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Once upon a time, there was a Type III effector… TALEs (Transcription-Activator-Like Effectors) are a family of effector molecules first discovered in Xanthomonas. The first TAL effector gene AvrBs3 was described more than 25 years ago. It consists of two constant domains, one localized on the C-Terminus and N-Terminus each and a central domain consisting of 17 repeats of 34 amino acids with two hypervariable positions, called RVDs (Repeat-Variable Di-residues). (Bonas, Stall et al., 1989). The actual number of tandem repeats in TALEs is variable and varies in accordance to the target sequence (Marois et al., 2002). TAL effectors were found to be able to induce gene expression in plants by binding a specific sequence in the promoter region (Römer et al., 2007; Kay et al., 2007) though the exact mechanism by which they did so remained unknown until 2009, when a correlation between the DNA sequence necessary for gene regulation by AvrBs3 and the sequence of RVDs in the effector protein was discovered (Boch et al., 2009). By breaking the code of TAL specifity it was now possible to create new TAL effectors binding specific, purposefully targeted DNA sequences. By the way, they’re actually useful: Perhaps the most commonly known application of TALEs are TALENs (TAL effector nucleases). By fusing a TALE to a catalytic domain of the endonuclease FokI it is possible to induce sequence specific double-strand breaks, thus yielding an effective tool for genome editing (Christian et al., 2010). Another application for TALEs is gene regulation. By nature, TAL effectors are regulators of gene expression and exist only to make host plants accommodate pathogens. They induce expression likely by replacing the TATA-binding protein and recruiting transcription factors (Römer et al., 2009). However, in place of direct gene regulation it has also been shown that by fusing a TALE to a histone modifying enzyme it is possible to induce chromatin modifications, thus altering chromatin structure and gene expression (Mendenhall et al., 2013). References Boch, Jens, and Ulla Bonas. “Xanthomonas AvrBs3 Family-Type III Effectors: Discovery and Function.” Annual Review of Phytopathology 48, no. 1 (July 2010): 419–36. doi:10.1146/annurev-phyto-080508-081936. Boch, J., H. Scholze, S. Schornack, A. Landgraf, S. Hahn, S. Kay, T. Lahaye, A. Nickstadt, and U. Bonas. “Breaking the Code of DNA Binding Specificity of TAL-Type III Effectors.” Science 326, no. 5959 (December 11, 2009): 1509–12. doi:10.1126/science.1178811. Bonas, U., R. E. Stall, and B. Staskawicz. “Genetic and Structural Characterization of the Avirulence Gene avrBs3 from Xanthomonas Campestris Pv. Vesicatoria.” Molecular & General Genetics: MGG 218, no. 1 (July 1989): 127–36. Christian, Michelle, Tomas Cermak, Erin L. Doyle, Clarice Schmidt, Feng Zhang, Aaron Hummel, Adam J. Bogdanove, and Daniel F. Voytas. “Targeting DNA Double-Strand Breaks with TAL Effector Nucleases.” Genetics 186, no. 2 (October 2010): 757–61. doi:10.1534/genetics.110.120717. Kay, S., S. Hahn, E. Marois, G. Hause, and U. Bonas. “A Bacterial Effector Acts as a Plant Transcription Factor and Induces a Cell Size Regulator.” Science 318, no. 5850 (October 26, 2007): 648–51. doi:10.1126/science.1144956. Marois, Eric, Guido Van den Ackerveken, and Ulla Bonas. “The Xanthomonas Type III Effector Protein AvrBs3 Modulates Plant Gene Expression and Induces Cell Hypertrophy in the Susceptible Host.” Molecular Plant-Microbe Interactions 15, no. 7 (July 2002): 637–46. doi:10.1094/MPMI.2002.15.7.637. Mendenhall, Eric M, Kaylyn E Williamson, Deepak Reyon, James Y Zou, Oren Ram, J Keith Joung, and Bradley E Bernstein. “Locus-Specific Editing of Histone Modifications at Endogenous Enhancers.” Nature Biotechnology 31, no. 12 (September 8, 2013): 1133–36. doi:10.1038/nbt.2701. Römer, Patrick, Tina Strauss, Simone Hahn, Heidi Scholze, Robert Morbitzer, Jan Grau, Ulla Bonas, and Thomas Lahaye. “Recognition of AvrBs3-like Proteins Is Mediated by Specific Binding to Promoters of Matching Pepper Bs3 Alleles.” Plant Physiology 150, no. 4 (August 2009): 1697–1712. doi:10.1104/pp.109.139931. Romer, P., S. Hahn, T. Jordan, T. Strauss, U. Bonas, and T. Lahaye. “Plant Pathogen Recognition Mediated by Promoter Activation of the Pepper Bs3 Resistance Gene.” Science 318, no. 5850 (October 26, 2007): 645–48. doi:10.1126/science.1144958.