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Biological Significance of Domain-oriented DNA Repair in Xeroderma Pigmentosum Cells¹

Department of Biological Sciences, Wright State University, Dayton, Ohio 45435

The patterns (domain oriented versus a random location) and amounts of DNA excision repair, determined by standard density gradient techniques and sedimentation properties of partially repaired and UV-endonuclease-digested DNA in alkaline sucrose gradients, are reported for UV (254 nm)-irradiated nondividing xeroderma pigmentosum complementation group C or A (XP-C, XP-A) and normal cells. Repair synthesis in relatively UV-resistant XP-C (XP4RO) cells is domain oriented and limited (10% of normal values) while it is randomly located and not as limited in more sensitive XP-A (XP8LO) cells. Thus, greater UV resistance is associated with a very limited but domain-oriented pattern of repair. In XP-C cells, both total and domain-oriented repair syntheses, while limited, increase with UV dose and plateau at about 15–20 J/m², as observed for normal cells. We suggest that repair in XP-C is limited at the lower UV doses (less than 15–20 J/m²) by substrate levels in specific chromatin domains and not by availability of essential enzymes for domain-oriented repair. In contrast, the XP-A strain XP8LO exhibits normal repair activities for doses up to 5 J/m² and limited repair at higher doses, indicating that repair occurs through normal pathways that are limited by reduced availability of an essential enzyme.

¹ This work was supported by a Research Challenge Program award (66072) from the State of Ohio to G. J. Kantor and by funds from the Wright State University Biological Sciences Department.

² To whom requests for reprints should be addressed.

Received 7/30/87. Revised 11/ 9/87. Accepted 11/17/87.

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