title: Defective thymine dimer excision from xeroderma pigmentosum chromatin and its characteristic catalysis by cell-free extracts.
abstract: Specific excision of thymine dimers from isolated normal human and xeroderma pigmentosum (XP complementation groups A, C, D and G) chromatin was investigated under cell-free conditions. Crude extracts derived from unirradiated XP groups A, C and G cells were unable to excise dimers from their own nuclear sonicates, native chromatin and whole-cell sonicates prepared after exposure to 100 J/m2 of u.v. radiation at 254 nm, while normal-cell extracts were able to do so from all substrates including purified DNA. However, the extracts of XP groups A, C and G cells became capable of excising thymine dimers from chromatin preparations depleted of loosely bound nonhistone proteins with 0.35 M NaCl and from purified DNA. Extracts of XP group D cells catalyzed normal levels of excision from nuclear sonicates, native chromatin and 0.35 M NaCl-treated chromatin. These results suggest that none of the XP groups examined is deficient in a dimer-specific u.v. endonuclease. XP groups A, C and G cells are apparently defective in 'XP factors' present in the non-histone protein fraction, which are required for the excision of thymine dimers from chromatin. The XP group D factor appears to be different from the others. Extracts from XP groups A, C and G cells were able to complement each other with respect to dimer excision from chromatin. Novobiocin (200 micrograms/ml) completely inhibited dimer excision effected by extracts of normal cells or by complementing extracts of XP cells.
mesh_terms: D002460:Cell Line; D002843:Chromatin / Q000378:metabolism* / Q000528:radiation effects; D005260:Female; D005333:Fetus; D006801:Humans; D008168:Lung; D011247:Pregnancy; D011740:Pyrimidine Dimers / Q000378:metabolism*; D012867:Skin / Q000378:metabolism / Q000528:radiation effects; D014466:Ultraviolet Rays; D014983:Xeroderma Pigmentosum / Q000235:genetics* / Q000378:metabolism
chemical_list: D002843:Chromatin; D011740:Pyrimidine Dimers