AvrXa27 binding influences unwinding of the double-stranded DNA in the UPT box

Transcription-Activator Like (TAL) effectors, delivered by Xanthomonas pathogens bind specifically to UP-regulated by TAL effectors (UPT) box of the host gene promoter to arouse disease or trigger defense response. This type of protein-DNA interaction model has been applied in site-directed genome editing. However, the off-target effects of TAL have severely hindered the development of this promising technology. To better exploit the specific interaction and to deeper understand the TAL-induced host transcription rewiring, the binding between the central repeat region (CRR) of the TAL effector AvrXa27 and its UPT box variants was studied by kinetics analysis and TAL-blocked helicase unwinding assay. The results revealed that while AvrXa27 exhibited the highest affinity to the wild type UPT box, it could also bind to mutated UPT box variants, implying the possibility of non-specific interactions. Furthermore, some of these non-specific combinations restricted the helicase-elicited double-stranded DNA (dsDNA) separation to a greater extent. Our findings provide insight into the mechanism of TAL transcriptional activation and are beneficial to TAL-mediated genome modification.