## 4 UvrC is a Damage Specific Dual Nuclease  
UvrC is a dual endonuclease recruited to the UvrB\(\bullet\)DNA pre-incision complex to introduce incisions on either side of the lesion [23]. The first incision is damage-dependent and is made at the 4 th or 5 th phosphodieser bond 3\({}^{\prime}\) to the lesion by the N-terminal domain of UvrC, while the second incision is made at the 8 th phosphodieser bond 5\({}^{\prime}\) to the lesion by another endonuclease domain located towards the C-terminus [52]. Following both incisions, an additional damage-independent cleavage is often observed at the 15 th phosphodieser bond 5\({}^{\prime}\) to the lesion; this extra incision is carried out by the same catalytic domain responsible for the damage-dependent 5\({}^{\prime}\) incision [64].  
Biochemical experiments have shown that both incisions are made by a single UvrC molecule within one binding event and are regulated by binding and hydrolysis of ATP in the UvrB\(\bullet\)UvrC\(\bullet\)DNA complex [65]. The 3\({}^{\prime}\) incision is made by the N-terminal domain of UvrC, which harbors a GIV-YIG motif seen in other endonucleases [66]. [PERSON] and coworkers have proposed that the UvrC 3\({}^{\prime}\) endonuclease domain uses a metal-ion-based mechanism to catalyze cleavage of the phosphodieser bond [48]. On the other hand, the 5\({}^{\prime}\) incision is catalyzed by the C-terminal domain of UvrC. Structure-function studies [67], in combination with structural similarity to RNaseH enzymes, suggest involvement of a two-metal mechanism in the 5\({}^{\prime}\) incision [49].  
Although the complete structure of UvrC remains to be described, several additional structural features are notable. The first is an element (residues 191-233) that resides between the two endonuclease catalytic domains of UvrC (Figure 2C) and exhibits sequence similarity to the C-terminal domain of UvrB; this element is required for the interaction with UvrB. Although the structure of this UvrC element is unknown, a structure is available for the homologous element from UvrB [68; 69]. The arrangement in the crystal features two copies of UvrB C-terminal domain in a head-to-head dimeric configuration, with dimerization mediated by hydrophobic interactions and salt bridges between highly conserved residues located within and around the loop region. This crystallographic interaction provided a basis for constructing a model of contacts made by UvrB and UvrC in the complex; this model is supported by mutational analyses. Notably, interaction between these two segments on UvrB and UvrC is required to form the pre-incision complex and complete the 3\({}^{\prime}\) incision [70], as well as the additional damage-independent 5\({}^{\prime}\) incision [64].  
The other structural feature of UvrC is the DNA-binding domain comprising two helix-hairpin-helix motifs (HhH)\({}_{2}\) that is located at the C-terminus of UvrC [71] (Figure 2C). This domain is required for binding to ssDNA, the 5\({}^{\prime}\) incision and the additional damage-independent incision at the 15 th bond 5\({}^{\prime}\) to the lesion [64; 72]. In addition, it may also play a role in the 3\({}^{\prime}\) incision in certain sequence contexts [73]. The (HhH)\({}_{2}\) DNA-binding domain is observed to adopt significantly different positions relative to the C-terminal 5\({}^{\prime}\) catalytic domain of UvrC [49]. Intrinsic conformational flexibility between the 5\({}^{\prime}\) endonuclease domain and the other domains of UvrC could be important for its ability to incise DNA substrates containing different lesions by allowing UvrC to adjust its positioning of the active site for 5\({}^{\prime}\) incision. Based on the structural similarity [74], a model was proposed in which UvrC uses the conserved G\(\varphi\)G motif, where \(\varphi\) is a hydrophobic residue, in the (HhH)\({}_{2}\) domain to interact with DNA. The relationship between the two nuclease domains and the structural elements noted above during NER will require additional structural and biochemical analyses to elucidate.