TAL effectors: function, structure, engineering and applications.

TAL effectors are proteins secreted by bacterial pathogens into plant cells, where they enter the nucleus and activate expression of individual genes. TAL effectors display a modular architecture that includes a central DNA-binding region comprising a tandem array of nearly identical repeats that are almost all 34 residues long. Residue number 13 in each TAL repeat (one of two consecutive polymorphic amino acids that are termed 'repeat variable diresidues', or 'RVDs') specifies the identity of a single base; collectively the sequential repeats and their RVDs dictate the recognition of sequential bases along one of the two DNA strands. The modular architecture of TAL effectors has facilitated their extremely rapid development and application as artificial gene targeting reagents, particularly in the form of site-specific nucleases. Recent crystallographic and biochemical analyses of TAL effectors have established the structural basis of their DNA recognition properties and provide clear directions for future research.