An Exogenous Chloroplast Genome for Complex Sequence Manipulation in Algae

We demonstrate a system for cloning and modifying the chloroplast genome from the green alga, Chlamydomonas reinhardtii. Through extensive use of sequence stabilization strategies, the ex vivo genome is assembled in yeast from a collection of overlapping fragments. The assembled genome is then moved into bacteria for large-scale preparations and transformed into C. reinhardtii cells. This system also allows for the generation of simultaneous, systematic and complex genetic modifications at multiple loci in vivo. We use this system to substitute genes encoding core subunits of the photosynthetic apparatus with orthologs from a related alga, Scenedesmus obliquus. Once transformed into algae, the substituted genome recombines with the endogenous genome, resulting in a hybrid plastome comprising modifications in disparate loci. The in vivo function of the genomes described herein demonstrates that simultaneous engineering of multiple sites within the chloroplast genome is now possible. This work represents the first steps toward a novel approach for creating genetic diversity in any or all regions of a chloroplast genome.