FIG. 16.

Mutants of the RecBCD enzyme. The crystal structure of RecBCD is shown, with the main chains of RecB, RecC, and RecD shown in red/magenta, blue/cyan/grayish blue, and green, respectively. The DNA is shown in orange. This color coding is essentially the same as that described in the legend of Fig. 8, as is the orientation of the structure. Note that the RecC backbone is shown in three different shades of blue. The light blue/gray section represents residues 981 to 1084; deletion from the C terminus to within this region yields the “double-dagger” phenotype. The cyan section represents residues 790 to 922; further deletion from the C terminus to within this region yields the “dagger” phenotype. The remainder of RecC is dark blue. The locations of other individual amino acids that are mutated in several well-studied recBCD alleles are indicated by labeled surface representations of the wild-type residues shown in black. The “RecC1004” region (shown in orange) defines residues in the partial frameshift mutation that resulted in an altered specificity of Chi recognition. The “RecB motif 6” region includes R794, R800, Y803, V804, and T807. See the text for details of the phenotypes and/or in vitro enzyme properties associated with each mutation.