Figure 1 - Model for a ternary complex of the E. coli clamp loader, clamp, and nucleic acid derived from the structure of the RFC-PCNA complex (Bowman et al.). (a) E. coli clamp loader complex modeled with beta clamp and duplex A-form RNA-DNA hybrid (RNA primer strand in yellow, DNA template strand in blue) as described in the text. Position of possible extension for 5’ overhang from template strand is noted by blue circles. (b) Top view of the clamp loader spiral. Location of residues mutated in the gamma-BCD and delta’-E subunits are indicated by colored spheres (yellow for those near the clamp-interacting helix in gamma-BCD, red for those near the central helix in gamma-BCD, green for that near the SRC helix in gamma-BCD, purple for control mutations in gamma-BCD, blue for mutations in delta’-E). Modeled RNA-DNA heteroduplex is shown in the center of the clamp loader spiral. The C-terminal collar domains (Domain III) are not shown. (c). Domain I of the AAA+ ATPase module is shown for gamma-B and delta’-E modeled alongside nucleic acid. Because the arrangement of the clamp loader spiral is highly similar to the rise of duplex nucleic acid, subunits gamma-C and gamma-D also track the minor groove of the nucleic acid. All figures rendered with PyMol (DeLano Scientific).