**3. The role of XRCC1 protein in base excision DNA repair**

X-ray cross-complementing group 1 (XRCC1) is a 70- kDa protein comprising three functional domains; an N-terminal DNA binding domain, a centrally located BRCT I and a C-terminal BRCT II domain. It has no known enzymatic activity. Since it specifically interacts with nicked and gapped DNA *in vitro*[7-9], and rapidly and transiently responds to DNA damage in cells, it may serve as a strand-break sensor [10, 11].

DNA single-strand breaks (SSBs) are one of the most frequent types of DNA damage in cells [12]. SSBs can lead to the accumulation of mutations or can be converted from single to cytotoxic double-strand breaks. Thus, SSBs pose a critical threat to the genetic stability and survival of cells[13]. Various proteins have been identified that are part of the repair machinery for SSBs, including XRCC1 protein. XRCC1 has been shown to be critically involved in DNA SSB repair in studies using XRCC1-mutant cells and XRCC1 knockout mice[14], which have increased sensitivity to alkylating agents, ultraviolet and ionizing radiation [15], as well as elevated levels of sister chromatic exchange. Since XRCC1 interacts with many proteins known to be involved in BER and SSBR, it has been proposed that XRCC1 functions as a scaffold protein able to coordinate and facilitate the steps of various DNA repair pathways[11, 16]. For example, XRCC1 interacts with several DNA glycosylases involved in repair of both oxidative and alkylated base lesions, and stimulates their activity[17, 18]. This protein interacts with DNA ligase III, polymerase beta and poly (ADPribose) polymerase to participate in the base excision repair pathway. It is recruited to the site of DNA damage by several DNA glycosylases, e.g. OGG1 or NTH1 and remains at the site of repair till the last stage of ligation (*Fig. 2*), regulating and coordinating the whole process. XRCC1 facilitates exchange of DNA glycosylase with AP-endonuclease at the damaged substrate, which increases the excision rate of modified base, regulates pol ß interactions with APE1, and finally activates ligation step [17]. Binding of XRCC1 to Polynucleotide Kinase (PNK) enhances its capacity for damage discrimination, and binding of XRCC1 to DNA enables displacement of PNK from the phosphorylated product [19] thus accelerating SSBR of damaged DNA[20]. XRCC1 associates with Tyrosyl-DNA phosphodiesterase1(Tdp1) and enhances its activity required for repair of Top1-associated SSBs. It may act to recruit Tdp1 to these damaged sites[21]. Biochemical and nuclear magnetic resonance (NMR) experiments have demonstrated protein-protein interaction between the N-terminal domain of XRCC1 and the polymerase domain of pol β[22-25]. Additionally, stabilization of DNA ligase IIIα is dependent on its interaction with the BRCT II domain of XRCC1[26]. Aprataxin also interacts with XRCC1 and functions to maintain

The Involvement of E2F1 in the Regulation of XRCC1-Dependent Base Excision DNA Repair 131

E2F1 is a member of E2F family of transcription factors which plays an important role in promoting both cellular proliferation and cell death. E2F1 is important for regulating Sphase specific genes as well as promoting apoptosis, just as other "activating" E2F family members [32, 33]. Simultaneously, E2F1 regulates DNA repair through interaction with other factors including RB family proteins, p53 and X-ray repair cross-complementing group

The E2F transcription factor family consists of at least seven distinct genes divided into two groups. E2F1, E2F2, E2F3, E2F4, and E2F5 constitute one group, while the related DP1 and DP2 genes constitute the other group. Several forms of the DP2 (also referred to as DP3) protein can be produced as the result of alternative splicing, thus providing additional complexity to the E2F family. A functional E2F transcription factor consists of a heterodimer containing an E2F polypeptide and a DP polypeptide. Each of the five E2F polypeptides can heterodimerize with either DP1 or DP2 (DP3). Furthermore, each of these E2F/DP heterodimers (referred to as E2F factors hereafter) can bind consensus E2F sites *in vitro* and

All of the E2F subgroup proteins have a similar structure although E2F1, E2F2, and E2F3 are more closely related to each other than to E2F4 and E2F5 (*Fig. 3*). The DNA-binding domain found in the amino terminus represents the area of greatest homology between the five E2F species. Adjacent to the DNA-binding domain is the DP dimerization domain, which contains within it a leucine heptad repeat. The carboxy termini of the five E2F polypeptides contain the defined transcriptional activation domains, which are characterized by an abundance of acidic residues. Embedded within the transactivation domain of each E2F is a

**4. The role of E2F1 in XRCC1 associated base excision DNA repair** 

1 (XRCC1) protein.

Fig. 3. the members of E2F

stimulate transcription when overexpressed[34].

**4.1 E2F family** 

XRCC1 stability, thus further linking the neurological degeneration associated with ataxia to an inefficiency of SSBR[27-29].

Fig. 2. Coordinative role of XRCC1 protein in BER

Several additional proteins participate in BER and play regulative and coordinative role. The most important proteins are: PARP1 (polyADP ribose polymerase, which binds to free DNA ends and protects them against degradation, participates in chromatin relaxation and probably modulates binding of repair proteins to the site of damage by interaction with poly(ADP-ribose) chains [22, 30, 31], PCNA (proliferating cells nuclear antigen, DNA polymerase processivity subunit in LP-BER), RFC (replication factor C, loading PCNA on DNA), WRN (helicase deficient in Werner syndrome, a premature aging disease) or CSB (helicase deficient in Cockayne syndrome, neurodevelopmental and premature aging disease).
