Practice question

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 and predictable enabling assembly of arrays recognizing 15-20 bp with straightforward cloning. N-terminal noncanonical repeats 0 and -1 contact 5' T required for optimal binding due to cryptic repeat. Some RVDs tolerate degeneracy broadening possibilities but overall specificity high. This modularity allowed Golden Gate assembly kits where RVD modules multimerized into full TALE in single reaction within days facilitating high-throughput genome editing in zebrafish, human pluripotent stem cells and tomato, overcoming context dependence bottleneck of ZFNs and launching second generation designer nucleases before RNA-guided systems emerged.