FIG. 12.

Locations of tunnels in the RecC protein for the nascent ssDNA strands. (A) Cartoon representation of the RecC protein revealing its subdomain structure. Four subdomains (green, red, yellow, and blue) are characteristic of SF1 DNA helicases, although the RecC protein is not active as a DNA helicase and does not possess the conserved motifs. The C-terminal domain (magenta) (“Nuc”) has a nuclease-like fold, but note that the RecC protein is not a nuclease and does not possess the conserved motifs expected for a nuclease. The 5′ ssDNA tail of the DNA substrate (black) is pointing toward the viewer and is in a tunnel in the RecC protein. The 3′ ssDNA tail runs underneath RecC (where it is in contact with the RecB helicase domain) (not shown) and toward a second small tunnel in RecC. The duplex DNA points away from the viewer below the 5′ ssDNA strand and the RecC protein. (B) Space-filling model of the RecC protein, colored as described above for A. This illustration clearly shows the two small tunnels for the nascent ssDNA strands and the large central hole that accommodates RecB auxiliary subdomain 2B. In particular, note that the 3′ tunnel is formed at the top surface of the core helicase-like domains (green and red) and that mutations (recC1004 allele) associated with altered Chi recognition specificity (black) map to this region. The 5′ tunnel is formed by the nuclease-like domain. (C) View equivalent to those of A and B with the RecC protein shown as a space-filling representation completely in blue. Cartoon representations of the RecB and RecD proteins have been added. The color scheme is described in the legend of Fig. 4. Note that the RecD protein and the RecB nuclease are positioned above the 5′ ssDNA and 3′ ssDNA tunnels, respectively. RecB subdomain 2B occupies the large hole in the middle of the RecC protein.