Exploiting the sequence diversity of TALE-like repeats to vary the strength of dTALE‐promoter interactions

Naturally TALEs are pathogenicity factors secreted by Xanthomonas spp. into host plant cells to bind specific promoters and manipulate host transcription. Sequence specific DNA binding is conferred by the TALE repeat array. Natural TALE repeat arrays are formed of 10-30 repeats, each of which pairs with one DNA base. Collectively the repeats of a TALE array form a righthanded superhelix, enfolding the DNA, allowing repeats to contact their target bases . Although each repeat is 33-35 amino acids long, polymorphisms are mostly restricted to the residues occupying positions 12 and 13, termed the Repeat Variable Di-residue (RVD). RVDs are oriented in close proximity with target bases and they define the base preference of each repeat. The sequence of all RVDs from the N to the C-terminal end of the repeat-array, known as the RVD composition, determines the 5’-3’ DNA sequence specificity. The flanking residues of each repeat are termed non-RVDs and are generally seen as fixed scaffolds to house RVDs. Natural TALE genes seem to evolve rapidly with respect to the number and RVD composition of the repeats they encode, forming TALEs with new target preferences. Non-RVDs are contrastingly conserved, a factor that may speed the evolution of TALEs through repeat recombinations.