A simple technology for generating marker-free chloroplast transformants of the green alga Chlamydomonas reinhardtii

The availability of routine methods for the genetic engineering of the chloroplast genome of Chlamydomonas reinhardtii is allowing researchers to explore the use of this microalga as a phototrophic cell platform for synthesis of high value recombinant proteins and metabolites. However, the established method for delivering transforming DNA into the algal chloroplast involves microparticle bombardment using an expensive “gene gun”. Furthermore, selection of transformant lines most commonly involves the use of a bacterial antibiotic resistance gene. In this chapter, we describe a simple and cheap delivery method in which cell–DNA suspensions are agitated with glass beads: a method that is more commonly used for nuclear transformation of Chlamydomonas. We also describe the use of plasmid expression vectors that target transgenes to a neutral site within the chloroplast genome between psbH and trnE2, and employ psbH as the selectable marker—thereby avoiding issues of unwanted antibiotic resistance genes in the resulting transgenic lines. Finally, we highlight a feature in our latest vectors in which the presence of a novel tRNA gene on the plasmid results in recognition within the chloroplast of UGA stop codons in transgenes as tryptophan codons. This feature simplifies the cloning of transgenes that are normally toxic to E. coli, serves as a biocontainment strategy restricting the functional escape of transgenes from the algal chloroplast to environmental microorganisms, and offers a simple system of temperature-regulated translation of transgenes.