Several lines of evidence indicate that eukaryotic DNA is arranged in highly supercoiled domains or loops, and that the repeating loops are constrained by attachment to a nuclear skeletal structure termed the nuclear matrix. Active genes are transcribed at the nuclear matrix and during replication the loops are reeled through fixed matrix-associated replication complexes. We have investigated whether the repair of DNA damage also occurs in the nuclear matrix compartment. Biochemical analysis of confluent normal human fibroblasts, ultraviolet (u.v.)-irradiated with 30 J m−2 and post-u. v. incubated in the presence of hydroxyurea, did not show any evidence for the occurrence of repair synthesis at the nuclear matrix either 30min or 13 h after irradiation. Autoradiographic visualization of repair events in single DNA halo–matrix structures confirmed the biochemical observations. At a biologically more relevant dose of 5 J m−2 repair synthesis seems to initiate at the nuclear matrix, although only part of the total repair could be localized there. In u.v.-irradiated (30 J m−2) normal human fibroblasts post-u.v. incubated in the presence of hydroxyurea and arabinosylcytosine for 2h, multiple single-stranded regions are generated in a DNA loop as a result of the inhibition of the excision repair process. Different biochemical approaches revealed that most of the single-stranded regions are clustered, indicating that the repair process itself is non-random or that domains in the chromatin are repaired at different rates. Preferential repair of certain domains in the chromatin was shown to occur in xeroderma pigmentosum cells of complementation group C (XP-C). In XP-C cells these domains are localized near the attachment sites of DNA loops at the nuclear matrix. In contrast, xeroderma pigmentosum cells of complementation group D as well as Syrian hamster embryonic cells with limited excision-repair capacities, revealed a random distribution of repair events in DNA loops. The preferential repair of matrix-associated DNA in XP-C cells may be related partly to repair of transcriptionally active DNA and this may account for the ability of XP-C cells, in contrast to XP-D cells, to recover u.v.-inhibited synthesis of DNA and RNA.

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