A buffer-tuning strategy to profile domain-specific activity of chimeric I-TevI/CRISPR gene editors in vitro

Protein-DNA interactions can be manipulated in vitro by changing buffer conditions. Here, we develop a methodology to map the cleavage preferences of chimeric gene editors that are fusions of the I-TevI nuclease domain to CRIPSR nucleases by manipulating in vitro salt concentrations. We found that DNA cleavage by the I-TevI (Tev) nuclease domain at CNNNG sites was de-coupled from the gRNA-targeted site in low salt buffers. For TevCas12a, this non-targeted cleavage activity was enriched at Tev CNNNG cleavage motifs optimally positioned within a 30-bp window upstream of a Cas12a TTTV PAM site. Non-targeted cleavage did not require Cas12a nuclease activity or specific Cas12a gRNA targeting. Similar non-targeted products were observed in low salt buffer conditions for TevSaCas9, Tev-meganuclease and Tev-zinc finger editors. Cas12a and SaCas9 activity at gRNA-directed sites and sites with multiple mismatches were also sensitive to buffer salt concentration. Oxford Nanopore sequencing revealed a remarkably similar Tev CNNNG cleavage preference at different salt concentrations and in different fusion contexts, emphasizing the robustness and specificity of Tev activity. More generally, our work highlights the sensitivity of gene editors to in vitro reaction conditions and how these conditions can be leveraged to functionally dissect the activity of individual domains of chimeric gene editors.