TALE repeat variable diresidues (RVDs) recognize:
TALE repeat specificity is encoded by repeat variable diresidues RVDs located at positions 12 and 13 within each 33-35 aa repeat. Repeats fold as two helix bundle forming superhelical structure wrapping major groove around B-form DNA in contiguous tracking left-handed. Crystal structure solved 2012 shows each repeat contacts single base pair phosphate backbone via conserved residues while RVD at tip reads base via side chain interactions: hydrogen bonding and steric complementarity. Unlike zinc fingers recognizing triplets with neighbor influence, one repeat one base makes code highly modular
Ref: Mak et al. Science 2012 TALE DNA crystal; Moscou Bogdanove Science 2009 RVD code DNA bases.