Structural basis of the TAL effector-DNA interaction.

Phytopathogen transcription activator-like effectors (TALEs) bind DNA in a sequence specific manner in order to manipulate host transcription. TALE specificity correlates with repeat variable diresidues in otherwise highly stereotypical 34-35mer repeats. Recently, the crystal structures of two TALE DNA-binding domains have illustrated the molecular basis of the TALE cipher. The structures show that the TALE repeats form a right-handed superhelix that is wound around largely undistorted B-DNA to match its helical parameters. Surprisingly, repeat variable residue 1 is not in contact with the bases. Instead, it is involved in hydrogen bonding interactions that stabilize the overall structure of the protein. Repeat variable residue 2 contacts the top strand base and forms sequence-specific hydrogen bonds and/or van der Waals contacts. Very unexpectedly, bottom strand bases are exposed to solvent and do not make any direct contacts with the protein. This review contains a summary of TALE biology and applications and a detailed description of the recent breakthroughs that have provided insights into the molecular basis of the TALE code.