DNA end joining capacity of LIG12/2, LIG42/2, and LIG12/2LIG42/2 extracts is similar to the wt, in line with our 19111597 earlier observations that DNA ligation in this assay mainly reflects LIG3 activity. For further biochemical analysis of the contribution of LIG3 to plasmid DNA end joining, we prepared extracts from 4HT-treated LIG32/2loxP cells. DNA end joining activity is barely detectable 3 d after incubation with 4HT and remains at similarly low levels up to 4 d. Since LIG32/2loxP cells have normal levels of LIG1 and LIG4, the result confirms the dominant role of LIG3 in this version of the assay. A reduction in plasmid DNA end joining after treatment with 4HT similar in magnitude to that measured in LIG32/2loxP is also observed in extracts of LIG32/2loxPLIG42/2 cells. Finally, there is no detectable LIG3 activity in the extracts of LIG32/ 2 LIG42/2mts-hLIG1 cells. This biochemical data in aggregate allows us to conclude that LIG3, as a result of its amino terminal Zn-finger domain, has a dominant contribution to DNA end joining under the reaction conditions employed. LIG3 and LIG1 Contribute to the Survival of IR-exposed Cells Albeit Less Efficiently than LIG4 Exposure of wt DT40 cells to IR compromises their reproductive integrity. Deletion of LIG1 has no effect on this endpoint suggesting that LIG1 is not essential for any of the repair pathways that support the survival of irradiated DT40 cells. As expected, deletion of LIG4 22431203 causes a marked radiosensitization, which is not exacerbated by a concomitant deletion of LIG1. In the latter mutant all ligation BX 912 functions of the DNA metabolism including repair of all forms of radiation-induced DNA lesions are carried out by LIG3. The biphasic shape of the survival curve likely reflects pronounced differences in the radiosensitivity of cells throughout the cell cycle. LIG32/2loxP cells display radiosensitivity very similar to the wt, and treatment with the DNA-PKcs specific inhibitor NU7441 increases radiosensitivity to LIG42/2 levels . Notably, LIG32/2Cdc9 cells are more radiosensitive than the wt pointing to some unique function of LIG3 in cell survival after exposure to IR, or to dominant negative effects exerted by Cdc9. However, after treatment with NU7441, LIG32/2Cdc9 cells become indistinguishable from the LIG12/2LIG42/2 mutant suggesting that the increased radiosensitivity derives from compromised D-NHEJ. These observations indicate that LIG1 substitutes with similar efficiency LIG3 in repair functions supporting cell survival. DNA Ligases in Alternative NHEJ LIG32/M2I cells also show radiosensitivity to killing very similar to that of the wt and add further support to the notion that nuclear LIG3 is not essential for survival in IR exposed cells. Treatment of this mutant with NU7441 increases radiosensitivity to the same extent as in the wt or in LIG32/2loxP cells suggesting that the reduced level of nuclear LIG3 has in the presence of LIG1 no influence on cell radiosensitivity to killing, even when D-NHEJ is compromised. The double mutants LIG32/2loxPLIG42/2 and LIG32/M2ILIG42/2 show radiosensitivity indistinguishable from that of the double LIG12/2LIG42/2 mutant, further demonstrating that strong reduction in nuclear LIG3 leaves 9 DNA Ligases in Alternative NHEJ unchanged the 
ability of DT40 to deal with radiation lesions, presumably because these are processed by LIG1. To further delineate the function of LIG1 in cell survival after exposure to IR, we examined clone 3 that expresses
