Breaking the Code of DNA Binding Specificity of TAL-Type III Effectors

TAL Order Xanthomonas bacteria attack their plant hosts by delivering their own transcription-activator–like (TAL) proteins into the plant cell nucleus and alter the plant's gene regulation (see the Perspective by Voytas and Joung ). Moscou and Bogdanove (p. 1501 , published online 29 October: see the cover) and Boch et al. (p. 1509 , published online 29 October) have now discovered how the similar but not identical repeats in the TAL proteins encode the specificity needed for the proteins to find their targets. Each repeat is specific for one DNA base pair, a specificity encoded by hypervariable amino acid positions. Combining several repeats with different amino acids in the hypervariable positions allowed the production of new effectors that targeted new DNA sites.