Across the evolutionary spectrum, living organisms depend on high-fidelity DNA replication and recombination mechanisms to maintain genome stability and thus to avoid mutation and disease. The repair of severe lesions in the DNA such as double-strand breaks or stalled replication forks requires the coordinated activities of both the homologous recombination and DNA replication machineries. Growing evidence indicates that so-called “accessory proteins” in both systems are essential for the effective coupling of recombination to replication that is necessary to restore genome integrity following severe DNA damage. In this article we review the major processes of homology-directed DNA repair (HDR), including the double Holliday Junction (dHJ), synthesis-dependent strand annealing (SDSA), break-induced replication (BIR), and error-free lesion bypass pathways. Each of these pathways involves the coupling of a homologous recombination event to DNA synthesis. We highlight two major classes of accessory proteins in recombination and replication that facilitate HDR: Recombination mediator proteins exemplified by T4 UvsY, S. cerevisiae Rad52, and human BRCA2; and DNA helicases/translocases exemplified by T4 Gp41/Gp59, E. coli DnaB and PriA, and eukaryotic Mcm2-7, Rad54, and Mph1. We illustrate how these factors help to direct the flow of DNA and protein-DNA intermediates on the pathway from a double-strand break or stalled replication fork to a high-fidelity recombination-dependent replication apparatus that can accurately repair the damage.

Nucleotide excision repair (NER) is critical for the repair of DNA lesions induced by UV radiation, but its contribution in replicating cells is less clear. Here, we show that dual incision by NER endonucleases, including XPF and XPG, promotes the S-phase accumulation of the BRCA1 and Fanconi anemia–associated DNA helicase FANCJ to sites of UV-induced damage. FANCJ promotes replication protein A phosphorylation and the arrest of DNA synthesis following UV irradiation. Interaction defective mutants of FANCJ reveal that BRCA1 binding is not required for FANCJ localization, whereas interaction with the mismatch repair (MMR) protein MLH1 is essential. Correspondingly, we find that FANCJ, its direct interaction with MLH1, and the MMR protein MSH2 function in a common pathway in response to UV irradiation. FANCJ-deficient cells are not sensitive to killing by UV irradiation, yet we find that DNA mutations are significantly enhanced. Thus, we considered that FANCJ deficiency could be associated with skin cancer. Along these lines, in melanoma we found several somatic mutations in FANCJ, some of which were previously identified in hereditary breast cancer and Fanconi anemia. Given that, mutations in XPF can also lead to Fanconi anemia, we propose collaborations between Fanconi anemia, NER, and MMR are necessary to initiate checkpoint activation in replicating human cells to limit genomic instability.

Background: Gp59 is required to correctly load DNA helicase during T4 recombination-dependent DNA replication and repair transactions. Results: Gp59 forms helicase loading complexes with clusters of Gp32 (SSB) bound to ssDNA. Conclusion: Gp59 follows Gp32 to single-stranded regions of recombination intermediates to promote strand-specific helicase loading. Significance: Mediator proteins such as Gp59 play critical roles in the regulation of DNA recombination, repair, and replication.

Background: Helicase loading protein Gp59 coordinates recombination-dependent DNA replication in bacteriophage T4. Results: A DNA binding-deficient mutant, Gp59-I87A, loads helicase normally onto ssDNA covered with the ssDNA-binding protein, Gp32. Conclusion: Gp59-Gp32 interactions control helicase loading onto D-loops during recombination-dependent replication. Significance: Helicase loading proteins work in concert with single-stranded DNA binding proteins to control the location and timing of helicase assembly in DNA replication, recombination, and repair.

<p>PDF file - 260K, Supplemental Figure 2. MMR localizes FANCJ to UV induced LUDs</p>

<p>PDF file - 151K, Supplemental Figure 1. FANCJ is recruited to sites of UV-induced damage in an NER dependent manner</p>

<p>PDF file - 119K, Supplemental Figure 8. Ectopic expression of catalytic inactive FANCJK52R disrupts clearance of UV induced lesions in U2OS cells</p>

<p>PDF file - 228K, Supplemental Figure 3. FANCJ localizes to UV induced LUDs predominantly in S-Phase and contributes to the UV induced checkpoint</p>