Evolution of transcription activator-like effectors in Xanthomonas oryzae

Transcription activator-like effectors (TALEs) are secreted by plant-pathogenic Xanthomonas bacteria into plant cells where they act as transcriptional activators and, hence, aremajor drivers in reprogramming the plant for the benefit of the pathogen. TALEs possess ahighly repetitive DNA-bindingdomain of typically 34 amino acid(AA) tandemrepeats, where AA12 and 13, termed repeat variable di-residue (RVD), determine target specificity. Different Xanthomonas strains possess different repertoires of TALEs. Here, we study the evolution of TALEs from the level of RVDs determining target specificity down to the level of DNA sequence with focus on ricepathogenic Xanthomonas oryzae pv. oryzae (Xoo) and Xanthomonas oryzae pv. oryzicola (Xoc) strains. We observe that codon pairs coding for individual RVDs are conserved to a similar degree as the flanking repeat sequence. Wefind strong indications that TALEs may evolve 1) by base substitutions in codon pairs coding for RVDs, 2) by recombination of N-terminal or C-terminal regions of existing TALEs, or 3) by deletion of individual TALE repeats, and we propose possible mechanisms. We find indications that the reassortment of TALE genes in clusters ismediated by an integron-like mechanismin Xoc. We finally study the effect of the presence/ absence and evolutionary modifications of TALEs on transcriptional activation of putative target genes in rice, and find that even single RVD swaps may lead to considerable differences in activation. This correlation allowed a refined prediction of TALE targets, which is the crucial step to decipher their virulence activity.