TALENs comprise a powerful class of tools that are redefining the boundaries of biological research. TALEs are naturally occurring proteins from the plant patho­genic bacteria genus Xanthomonas, and contain DNA-binding domains composed of a series of 33-35 amino acid repeat domains that each recognizes a single base pair. TALE specificity is determined by two hypervariable amino acids that are known as the repeat variable di-residues (RVDs). TALEs can be quickly engineered to bind practically any desired DNA sequence (Boch 2011). TALENs can be used to edit genomes by inducing double-strand breaks (DSB) (Fig. 8.2b), which cells respond to with repair mechanisms (Boch 2011). Several methods have been developed that enable rapid assembly of custom TALE arrays such as golden gate cloning, high-throughput solid-phase assembly, and ligation-independent cloning techniques (Gaj et al. 2013).

Site-specific nucleases have enabled the introduction of targeted modifications in several model organisms common to biological research, including zebrafish, rat, mouse, Drosophila, Caenorhabditis elegans, and many other non-model species including the monarch butterfly, frogs and livestock (Gaj et al. 2013). In addition to valuable animal models, TALENs have been used to introduce targeted alterations in plants, including Arabidopsis and several crop species (Curtin et al. 2012), allowing the incorporation of valuable traits, such as disease and herbicide resistance.

In algae, studies of TALENs to modify the genome of Chlamydomonas have been initiated (Borchers et al. 2012; Spalding and Wright 2011). For instance, TALENs to knockout the Chlamydomonas Sta6 and CAH3 genes, which are responsible for starch production and CO2 uptake, have been designed. These early studies are of interest due to the status of Chlamydomonas as a model organism for biofuel production.

The diversity of organisms modified by these site-specific nucleases will undoubtedly continue to grow, expanding the repertoire of model systems for basic research. TALENs will also enhance research efforts in algal biomass production, thus opening new avenues for algal biofuels commercialization.

Custom-designed TALE arrays are commercially available through Cellectis Bioresearch (Paris, France), Transposagen Biopharmaceuticals (Lexington, KY, USA), and Life Technologies (brand of Thermo Fisher Scientific, Carlsbad, CA, USA).