green algae. Promoters were generated to mimic native cis-motif elements, structure, and overall nucleotide composition of top expressing genes from Chlamydomonas reinhardtii. Twenty five saps were used to drive expression of a fluorescent reporter in transgenic algae. A majority of the promoters were functional in vivo and seven were identified to drive expression of the fluorescent reporter better than the current best endogenous promoter in C. reinhardtii, the chimeric hsp70/rbs2 promoter. Further analysis of the best synthetic promoter, sap11, revealed a new DNA motif essential for promoter function that is widespread and highly conserved in C. reinhardtii. These data demonstrate the utility of synthetic promoters to drive gene expression in green algae, and lays the groundwork for the development of a suite of saps capable of driving the robust and complex gene expression that will be required for algae to reach their potential as an industrial platform for photosynthetic bio-manufacturing.