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XPG is a single strandspecific endonuclease with preference for single to doublestranded DNA junctions for the strand making the transition in the to direction. The protein also has a weak to exonuclease activity. Antibody inhibition experiments suggested that XPG is the nuclease that makes the incision, consistent with the specificity of junction cutting activity.XPFERCC is also a single strandspecific endonuclease for junctions where the singlestranded region makes the transition to doublestranded DNA in the to <a href="http://www.targetmol.com/compound/GS-7340"></a> direction, consistent with the previous finding that XPFERCC makes the incision. The biotinstreptavidin pulldown experiments described in this paper demonstrated that XPG is required for the formation of a stable preincision complex.Thus, in addition to its nuclease function, XPG has a structural role in maintaining a stable preincision complex, which then recruits XPFERCC.The extent of the incision observed in the defined system without XPFERCC is equal to that of uncoupled nicks plus excision by the complete system, indicating that the complex formed in the absence of XPFERCC is functionally similar to the one formed in the presence of XPFERCC.In light of the documented interactions between XPA and XPFERCC and data presented herein, it is tempting to speculate that XPA not only participates in damage recognition, but also acts as an anchor to recruit XPFERCC to the proper incision sites.In other words, this model predicts that XPA must be an integral part of the excision nucleaseDNA complex from the first step of damage recognition to the step prior to incision.Hence, the reaction occurs slowly compared with the normal excision.This mechanism of keeping tightly coupled dual incision may be accomplished by several means.The entire set of excision repair proteins may exist in a large complex termed repairosome, as has been suggested for yeast XPFERCC is actually recruited to the damage site early on, despite the failure to obtain supporting evidence in this study.An as yet unidentified protein can suppress incision by XPG.Upon displacement of this protein by XPFERCC, both and incisions occur in a concerted but nonsynchronous manner.More experiments are needed to differentiate these models.Surprisingly, in human   excinuclease, following the dual incision, the excised oligomer is set free from the gapped DNA.This is especially important for protection of gapped DNA from attack by nonspecific nucleases, which would otherwise produce double strand breaks.At present, we do not know the identity of the proteins associated with either the excised oligomer or the gapped DNA.The excised oligomer might be complexed with XPC since it undergoes degradation in the reactions where excision takes place without XPC.Biol. Chem. doi. jbc. Access the most updated version of this article at http:www.jbc.orgcontent When a correction for this article is posted Click here to choose from all of JBCs email alerts This article cites references, of which can be accessed free at http:www.jbc.orgcontent.full.htmlreflist Downloadedfromhttp: www.jbc.orgbyguestonSeptember, The RAD encoded protein is a ubiquitin conjugating enzyme, and RAD encodes a protein containing a RING finger motif and a nucleotide binding motif.Exposure of cells to ultraviolet light and to many other agents causes the formation of lesions in the DNA.

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