DNA repair defect in xeroderma pigmentosum group C and complementing factor from HeLa cells.

A predominant form of the inherited syndrome xeroderma pigmentosum is genetic complementation group C (XP-C). XP-C cells are defective in DNA nucleotide excision repair in the bulk of the genome but can repair transcribed strands of active genes. An activity that can complement the repair deficiency of extracts from XP-C cells has been purified approximately 2,000-fold from HeLa cells. The factor also increases the unscheduled DNA synthesis of XP-C fibroblasts in vivo after microinjection. Hydrodynamic measurements show that the XP-C complementing factor has a native molecular mass of approximately 160 kDa. The factor binds tightly to single-stranded DNA cellulose, eluting in approximately 1.3 M NaCl. No incision or ATPase activity of the protein alone was detected. XP-C protein is involved in an early stage of repair since its presence was required before the start of gap-filling repair synthesis. In vitro complementation was achieved with naked DNA substrates, and so a primary role in processing chromatin to allow access for repair enzymes seems unlikely. Surprisingly, however, extracts from an XP-C cell line introduced some incisions in UV-irradiated DNA; these were unstable in cell extracts and did not lead to complete repair. The data can be explained by a model in which XP-C factor participates in forming one of the repair incisions flanking DNA damage but not the other. In transcribed DNA, its role is subsumed by RNA polymerase and/or transcription coupling factors.