Ahmad I, Kulkarni M, Gopinath A, Saikrishnan K.

Nucleic Acids Res. 2018 Jul 6;46(12):6229-6237. doi:10.1093/nar/gky344

Single-site DNA cleavage by Type III restriction endonuclease requires a site-bound enzyme and a trans-acting enzyme that are ATPase-activated.

Single-site cleavage happens by cooperation between activated enzymes contacting each other by 3D diffusion, and therefore requires higher concentrations of enzyme than two-site cleavage, where contacts happen through 1D diffusion or looping.