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Originally, TAL proteins are virulence factors of the plant-pathogenic ''Xanthomonas spp.'' that are injected into plant cells via a type III secretion system in order to modulate transcription¹. For this purpose, their c-terminal end contains a nuclear localisation signal (NLS) and an acidic activation domain. The central part of the TAL protein contains a number of almost similar repeats that mediate specific binding to target loci in the genome. In 2009, two groups have simultaneously pointed out that each of these repeats specifically binds to one base of the target DNA via two amino acids (aa 12 and 13), named the repeat variable diresidues (RVD)² (see figure 1). Moreover, it has been shown that DNA binding of these proteins is highly modular, i.e. the number of bases or sequence of the target DNA can be altered by adjusting the number or order of the repeats in the TAL protein, respectively.




The minimal condition for TALE activity is a thymine at the 5’ end of the target sequence. Further target sequence requirements that allow for one TALEN pair binding site every 35 bp (published by the Voytas lab in 2011³) have recently been questioned by Reyon et al.⁴ In summary, it is very likely that you can find multiple potential TALE binding sites in any sequence you want to target. This, obviously, is very promising for biotechnological and clinical applications. Thus, two major classes of TAL Effectors have been created by replacing the natural acidic activation domain either by other transcription factors (TALE-TFs) ⁵ or by a monomer of the non-sequence specific nuclease FokI, resulting in TAL Effector Nucleases (TALENS).⁶ A pair of TALENs can be designed to bind adjacent DNA sequences in a way that the two monomers are able to form a functional FokI dimer that produces a double strand break (DBS) within the spacer between the TAL-Effectors (see figure 2).




Subsequently, the cell repairs the DBS by either non-homologous end joining (NHEJ, which results in indels at the DSB-site) or homologous recombination of exogenously added genetic material. That way, TALENs allow researchers to introduce genes into a genome with much higher efficiency than before. In this context, TALENs and TALE-TF have successfully been applied for manipulation of a series of genes in different organisms such as yeast⁷, tobacco³, fruitflies^6,8,worms^6,9, zebrafish¹⁰, rats¹¹ and various human cell types, including human stem cells¹². Moreover, TALENs are already applied for gene therapy in preclinical trials.






1. Scholze, H. & Boch, J. TAL effectors are remote controls for gene activation. ''Current Opinion in Microbiology'' 14, 47–53 (2011).
2. Moscou, M. J. & Bogdanove, A. J. A Simple Cipher Governs DNA Recognition by TAL Effectors. ''Science'' 326, 1501–1501 (2009).
3. Cermak, T. et al. Efficient design and assembly of custom TALEN and other TAL effector-based constructs for DNA targeting. ''Nucleic Acids Res'' 39, e82 (2011).
4. Reyon, D. et al. FLASH assembly of TALENs for high-throughput genome editing. ''Nature Biotechnology'' 30, 460–465 (2012).
5. Zhang, F. et al. Efficient construction of sequence-specific TAL effectors for modulating mammalian transcription. ''Nature biotechnology'' 29, 149–153 (2011).
6. Miller, J. C. et al. A TALE nuclease architecture for efficient genome editing. ''Nature Biotechnology'' 29, 143–148 (2010).
7. Boch, J. et al. Breaking the Code of DNA Binding Specificity of TAL-Type III Effectors. ''Science'' 326, 1509–1512 (2009). 8. Liu, J. et al. Efficient and Specific Modifications of the Drosophila Genome by Means of an Easy TALEN Strategy. ''Journal of Genetics and Genomics'' 39, 209–215 (2012).
9. Wood, A. J. et al. Targeted Genome Editing Across Species Using ZFNs and TALENs. ''Science'' 333, 307–307 (2011).
10. Sander, J. D. et al. Targeted gene disruption in somatic zebrafish cells using engineered TALENs. ''Nat Biotechnol'' 29, 697–698 (2011).
11. Tesson, L. et al. Knockout rats generated by embryo microinjection of TALENs. ''Nature Biotechnology'' 29, 695–696 (2011).
12. Hockemeyer, D. et al. Genetic engineering of human pluripotent cells using TALE nucleases. ''Nature Biotechnology'' 29, 731–734 (2011).
13. Sanjana, N. E. et al. A transcription activator-like effector toolbox for genome engineering. Nature Protocols 7, 171–192 (2012).

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