TAL effectors delivered by phytopathogenic Xanthomonas species are DNA-sequence-specific transcriptional activators of host susceptibility genes and sometimes resistance genes. The modularity of DNA recognition by TAL effectors makes them important also as tools for gene targeting and genome editing. Effector binding elements (EBEs) recognized by native TAL effectors in plants have been identified only on the forward strand of target promoters. Here, we demonstrate that TAL effectors can drive plant transcription from EBEs on either strand and in both directions. Furthermore, we show that a native TAL effector from Xanthomonas oryzae pv. oryzicola drives expression of a target with an EBE on each strand of its promoter. By inserting that promoter and derivatives between two reporter genes oriented head to head, we show that the TAL effector drives expression from either EBE in the respective orientations, and that activity at the reverse-strand EBE also potentiates forward transcription driven by activity at the forward-strand EBE. Our results reveal new modes of action for TAL effectors, suggesting the possibility of yet unrecognized targets important in plant disease, expanding the search space for off-targets of custom TAL effectors, and highlighting the potential of TAL effectors for probing fundamental aspects of plant transcription.
Keywords: bidirectional reporter; enhancer element; off-target; transcription activator-like effector; xanthomonas.
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