Figure 6.

A model for the two-step inactivation of RecBCD enzyme by Chi. RecBCD enzyme binds to a dsDNA end (A) and unwinds the DNA with the production of an ssDNA loop and tail (B). At Chi it produces a 3′ end (C); continued unwinding produces the 3′ Chi tail (D) onto which RecA protein is loaded (not shown). After encountering Chi, RecBCD enzyme is depicted in an altered form (C and D; the first change). During continued unwinding or upon reaching the end of the DNA the enzyme disassembles into separate subunits (E; the second change). Reassembly is prevented by the DNA entrapped within RecB (E). Removal of this DNA, via nuclease treatment or incubation with excess Mg²⁺, allows reassembly and reactivation of the enzyme (F). (See text for details).