Transcription Activator-Like Effector Nucleases
TALEN was first discovered in 2009 and successfully used for genome editing work in 2011.
Proteins called TALENs (Transcription Activator-like Effector Nucleases) function as transcription factors that turn on genes in plant hosts that support their successful infection.
TALENs are natural proteins made and used by harmful plant bacteria to control plant genes during infection.
TALEs represent a family of type III effector proteins from Xanthomonas spp., a genus of gram-negative Proteobacteria often associated with plant diseases. These intelligent microbial creatures have developed their own way to control gene expression in plants, primarily through TALEs.
Figure: Schematics for the reconstitution of TALE. The central binding domain contains multiple regions of repeated amino acid sequences (positions 12 and 13 are the most important for specific nucleotide recognition). DBD, DNA binding domain. FokI, Fok 1 nuclease.
Structure
A TALE is made up of successively repeated segments, each usually consisting of 34 amino acids. Each of these pairs of amino acids binds to a specific nucleotide in the DNA. This usually enhances the transcription of that gene.
If the nucleotide sequence of the region to be modified is known, a TALE can be synthesized by combining the corresponding repeat sequence. To this construct, an endonuclease is added that can cut the DNA to which the TALE binds.
Once TALEN enters the nucleus of the cell whose genome you want to change, it cuts the DNA creating a double-stranded break (DSB). This can be repaired by non-homologous end joining (NHEJ) or homologous recombination (HR).
The key technology behind TALEN is the use of transcription activator-like effectors (TALE) to bind a specific DNA sequence. When combined with a nuclease such as FokI, the precision of the cutting site becomes extremely efficient.
Applications
– So far, TALENs have been used to alter genes in many species: plants, animals, fungi and monkeys.
– In vitro mutagenesis studies using cell lines
– In vivo genome editing
– Sugarcane for use as biofuel
– Crop improvement applications
TALENs How to use it to improve our crops?
TALENs are used by scientists to help improve food crops.
TALENs are a tool used to extend the characteristics of crops and allow breeders to selectively modify the qualities they want; such as increased yield and quality, tasty foods, resistance to disease. Resistance to environmental factors.
To date, TALENs have been able to produce soybeans that produce superior quality oil, rice that is more aromatic and resistant to bacterial blight, potatoes with better taste and less carcinogenic acrylamide, and wheat that is fully resistant to powdery mildew.
With the need to double food production to feed a projected population of 10 billion by 2050, TALENs and other new breeding innovations are essential tools to ensure food security.
Outside of plants and algae, TALENs have been successfully used to modify genes in yeast, fruit flies, roundworms, crickets, zebrafish, frogs, rats, pigs, cows, silkworms and humans.
Conclusion
TALENs remain the most specific gene editing tool today.
TALENs are highly precise gene editing tools that can target any sequence and in some cases outperform CRISPR.
However, the technology requires the creation of engineered proteins.
While the technology is well established, proven and specific, protein engineering is time consuming and expensive. Only a fraction have high performance. And simultaneous editing is difficult.
TALENs are great for molecular genome engineers and newer and better ways of using TALENs will emerge every day.