Figure 1.
DNA unwinding by RecBCD enzyme and structure of the enzyme bound to a dsDNA end. The RecB subunit is orange, RecC is blue, and RecD is green. (A) Loop–tail structure formed during DNA unwinding by RecBCD. The lengths of the short (x) and long (y) tails are proportional to the rates of the slower (RecB) and faster (RecD) helicases, respectively (Taylor and Smith 2003). See Figure 3 for examples of electron micrographs of such structures. (B) Model for Chi-stimulated recombination. RecBCD binds a duplex DNA end (a) and unwinds the DNA with the formation of a loop–tail structure (b). (c) The loop and tails enlarge as RecBCD unwinds; the tails can anneal to form a twin-loop structure. Upon encountering Chi, the enzyme cuts the top strand (d) and loads RecA onto the 3′-ended strand (e). (f) The RecA-ssDNA filament forms a D-loop with a homologous duplex. (g) The DNA loop can be cut, with the formation of a Holliday junction, which can be resolved into crossover-type recombinants. (h) Alternatively, the D-loop can prime DNA synthesis, with the formation of a replication fork and a break-induced recombinant (BIR). For discussion of alternative models see Amundsen and Smith (2007) (adapted with permission from the Annual Review of Genetics, Volume 35 © 2001 by Annual Reviews, http://www.annualreviews.org) and Smith (2001) (adapted with permission), from which this figure is adapted. (C) Surface representation of RecBCD-dsDNA complex. The four terminal base pairs of DNA are unwound, and the 3′ end lies within RecB. During unwinding, this strand is postulated to pass through a tunnel in RecB and RecC on its way to the nuclease domain of RecB. Chi is postulated to be recognized by parts of the tunnel in RecC (red arrow). The 5′ end of the DNA lies within RecC and extends toward an unordered part of RecD (amino acids 245–255) lying behind the surface shown. Data from Singleton et al. (2004). (D) Ribbon representation of part of the RecBCD-dsDNA complex. Helicase motifs are in red. The RecB α-helix composed of residues 785–807 contains the conserved motif VI common to helicases and the amino acid substitutions Y803H and V804E described here. Data from Singleton et al. (2004).