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Binding of the XPC:RAD23:CETN2 complex to distorted DNA is enhanced in the presence of the DDB1:DDB2 complex, also known as the UV-DDB complex. The UV-DDB complex preferentially binds UV-generated lesions, such as pyrimidine-pyrimidone photodimers (6-4 PPDs) and cyclobutane pyrimidine dimers (CPDs), but also recognizes DNA with apurinic/apyrimidinic (AP) sites, and 2-3 bp mismatches (Fujiwara et al. 1999, Wittschieben et al. 2005). The DDB2 subunit of the UV-DDB complex is a WD40 repeat beta-propeller protein. The beta-propeller domain of DDB2 binds the damaged DNA strand (Scrima et al. 2008). The UV-DDB complex is part of a larger ubiquitin ligase complex that, besides DDB1 and DDB2, also contains CUL4A or CUL4B and RBX1 (Groisman et al. 2003, Sugasawa et al. 2005). In the case of 6-4 PPDs and CPDs, UV-DDB binding to damaged DNA probably precedes the binding of the XPC:RAD23:CETN2 complex. However, in the case of 6-4 PPDs, the XPC:RAD23:CETN2 complex may also recognize damaged DNA in the absence of the UV-DDB complex (Fitch et al. 2003, Moser et al. 2005, Wang et al. 2004), but the UV-DDB complex may be important for retention of DNA repair proteins at the DNA damage site (Oh et al. 2011).
The INO80 chromatin remodelling complex positively regulates GG-NER. INO80 and ACTR5 (ARP5) subunits of the INO80 complex are enriched at GG-NER sites, probably via interaction with DDB1. Chromatin relaxation by the INO80 complex at DNA damage site may be necessary for XPC recruitment (Jiang et al. 2010). In yeast, the interaction between INO80 and the orthologs of XPC and RAD23 has been reported and it was suggested that this interaction is important for the restoration of chromatin structure after GG-NER completion (Sarkar et al. 2010).
TP53 directly stimulates transcription of several genes involved in DNA mismatch repair, including MSH2 (Scherer et al. 2000, Warnick et al. 2001), PMS2 and MLH1 (Chen and Sadowski 2005). TP53 also directly stimulates transcription of DDB2, involved in nucleotide excision repair (Tan and Chu 2002), and FANCC, involved in the Fanconi anemia pathway that repairs DNA interstrand crosslinks (Liebetrau et al. 1997). Other p53 targets that can influence DNA repair functions are RRM2B (Kuo et al. 2012), XPC (Fitch et al. 2003), GADD45A (Amundson et al. 2002), CDKN1A (Cazzalini et al. 2010) and PCNA (Xu and Morris 1999). Interestingly, the responsiveness of some of these DNA repair genes to p53 activation has been shown in human cells but not for orthologous mouse genes (Jegga et al. 2008, Tan and Chu 2002). Contrary to the positive modulation of nucleotide excision repair (NER) and mismatch repair (MMR), p53 can negatively modulate base excision repair (BER), by down-regulating the endonuclease APEX1 (APE1), acting in concert with SP1 (Poletto et al. 2016).
Expression of several DNA repair genes is under indirect TP53 control, through TP53-mediated stimulation of cyclin K (CCNK) expression (Mori et al. 2002). CCNK is the activating cyclin for CDK12 and CDK13 (Blazek et al. 2013). The complex of CCNK and CDK12 binds and phosphorylates the C-terminal domain of the RNA polymerase II subunit POLR2A, which is necessary for efficient transcription of long DNA repair genes, including BRCA1, ATR, FANCD2, FANCI, ATM, MDC1, CHEK1 and RAD51D. Genes whose transcription is regulated by the complex of CCNK and CDK12 are mainly involved in the repair of DNA double strand breaks and/or the Fanconi anemia pathway (Blazek et al. 2011, Cheng et al. 2012, Bosken et al. 2014, Bartkowiak and Greenleaf 2015, Ekumi et al. 2015).
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