TAL (transcription activator-like) effectors, secreted by phytopathogenic bacteria, recognize host DNA sequences through a central domain of tandem repeats. Each repeat comprises 33 to 35 conserved amino acids and targets a specific base pair by using two hypervariable residues [known as repeat variable diresidues (RVDs)] at positions 12 and 13. Here, we report the crystal structures of an 11.5-repeat TAL effector in bothDNA-free and DNA-bound states. Each TAL repeat comprises two helices connected by a short RVD-containing loop. The 11.5 repeats form a right-handed, superhelical structure that tracks along the sense strand of DNA duplex, with RVDs contacting the major groove. The 12th residue stabilizes the RVD loop, whereas the 13th residue makes a base-specific contact. Understanding DNArecognition by TALeffectors may facilitate rational design of DNA-binding proteins with biotechnological applications.

Deng D, Yan CY, Pan XJ, Mahfouz M, Wang JW, Zhu JK, Shi YG*, Yan NE*. (2012) Structural Basis for Sequence-Specific Recognition of DNA by TAL Effectors. Science, 335(6069):720-723.