Practice question

Like zinc finger nucleases, transcription activator-like effector nucleases utilize catalytic domain of FokI endonuclease for cleavage because DNA binding domain lacks nuclease activity. TALE repeats provide programmable anchoring but cannot cut phosphodiester backbone; fusion via flexible linker to C-terminal 196 aa FokI cleavage domain confers scission capability. FokI domain requires dimerization, so two TALEN monomers engineered to bind opposite strands in tail-to-tail orientation with spacer 12 to 21 bp allow FokI domains to associate forming active nuclease complex generating double-strand break with variable overhang within spacer. Truncation analysis identified optimal N-terminal 152 aa upstream of repeats and C-terminal 63 aa linker downstream essential for activity, influencing binding affinity and steric positioning of FokI. Introduction of heterodimeric FokI mutations ELD KKR improves specificity reducing homodimer off-target cutting. Use of shared FokI domain enables transfer of knowledge about dimerization requirement, spacer length optimization, and fidelity enhancements from ZFN to TALEN, streamlining both platforms and establishing architecture reused in dimeric CRISPR-FokI dCas9-FokI fusions for improved specificity via dual guide requirement.